Patent Application
20120312192
The present invention generally relates to antifouling sol-gel films. More particularly, the invention relates to ORMASIL (organically modified silica) antifouling films.
Biofouling on ships' hulls is a significant problem worldwide causing an increase in fuel consumption due to drag as well as mediating the spread of non-indigenous species. The economic impact of biofouling has been estimated to be $56M per year ($1 B over 15 years) for a single class of naval vessel. Biocides have been used in the past to combat biofouling, but the use of biocides in antifouling (AF) paints is becoming increasingly restricted.
The settlement of barnacle cyprids and algal zoospores is highly correlated with surface energies and surface wettabilities as is the removal of algal (Ulva) sporelings (young plants) and adhered diatoms. For example, cypris larvae of B. Amphitrite prefer to settle on surfaces with high wettability and high surface energy while algal zoospores prefer to settle on hydrophobic surfaces with low surface energy. Algal sporelings are more readily removed from surfaces with high wettability and high surface energy while diatoms are more readily removed from surfaces with high wettability and high surface energy. The adhesion of proteins to the xerogel surfaces follows the “Baier curve” with higher adhesion at both low and high surface energies and minimal adhesion near a critical surface tension, γC, of 20 to 25 mN m−1. NATURE in her diversity has created a situation where no previous single surface can have appropriate wettability/surface energy to prevent and/or release micro- and macrofouling of all types.
The present invention provides sol-gel matrix surface coatings (also referred to as ORMASIL films). The present invention also provides methods of preventing fouling of surfaces subjected to an aqueous environment using the coatings of the present invention. The organically-modified, hybrid xerogel coatings of the present invention have been shown to possess antifouling and fouling release characteristics.
Fluoroalkane functionality can be incorporated within the xerogel coatings using the sol-gel process. Mixed alkane and perfluoroalkane modifications can be incorporated from appropriate perfluoroalkyl- and alkyltrialkoxysilane precursors.
In an aspect, the present invention provides sol-gel matrix based surface coatings. The sol-gel matrix is a sol-gel processed xerogel film formed from a sol-gel processed coating precursor (e.g., long-chain alkyltrialkoxysilanes, short-chain alkyltrialkoxysilanes, aminoalkyltrialkoxysilanes, alkylaminoalkyltrialkoxysilanes, dialkylaminoalkyltrialkoxysilanes, and perfluororalkyltrialkoxysilanes) composition. The surface coatings have anti-fouling properties. The coatings are three- or four-component ORMASIL (organically modified silica) xerogel films (also referred to herein as hybrid films). The xerogel films can be formed by sol-gel methods, such as the methods disclosed herein. In an embodiment, an anti-fouling surface coating composition comprises a sol-gel matrix. The precursor composition comprises three or four sol-gel precursor components.
In another aspect, the present invention provides methods for preventing fouling of surfaces subjected to a aqueous environments. The methods can prevent fouling by reducing (or completely preventing) the attachment or growth of biofoulants on a surface.
The present invention provides sol-gel matrix surface coatings. The present invention also provides methods of preventing fouling of surfaces subjected to an aqueous environment using the coatings of the present invention.
Alkyl group as used herein, unless otherwise expressly stated, refers to branched or unbranched saturated hydrocarbons. Examples of alkyl groups include methyl groups, ethyl groups, n-propyl groups, i-propyl groups, n-butyl groups, i-butyl groups, s-butyl groups, pentyl groups, hexyl groups, octyl groups, nonyl groups, and decyl groups and octadecyl groups. The alkyl group can be unsubstituted or substituted with groups such as halides (—F, —Cl, —Br, and—I), alkenes, alkynes, aliphatic groups, aryl groups, alkoxides, carboxylates, carboxylic acids, and ether groups. For example, the alkyl group can be perfluorinated.
Alkoxy group as used herein, unless otherwise expressly stated, refers to—OR groups, where R is an alkyl group as defined herein. Examples of alkyoxy groups include methoxy groups, ethoxy groups, n-propoxy groups, i-propoxy groups, n-butoxy groups, i-butoxy groups, and s-butoxy groups.
The organically-modified, hybrid xerogel coatings of the present invention have been shown to possess antifouling and fouling release characteristics. The xerogel surfaces are inexpensive and robust, have desirable surface roughness/topography, and cover a range of wettabilities (e.g., 35 to 105°, as measured by the static water contact angle, θWs) and surface energies (e.g., 21 to 55 mN m−1).
Fluoroalkane functionality can be incorporated within the xerogel coatings using the sol-gel process. Mixed alkane and perfluoroalkane modifications can be incorporated from appropriate perfluoroalkyl- and alkyltrialkoxysilane precursors. Surface segregation into nm- and/or mm scale structural features on surfaces containing hydrocarbon and fluorocarbon functionality from xerogel coatings prepared from sol-gel precursors incorporating 1 mole % C18 and 1 to 24 mole % tridecafluorooctyltriethoxysilane (TDF) in combination with C8 and 50 mole % TEOS is shown. These coatings can have values of γS outside the 20 to 25 mN m−1 minimaladhesion zone of the Baier curve and behave as AF/FR (anti-fouling/fouling release) coatings.
For example, hybrid three-component xerogels made from combinations of 1,1,1-trifluoropropyltrimethoxysilane (TFP) with phenyltriethoxysilane (PH), n-propyltrimethoxysilane (C3), or n-octyltriethoxysilane (C8) and with tetraethoxysilane (TEOS) as the third component gave uniformly smooth surfaces by time of flight—secondary ion mass spectrometry (ToF-SIMS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). There was no phase segregation and no distinct topographical features were apparent with short-chain perfluoroalkyltrialkoxysilanes and short-chain (e.g., chains of 3 and 8 carbons) alkyltrialkoxysilanes.
The organically-modified, hybrid xerogel coatings have AF and FR characteristics. The xerogel materials have tunable surface energies (by selection of appropriate sol-gel precursors) and are thinner (1-10 μm) with higher elastic modulus than silicone films. For example, a xerogel surface incorporating 1 mole % of an n-octadecyltrimethoxysilane (C18) precursor in combination with n-octyltriethoxysilane (C8) and tetraethoxysilane (TEOS) released juvenile barnacles and sporelings of the macrofouling algae Ulva linza efficiently with performance comparable to that of a poly(dimethylsiloxane)elastomer standard. Other examples of xerogel surfaces incorporating 1:4:45:50 mole % and 1:14:35:50 mole %, respectively, of C18, tridecafluoro-1,1,2,2-tetrahydrooctyl-triethoxysilane (TDF), C8, and TEOS performed desirably with respect to the release of juvenile barnacles and Ulva sporelings. The xerogel surfaces are optically transparent and require no “tie” coat for bonding to a variety of substrates, which makes them viable AF/FR candidates for specialized applications on surfaces involved in optics, sensors, solar panels, and related applications.
For example, the incorporation of low levels (e.g., 1 to 5 mole %) of the long chain n-octadecyltriethoxysilane gave unexpected results with respect to surface topography and the separation of phases on the xerogel surfaces. These surfaces were rougher (root-mean-square roughness>1 nm) and had chemically distinct phases as observed by IR microscopy and AFM. Biological properties that were unexpected included reduced settlement of fouling organisms and easier removal of fouling organisms relative to xerogel coatings lacking the 1 to 5 mole % long chain alkylsiloxane.
In an aspect, the present invention provides sol-gel matrix based surface coatings. The sol-gel matrix is a sol-gel processed xerogel film formed from a sol-gel processed precursor coating composition. The surface coatings have anti-fouling properties. The coatings are three- or four-component ORMASIL (organically modified silica) xerogel films (also referred to herein as hybrid films). The xerogel films can be formed by sol-gel methods, such as the methods disclosed herein.
In an embodiment, an anti-fouling surface coating composition comprises a sol-gel matrix. The precursor composition comprises three or four sol-gel precursor components. In another embodiment, the anti-fouling coating consists essentially of a sol-gel matrix and the compositions consists essentially of three sol-gel precursor components. In another embodiment, the anti-fouling coating consists essentially of a sol-gel matrix and the compositions consists essentially of four sol-gel precursor components. In yet another embodiment, the anti-fouling coating consists of a sol-gel matrix and the composition consists of three sol-gel precursor components. In yet another embodiment, the anti-fouling coating consists of a sol-gel matrix and the composition consists of four sol-gel precursor components.
In an embodiment, a first precursor component is a long-chain alkyltrialkoxysilane, a perfluoalkyltrialkoxysilane, or is selected from an aminoalkyltrialkyoxysilane, alkylaminoalkyltrialkoxysilane, and dialkylaminoalkyltrialkoxysilane. A second precursor component is a shorter-chain alkyltrialkoxysilane, or, if the first precursor component is an aminoalkyltrialkyoxysilane, alkylaminoalkyltrialkoxysilane, or dialkylaminoalkyltrialkoxysilane, then the second precursor is a long-chain alkyltrialkoxysilane. A third component is a tetraalkoxysilane.
In another embodiment, where the first component is a long-chain alkyltrialkoxysilane, the sol-gel processed composition further comprises a fourth component that is a perfluoroalkyltrialkoxysilane.
In an embodiment, the third precursor component makes up the remainder of the precursor composition.
In an embodiment, the three-component xerogel surface incorporates 0.25 mole % to 5.0 mole % of a long-chain alkyltrialkoxy silane (where long-chain refers to ten (10) or more carbons, such as, but not limited to, n-dodecyltriethoxysilane (C12) or n-octadecyltriethoxysilane (C18)) precursor in combination with 20 mole % to 55 mole % of a shorter-chain alkyltrialkoxysilane (such as, but not limited to, n-propyltrimethoxysilane (C3) or n-octyltriethoxysilane (C8)) and a tetraalkoxysilane (such as, but not limited to, tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), or tetraisopropoxysilane (TIPOS)).
In embodiment, 1 mole % to 45 mole % of a long-chain perfluoroalkyltrialkoxysilane (where long-chain refers to eight (8) or more carbons such as, but not limited to, tridecafluorooctyltriethoxysilane (TDF) or tridecafluorooctyltrimethoxysilane) in combination with 20 mole % to 55 mole % of a shorter-chain alkyltrialkoxysilane (such as, but not limited to, n-propyltrimethoxysilane (C3) or n-octyltriethoxysilane (C8)) and a tetraalkoxysilane (such as, but not limited to, tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), or tetraisopropoxysilane (TIPOS)) are incorporated in the surface.
In an embodiment, 1 mole % to 20 mole % of an aminoalkyl-, alkylaminoalkyl-, or dialkylaminoalkyltrialkoxysilane (such as, but not limited to, aminopropyltriethoxysilane (AP), methylaminopropyltriethoxysilane (MAP), or dimethylaminopropyltriethoxysilane (DMAP)) in combination with 1 mole % to 45 mole % of a long-chain perfluoroalkyltrialkoxysilane (where long-chain refers to eight (8) or more carbons such as, but not limited to, tridecafluorooctyltriethoxysilane (TDF) or tridecafluorooctyltrimethoxysilane) and a tetraalkoxysilane (such as, but not limited to, tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), or tetraisopropoxysilane (TIPOS)) are incorporated in the surface.
In an embodiment, 1 mole % to 20 mole % of an aminoalkyl-, alkylaminoalkyl-, or dialkylaminoalkyltrialkoxysilane (such as, but not limited to, aminopropyltriethoxysilane (AP), methylaminopropyltriethoxysilane (MAP), or dimethylaminopropyltriethoxysilane (DMAP)) in combination with 1 mole % to 45 mole % of a longer-chain alkyltrialkoxysilane (where longer-chain refers to eight (8) or more carbons, such as, but not limited to, n-octyltriethoxysilane (C8), n-dodecyltriethoxysilane (C12), or n-octadecyltriethoxysilane (C18)) and a tetraalkoxysilane (such as, but not limited to, tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), or tetraisopropoxysilane (TIPOS)) are incorporated in the surface.
The sol-gel precursors are long-chain alkyltrialkoxysilanes, short-chain alkyltrialkoxysilanes, aminoalkyltrialkoxysilanes, alkylaminoalkyltrialkoxysilanes, dialkylaminoalkyltrialkoxysilanes, and perfluororalkyltrialkoxysilanes. The sol-gel precursors can be obtained from commercial sources or synthesized by known methods.
The long-chain alkyltrialkoxysilane has a long-chain alkyl group and three alkoxy groups. The long-chain alkyltrialkoxysilane has the following structure:
where, in this structure, R′ is a long-chain alkyl group and R is an alkyl group of an alkoxy group. The long chain alkyl group is a C10 to C25, including all integer numbers of carbons and ranges therebetween, alkyl group. The alkoxy groups are, independently, C1, C2, or C3 alkoxy groups. The alkoxy groups can have the same number of carbons. The long-chain alkyltrialkoxysilane is present as a first component at from 0.25 mole % to 5.0 mole %, including all values to the 0.1 mole % and ranges therebetween, or as a second component at 1 mole % to 45 mole %, including all integer mole % values and ranges therebetween. Examples of suitable long-chain alkyltrialkoxysilanes include n-dodecyltriethoxysilane, n-octadecyltriethoxysilane, and n-decyltriethoxysilane.
The short-chain alkyltrialkoxysilane has the following structure:
where, in this structure, R′ is a short-chain alkyl group and R is an alkyl group of an alkoxy group. The short-chain alkyltrialkoxysilane has a short-chain alkyl group and three alkoxy groups. The short-chain alkyl group is a C3 to C8, including all integer numbers of carbons and ranges therebetween, alkyl group The alkoxy groups are, independently, C1, C2, or C3 alkoxy groups. The alkoxy groups can have the same number of carbons. The short-chain alkyltrialkoxysilane is present at 20 mole % to 55 mole %, including all integer mole % values and ranges therebetween. Examples of suitable short-chain alkyltrialkoxysilanes include n-propyltrimethoxy silane, n-butyltriethoxysilane, n-pentyltriethoxysilane, n-hexyltriethoxysilane, n-heptyltriethoxysilane, n-octyltriethoxysilane, and branched analogues thereof.
The aminoalkyltrialkoxysilane has an aminoalkyl group and three alkoxy groups. The aminoalkyltrialkoxysilane has the following structure:
where, in this structure, R′ is a an alkyl group of the aminoalkyl group and R is an alkyl group of an alkoxy group. The aminoalkyl group has a C1 to C10, including all integer numbers of carbons and ranges therebetween, aminoalkyl group. The alkoxy groups are, independently, C1, C2, or C3 alkoxy groups. The alkoxy groups can have the same number of carbons. The aminoalkyltrialkoxy silane is present at 1 mole % to 20 mole %, including all integer mole % values and ranges therebetween. Examples of suitable aminoalkyltrialkoxysilanes include aminomethyltriethoxysilane, aminoethyltriethoxysilane, aminopropyltriethoxysilane, aminobutyltriethoxysilane, aminopentyltriethoxysilane, and aminohexyltriethoxysilane.
The alkylaminoalkyltrialkylsilane has an alkylamino group, aminoalkyl group, and three alkoxy groups. The alkylaminoalkyltrialkoxysilane has the following structure:
where, in this structure, R″ is the alkyl group of the alkylamino group and R′ is a the alkyl group of the alkylaminoalkyl group and R is an alkyl group of a alkoxy group. The aminoalkyl group has a C1 to C10, including all integer numbers of carbons and ranges therebetween, alkyl group. The aminoalkyl group has a C1 to C10, including all integer numbers of carbons and ranges therebetween, alkyl group. The alkoxy groups are, independently, C1, C2, or C3 alkoxy groups. The alkylaminoalkyltrialkoxysilane is present at 1 mole % to 20 mole %, including all integer mole % values and ranges therebetween. The alkoxy groups can have the same number of carbons. Examples of suitable alkylaminoalkyltrialkoxysilanes include methylaminoethyltriethoxysilane, methylaminopropyltriethoxysilane, methylaminobutyltriethoxysilane, methylaminopentyltriethoxysilane, methylaminohexyltriethoxysilane, and ethyl and propyl amino analogues thereof.
The dialkylaminoalkyltrialkoxysilane has the following structure:
where, in this structure, R′ and R″ are each an alkyl group of the alkylamino group and R′″ is the alkyl group of the dialkylaminoalkyl group and R is an alkyl group of a alkoxy group. The dialkylaminoalkyltrialkylsilane has a dialkylamino group, aminoalkyl group, and three alkoxy groups. The alkyl groups of the diaminoalkyl group are, independently, C1 to C10, including all integer numbers of carbons and ranges therebetween, alkyl groups. The dialkylamino alkyl groups can have the same number of carbons. The aminoalkyl group has a C1 to C10, including all integer numbers of carbons and ranges therebetween, alkyl group. The alkoxy groups are, independently, C1, C2, or C3 alkoxy groups. The alkoxy groups can have the same number of carbons. The dialkylaminoalkyltrialkoxysilane is present at 1 mole % to 20 mole %, including all integer mole % values and ranges therebetween. Examples of suitable dialkylaminoalkyltrialkoxysilanes include dimethylaminoethyltriethoxysilane, dimethylaminopropyltriethoxysilane, dimethylaminobutyltriethoxysilane, dimethylaminopentyltriethoxysilane, dimethylaminohexyltriethoxysilane, and diethylamino and dipropylamino analogues thereof.
The perfluoroalkyltrialkoxysilane has the following structure:
where, in this structure, R′ is a perfluoroalkylalkyl group and R is an alkyl group of an alkoxy group. The perfluoroalkyltrialkoxysilane has a perfluoroalkyl group and three alkoxy groups. The pefluoroalkyl group is a C8 to C25, including all integer numbers of carbons and ranges therebetween, alkyl group. The alkoxy groups are, independently, CI, C2, or C3 alkoxy groups. The alkoxy groups can have the same number of carbons. The perfluoroalkyltrialkoxysilane is present at 1 mole % to 45 mole %, including all integer mole values and ranges therebetween. Examples of suitable perfluoroalkyltrialkoxysilanes include tridecafluorooctadecyltriethoxysilane and tridecafluorooctyltrimethoxysilane.
The tetraalkoxysilane has the following structure:
where, in this structure, R is an alkyl group of an alkoxy group.
The tetraalkoxysilane has four alkoxy groups. The alkoxy groups are, independently, C1, C2, or C3 alkoxy groups. The alkoxy groups can have the same number of carbons. The tetraalkoxysilane is present at 1 mole % to 90 mole %, including all integer mole % values and ranges therebetween. Examples of suitable tetralkoxysilanes include tetramethoxysilane, tetraethoxysilane, and tetraisopropoxysilane.
The sol-gel matrix surface coatings comprise functional groups derived from the precursor silanes. For example, coatings formed using perfluoroalkyltrialkoxysilanes have perfluoroalkyl groups. The surface coatings also have residual silanol functional groups. The groups can be on the surface of the film or in the bulk matrix of the film.
The thickness of the sol-gel matrix surface coatings can be varied based on the deposition method and/or parameters of the deposition process (e.g., concentrations of the precursor components). For example, the film can have a thickness of 1 micron to 20 microns, including all integer thickness values and ranges therebetween.
The sol-gel matrix surface coatings have desirable properties. For example, the coatings have desirable wetting properties (which can be measured by, for example, contact angle) and surface roughness. In various examples, the contact angle of the film is greater than 95 degrees or greater than 100 degrees. For example, the contact angle of the coating is between 90 and 150 degrees, including all integer degree values and ranges thereof. For example, the surface roughness is greater than 1 nm. For example, the surface roughness is between 1 and 20 nm, including all values to the nm and ranges thereof.
The surface roughness can lead to topographical features, such as nanopores, as is observed with the 1:49:50 C18/C8/TEOS xerogel, while smooth or rough surfaces can have phase segregation of hydrocarbon, fluorocarbon and silicon oxide features as observed for 1:49:50 C18/C8/TEOS, 1:4:45:50 C18/TDF/C8/TEOS and 1:14:35:50 C18/TDF/C8/TEOS xerogels.
In an embodiment, an anti-fouling surface coating composition comprises a sol-gel matrix made by a method comprising the following steps: forming a precursor composition comprising three or four sol-gel precursor components, coating the precursor composition on a surface such that a sol-gel matrix film is formed on the surface.
Generally, the precursor composition (referred to herein as a sol) is formed by combining three or four sol-gel precursor components and allowing the components to stand for a period of time such that a desired amount of hydrolysis and polymerization of the precurors occurs. This precursor composition is coated on a surface and surface allowed to stand for a period of time such that a xerogel film having is formed. The determination of specific reaction conditions (e.g., mixing times, standing times, acid/base concentration, solvent(s)) for forming the xerogel film is within the purview of one having skill in the art.
In another aspect, the present invention provides methods for preventing fouling of surfaces subjected to a aqueous environments. The methods can prevent fouling by reducing (or completely preventing) the attachment or growth of biofoulants on a surface.
Biofoulants are organisms that attach and grow on surfaces in aqueous environments. Examples of biofoulants include algae, diatoms, bacteria, barnacles, and biofilms.
Aqueous environments are any aqueous media in which biofoulants are, or may be, present. Examples of such aqueous environments include freshwater and saltwater environments. The aqueous environments can be naturally occurring or man made. Examples of aqueous environments include rivers, lakes, and oceans. Additional examples of aqueous environments include tanks of freshwater or saltwater.
The surface is any surface that can be contacted with an aqueous environment. The surfaces can be materials such as metals (such as marine grade aluminum), plastics, composites (such as fiberglass), glass, wood, or other natural fibers. Examples of suitable surfaces include surfaces of a water-borne vessel such as a boat, ship, personal watercraft, . . . , and surfaces of devices such as optics (e.g., underwater cameras), sensors (e.g., marine sensors), submersible solar panels, and related applications. Another example of a surface is the surface of a monitoring system of an underwater archaeological site.
In an embodiment, the method comprises the step of applying a coating of anti-fouling coating composition as described herein to at least a portion of a surface subjected to an aqueous environment such that such an ORMOSIL xerogel film is formed on the surface and the xerogel film prevents or reduces fouling of the surface.
The coating of anti-fouling coating composition can be applied by a variety of coating methods. Examples of suitable coating methods including spray coating, dip coating, brush coating, or spread coating.
The sol-gel matrix coating can be formed by acid-catalyzed hydrolysis and polymerization of the precursor components. In an embodiment, the antifouling precursor composition further comprises an acidic component that makes the pH of the composition sufficiently acidic so that the components undergo acid-catalyzed hydrolysis to form the sol-gel matrix. Examples of suitable acidic components include aqueous acids such as hydrochloric acid, hydrobromic acid and trifluoroacetic acid. Conditions and components required for acid-based hydrolysis of sol-gel components are known in the art.
After applying the coating of anti-fouling coating composition, the coating is allowed to stand for a time sufficient to form the sol-gel matrix. Depending on the thickness of the coating, the standing time is, for example, from 1 hour to 72 hours, including all integer numbers of hours and ranges therebetween.
By preventing fouling it is meant that the attachment or growth of organisms is completely eliminated (i.e., no growth or attachment is detectible by methods, such as optical or fluorescence methods known in the art). By reducing fouling it is meant that the attachment or growth of organisms is reduced by more than 70%, 80%, 90%, 95%, 96%, 97%, 98%, or 99%. For example, the reduction in fouling (e.g., fouling rating) can be measured by determining difference in the amount of biofoulant (e.g., the number of settled organisms) on xerogel coated and uncoated surfaces under the same conditions by optical or fluorescence methods. As another example, the reduction in fouling (e.g., fouling release) is determined by the difference in water pressure required to remove biofoulant (e.g., settled organisms) from a xerogel coated and non-xerogel coated surface under the same conditions.
The steps of the methods described in the various embodiments and examples disclosed herein are sufficient to practice the methods of the present invention. Thus, in an embodiment, the method consists essentially of a combination of the steps of a method disclosed herein. In another embodiment, the method consists of such steps.
The following examples are presented to illustrate the present invention. They are not intended to limiting in any manner.
In this example, Three- and four-component, hybrid xerogel surfaces that have high contact angles (>95°) and that perform as foul-release surfaces are described.
Three-component Xerogels. Twenty-five (25) examples of three-component, hybrid xerogel surfaces incorporating the embodiments described above that perform as foul-release surfaces for the release of juvenile barnacles of Balanus amphitrite and/or 7-day old sporelings of the macrofouling alga Ulva linza are compiled in Table 1 as entries 9-33. In Table 1, entries 1-8 represent glass and PDMSE standards (entries 1 and 2, respectively), examples of two-component hybrid xerogels containing short-chain alkyl groups (entries 3 and 4), short-chain perfluoroalkyl groups (entry 5), long-chain perfluoroalkyl groups (entries 6 and 7), and a dialkylaminoalkyl group (entry 8) in combination with a tetraalkoxysilane (TEOS).
aMean of five (5) independent measurements for coatings stored in air prior to measurement. ± one standard deviation.
bAverage of five (5) replicate measurements. ± one standard deviation.
cApproximate surface water pressure (kPa) to give 50 percent removal of 7-day old sporelings from xerogel coatings and glass and PDMSE standards.
dNumber of juvenile barnacles completely removed with n as the total number of barnacles removed.
eAverage critical removal stress (CRS); ± standard error of the mean.
fAverage percentage of basal plate remaining after barnacle removal where n - CR (complete removal) is the number of barnacles leaving behind a partial base plate; ± standard error of the mean.
gSingle point datum.
Many of the surfaces are compared in Table 1 with respect to the water pressure required to remove 50% of 7-day-old Ulva sporeling growth and all of the surfaces in Table 1 are compared for the removal of juvenile barnacles of B. Amphitrite. For sporeling removal, PDMSE, a comparative example, is an industry standard and gave 50% removal with a water pressure of 69 kPa. The two-component xerogel surfaces in comparative example entries 3-8 gave 50% sporeling removal with a range of pressures from 75 kPa to >110 kPa. With respect to barnacle removal, three values are important: 1) the percentage of barnacles removed completely (i.e., no base plate remaining), 2) the critical removal stress (CRS) to remove the intact barnacle, and 3) for those barnacles that were not removed completely, the percentage of the basal plate remaining. PDMSE gave complete removal of 87% of the barnacles with an average CRS of 0.18 N mm−2 and, for the barnacles not removed completely, an average of 2% of the basal plate remained. For the xerogel surfaces of entries 3-8 used as standards, none of the barnacles was removed intact and an average of >85% of the basal plate remained on all surfaces.
The three-component, hybrid xerogel surfaces of entries 9-33 of Table 1 incorporating long-chain perfluoroalkyltrialkoxysilanes and/or long-chain alklytrialkoxysilanes outperformed the xerogel surfaces of entries 3-8 as foul-release surfaces. With the exception of entries 9 and 11 (90 and 91 kPa, respectively), the pressures required for the 50% removal of Ulva sporelings from the xerogel surfaces were ≦75 kPa. Importantly, entries 13, 15, 17-23, 25, and 27 were comparable to or outperformed the PDMSE standard. For barnacle removal, all of the three-component xerogels (entries 9-33) outperformed the two-component xerogels (entries 3-8). For entries 9-33, the basal plate remaining was ≦75% for the barnacles not removed intact while for entries 3-8 left >85% of the basal plate. Entries 3-8 showed zero (0) barnacles removed intact while entries 10, 12-17, and 22-33 gave complete removal of some barnacles. The values of CRS for barnacles removed completely from these surfaces covered the range of 0.19-0.26 N mm−2.
The 1:49:50 C18/C8/TEOS xerogel (entry 13 of Table 1) was statistically identical to the PDMSE standard (entry 2 of Table 1). Water pressures for the removal of 50% of 7-day-old sporeling growth were essentially identical (69 kPa for PDMSE and 67 kPa for 1:49:50 C18/C8/TEOS), comparable numbers of barnacles were removed intact from each surface (87% for PDMSE and 79% for 1:49:50 C18/C8/TEOS) with comparable values of CRS (0.18 N mm−2 for PDMSE and 0.20 N mm−2 for 1:49:50 C18/C8/TEOS).
Four-component Xerogels. The addition of a long-chain alkyl component to three-component xerogel formulations and found improved performance for the release of barnacles as shown in Table 2 was examined. Entries 14-17 of Table 1 show the performance of TDF/C8/TEOS xerogel formulations. The addition of 1 mole % C18 to these formulations as shown in entries 3-8 of Table 2 facilitated the release of barnacles as shown by an increase in the number of barnacles removed intact (30-100% vs. 5-20% for entries 14-17 of Table 1), a decrease in CRS for barnacle removal (0.12-0.24 N mm−2 vs. 0.19-0.26 N mm−2 for entries 14-17 of Table 1), and a decrease in the partial basal plate remaining on those barnacles not removed intact (0-12% vs. 26-41% for entries 14-17 of Table 1).
Similar results were found for the addition of 1 mole % C18 to the DMAP/C8/TEOS formulations as shown for entries 9-12 of Table 2. Entries 24-27 of Table 1 illustrate the performance of the DMAP/C8/TEOS xerogels. The addition of 1 mole % C18 to these formulations as shown in entries 9-12 of Table 2 facilitated the release of barnacles as shown by an increase in the number of barnacles removed intact (30-80% vs. 17-65% for entries 24-27 of Table 1).
aMean of five (5) independent measurements for coatings stored in air prior to measurement. ± one standard deviation.
bNumber of juvenile barnacles completely removed with n as the total number of barnacles removed.
cAverage critical removal stress (CRS); ± standard error of the mean.
dAverage percentage of basal plate remaining after barnacle removal where n-CR (complete removal) is the number of barnacles leaving behind a partial base plate; ± standard error of the mean.
eSingle point datum.
Entries 1 and 2 are comparative examples.
Several of the xerogel formulations of Table 2 were comparable to the PDMSE standard: the 1:19:30:50 C18/TDF/C8/TEOS xerogel (entry 7), the 1:24:25:50 C18/TDF/C8/TEOS xerogel (entry 8), and the 0.5:1:48.5:50 DMAP/C18/C8/TEOS xerogel (entry 9 of Table 1) formed one group. These surfaces had comparable numbers of barnacles removed intact from each surface (87% for PDMSE and 69 to 85% for the four-component xerogels), comparable values of CRS (0.18 N mm−2 for PDMSE and 0.16 to 0.20 N mm−2 for the four-component xerogels), and comparable percentages for the percentage of basal plate remaining from barnacles not removed intact (2% for PDMSE and 3 to 12% for the four-component xerogels).
A second group of four-component xerogel surfaces, the 1:4:45:50 C18/TDF/C8/TEOS xerogel (entry 4 of Table 2), the 1:9:40:50 C18/TDF/C8/TEOS xerogel (entry 5 of Table 2) and the 1:14:35:50 C18/TDF/C8/TEOS xerogel (entry 6 of Table 2), while similar to the PDMSE standard, displayed improved characteristics for barnacle removal with respect to at least one parameter: the number of barnacles removed intact from each surface (87% for PDMSE and 75-100% for the four-component xerogels), values of CRS (0.18 N mm−2 for PDMSE and 0.12-0.16 N mm−2 for the four-component xerogels), and the percentage of basal plate remaining from barnacles not removed intact (2% for PDMSE and 0-4% for the four-component xerogels).
Changes in Surface Topography. All of the three- and four-component, hybrid xerogel surfaces of Tables 1 and 2 have values of the static water contact angle (θWs) that are ≧95°. The contact angle appears to be a critical indicator for the release of juvenile barnacles and Ulva sporelings. Another critical feature of the long-chain xerogel components is the introduction of topographical features on the nanoscale.
The incorporation of 1 to 5 mole % C18 in the C8/TEOS sol (a relatively small change in formulation) dramatically impacted the distribution of chemical functionality on the xerogel surface (from transmission IR microscopy images) and the surface topography (AFM studies). The IR images of the 50:50 C8/TEOS and 1:49:50 C18/C8/TEOS xerogels films shown in
The numerical data for the comparison of xerogel surfaces containing 1 to 5 mole % C18 in the C8/TEOS sol, the 50:50 C8/TEOS xerogel and the PDMSE standard are compiled in Table 1. Graphical comparison of these surfaces is shown in
The segregation of chemical features into topographical features is also apparent in the four-component xerogels. As shown in
Materials and Methods. Chemical Reagents.
Deionized water was prepared to a specific resistivity of at least 18 MΩ using a Barnstead NANOpure Diamond UV ultrapure water system. Tetraethoxysilane or tetraethyl orthosilicate (TEOS), n-propyltrimethoxysilane (C3), n-octadecyltrimethoxysilane (C18), n-octyltriethoxy-silane (C8), 3,3,3-trifluoropropyltrimethoxysilane (TFP), tridecafluorooctyltriethoxysilane (TDF), 3-aminopropyltriethoxysilane (AP), methylaminopropyltriethoxysilane (MAP), and dimethylaminopropyltriethoxysilane were purchased from Gelest, Inc. and were used as received. Ethanol was purchased from Quantum Chemical Corp. Hydrochloric acid was obtained from Fisher Scientific Co. Borosilicate glass microscope slides were obtained from Fisher Scientific, Inc.
Sol Preparation.
The sol/xerogel composition is designated in terms of the molar ratio of Si-containing precursors. Thus, a 50:50 C8/TEOS composition contains 50 mole % C8 and 50 mole % TEOS.
Sol TEOS.
TEOS (3.96 g, 17.1 mmol, 3.35 mL), water (0.54 mL), ethanol (3.40 mL), and HCL (0.1 M, 15 μL) were stoppered in a glass vial and stirred at ambient temperature for 6 hours.
Sol AP. AP (2.544 g, mmol) was added dropwise to a stirred mixture of 6.67 M HCl (2.000 mL) and ethanol (10.56 ml). Once addition was complete the solution was mixed via sonication at ambient temperature for 40 min.
10:90 AP/TEOS.
A mixture of sol TEOS (3.353 mL) and sol AP (1.000 mL) was sonicated for 20 min at ambient temperature.
10:90 TMAP/TEOS.
A mixture of TEOS (2.4 g, 64.1 mmol), TMAP (0.50 g, 1.2 mmol), water (1.8 mL), ethanol (3.0 mL), and 12 M HCl (5.2 μL) was stirred at ambient temperature for 12 hours.
Sol DMAP.
DMAP (1.054 g, 4.827 mmol) was added dropwise to a mixture of 6.67 M HCl (0.955 mL) and ethanol (4.668 mL). The resulting solution was stirred at ambient temperature for 40 min.
10:90 DMAP/TEOS.
Sol DMAP (5.11 ml, 3.68 mmol) was added dropwise to sol TEOS (16.2 ml, 33.1 mmol). The mixture was stirred at ambient temperature for 20 min.
Sol MAP.
MAP (2.000 g, 10.34 mmol) was added dropwise to 6.67 M HCl (2.04 mL, 15 mmol) and ethanol (10.0 mL). The resulting solution was stirred at ambient temperature for 40 min.
10:90 MAP/TEOS.
Sol MAP (5.013 ml, 3.68 mmol) was added dropwise to sol TEOS (16.2 mL, 33.1 mmol). The resulting mixture was stirred at ambient temperature for 20 min.
50:50 TFP/TEOS.
A mixture of TEOS (1.82 g, 7.8 mmol), TFP (1.70 g, 7.8 mmol), H2O (0.563 ml, 31 mmol), and ethanol (3.5 ml, 60 mmol) was capped and sonicated at ambient temperature for 0.5 hour.
50:50 C3/TEOS.
A mixture of C3 (2.0 g, 12.17 mmol), TEOS (2.53 g, 12.17 mmol), ethanol (4.0 mL), and 0.1 N HCl (2.1 mL, 0.21 mmol) was capped and stirred at ambient temperature for 8 hours.
25:25:50 TFP/C8/TEOS.
A mixture of C8 (1.25 g, 4.5 mmol), TFP (1.0 g, 4.5 mmol), TEOS (1.8 g, 9.0 mmol), ethanol (3.0 mL), and 0.1 N HCl (1.6 mL, 0.16 mmol) was stirred at ambient temperature for 3 hours.
25:25:50 TFP/C3/TEOS.
A mixture of C3 (0.93 g, 4.5 mmol), TFP (1.0 g, 4.5 mmol), TEOS (1.87 g, 9.0 mmol), ethanol (3.0 mL), and 0.1 N HCl (1.6 mL, 0.16 mmol) was stirred at ambient temperature for 3 hours.
50:50 C8/TEOS.
A mixture of TEOS (2.70 g, 13 mmol), C8 (3.59 g, 13 mmol), ethanol (5.0 mL, 87 mmol) and 0.1 N HCl (1.6 mL, 0.16 mmol) was capped and stirred at ambient temperature for 24 hours.
5:45:50 C18/C8/TEOS.
A mixture of C18 (0.269 g, 0.72 mmol, 0.305 mL), C8 (1.79 g, 6.48 mmol, 2.03 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL), was stirred at ambient temperature for 24 hours.
4:46:50 C18/C8/TEOS.
A mixture of C18 (0.215 g, 0.58 mmol, 0.244 mL), C8 (1.83 g, 6.62 mmol, 2.08 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL), was stirred at ambient temperature for 24 hours.
3:47:50 C18/C8/TEOS.
A mixture of C18 (0.161 g, 0.43 mmol, 0.183 mL), C8 (1.87 g, 6.77 mmol, 2.12 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL), was stirred at ambient temperature for 24 hours.
2:48:50 C18/C8/TEOS.
A mixture of C18 (0.108 g, 0.29 mmol, 0.122 mL), C8 (1.91 g, 6.91 mmol, 2.17 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL), was stirred at ambient temperature for 24 hours.
1:49:50 C18/C8/TEOS.
A mixture of C18 (0.054 g, 0.14 mmol, 0.061 mL), C8 (1.95 g, 7.06 mmol, 2.21 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL), was stirred at ambient temperature for 24 hours.
10:90 TDF/TEOS.
TDF (0.288 g, 0.533 mmol, 0.213 mL), and TEOS (1.0 g, 4.80 mmol, 1.07 mL) were mixed. Ethanol (1.77 mL), and HCl (0.288 mL, 0.1 M), were added and the resulting solution was stirred at ambient temperature for 24 hours. At this time a 0.400 mL aliquot was removed and spun cast onto a glass microscope slide.
20:80 TDF/TEOS.
TDF (0.612 g, 1.2 mmol, 0.453 mL), and TEOS (1.07 g, 4.08 mmol) were mixed. Ethanol (2.0 mL), and HCl (0.583 mL, 0.1 M), were added and the resulting solution was stirred at ambient temperature for 24 hours. At this time a 0.400 mL aliquot was removed and spun cast onto a glass microscope slide.
10:40:50 TDF/C8/TEOS. C8 (1.06 g, 3.84 mmol, 1.21 mL), TDF (0.49 g, 0.96 mmol, 0.363 mL), and TEOS (1.0 g, 4.80 mmol, 1.07 mL) were mixed. Ethanol (3.2 mL), and HCl (0.52 mL, 0.1 M), were added and the resulting solution was stirred at ambient temperature for 24 hours. At this time a 0.400 mL aliquot was removed and spun cast onto a glass microscope slide.
20:30:50 TDF/C8/TEOS.
C8 (0.79 g, 2.88 mmol, 0.90 mL), TDF (0.98 g, 1.92 mmol, 0.725 mL), and TEOS (1.0 g, 4.80 mmol, 1.07 mL) were mixed. Ethanol (3.2 mL), and HCl (0.52 mL, 0.1 M), were added and the resulting solution was stirred at ambient temperature for 24 hours. At this time a 0.400 mL aliquot was removed and spun cast onto a glass microscope slide.
30:20:50 TDF/C8/TEOS.
C8 (0.53 g, 1.92 mmol, 0.60 mL), TDF (1.47 g, 2.88 mmol, 1.08 mL), and TEOS (1.0 g, 4.80 mmol, 1.07 mL) were mixed. Ethanol (3.2 mL), and HCl (0.52 mL, 0.1 M), were added and the resulting solution was stirred at ambient temperature for 24 hours. At this time a 0.400 mL aliquot was removed and spun cast onto a glass microscope slide.
40:20:50 TDF/C8/TEOS.
C8 (0.26 g, 0.26 mmol, 0.26 mL), TDF (1.96 g, 3.84 mmol, 1.45 mL), and TEOS (1.0 g, 4.80 mmol, 1.07 mL) were mixed. Ethanol (3.2 mL), and HCl (0.52 mL, 0.1 M), were added and the resulting solution was stirred at ambient temperature for 24 hours. At this time a 0.400 mL aliquot was removed and spun cast onto a glass microscope slide.
5:5:90 DMAP/TDF/TEOS.
Sol DMAP (2.489 ml, 1.792 mmol) was added dropwise to a stirring solution of TDF (0.915 g, 1.792 mmol), TEOS (6.72 g, 32.26 mmol), ethanol (5.039 ml), and 0.1M HCl (2.517 ml). The resulting mixture was stirred at ambient temperature for 24 hours.
2:48:50 C12/C8/TEOS.
C12 (0.214 g, 0.72 mmol), C8 (5.04 g, 17.3 mmol), TEOS (3.750 g, 18.0 mmol), and isopropanol (11.55 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
4:46:50 C12/C8/TEOS.
C12 (0.418 g, 1.44 mmol), C8 (4.579 g, 16.56 mmol), TEOS (3.750 g, 18.0 mmol), and isopropanol (11.55 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
5:45:50 C12/C8/TEOS.
C12 (0.523 g, 1.80 mmol), C8 (4.35 g, 12.4 mmol), TEOS (3.750 g, 18.0 mmol), and isopropanol (11.55 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
10:40:50 C12/C8/TEOS.
C12 (1.046 g, 3.60 mmol), C8 (3.981 g, 14.40 mmol), TEOS (3.750 g, 18.0 mmol), and isopropanol (11.55 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
20:30:50 C12/C8/TEOS.
C12 (2.092 g, 7.20 mmol), C8 (2.986 g, 10.80 mmol), TEOS (3.750 g, 18.0 mmol), and isopropanol (11.55 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
1:49:50 C18/TDF/TEOS.
C18 (0.135 g, 0.36 mmol), TDF (9.003 g, 17.64 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (10.90 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
1:1:48:50 C18/TDF/C8/TEOS.
C18 (0.135 g, 0.36 mmol), TDF (0.184 g, 0.36 mmol), C8 (3.750 g, 18.0 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (8.47 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
1:4:45:50 C18/TDF/C8/TEOS.
C18 (0.135 g, 0.36 mmol), TDF (0.735 g, 1.44 mmol), C8 (4.479 g, 16.2 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.9 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
1:9:40:50 C18/TDF/C8/TEOS.
C18 (0.135 g, 0.36 mmol), TDF (1.654 g, 3.24 mmol), C8 (3.981 g, 14.4 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.9 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
1:14:35:50 C18/TDF/C8/TEOS.
C18 (0.135 g, 0.36 mmol), TDF (2.572 g, 5.04 mmol), C8 (3.484 g, 12.6 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.46 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
1:19:30:50 C18/TDF/C8/TEOS.
C18 (0.135 g, 0.36 mmol), TDF (3.491 g, 6.84 mmol), C8 (2.986 g, 10.8 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.46 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
1:24:25:50 C18/TDF/C8/TEOS.
C18 (0.135 g, 0.36 mmol), TDF (4.410 g, 8.64 mmol), C8 (2.488 g, 9.0 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.46 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). The resulting solution was stirred at ambient temperature for 24 hours.
0.5:1:48.5:50 DMAP/C18/C8/TEOS.
C18 (0.135 g, 0.36 mmol), C8 (4.828 g, 17.46 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.835 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). Sol DMAP (0.249 mL, 0.18 mmol) was then added and the resulting solution was stirred at ambient temperature for 24 hours.
Preparation of 1:1:48:50 DMAP/C18/C8/TEOS.
C18 (0.135 g, 0.36 mmol), C8 (4.778 g, 17.28 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.64 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). Sol DMAP (0.499 mL, 0.36 mmol) was then added and the resulting solution was stirred at ambient temperature for 24 hours.
1.5:1:47.5:50 DMAP/C18/C8/TEOS.
C18 (0.135 g, 0.36 mmol), C8 (4.728 g, 17.10 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.45 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). Sol DMAP (0.748 mL, 0.54 mmol) was then added and the resulting solution was stirred at ambient temperature for 24 hours.
2:1:47:50 DMAP/C18/C8/TEOS.
C18 (0.135 g, 0.36 mmol), C8 (4.678 g, 16.92 mmol), TEOS (3.750 g, 18.0 mmol), and ethanol (11.26 mL) were mixed together followed by the addition of 0.1 M HCl (2.268 mL). Sol DMAP (0.997 mL, 0.723 M) was then added and the resulting solution was stirred at ambient temperature for 24 hours.
Xerogel Film Formation.
Xerogel films were formed by spin casting 400 μL of the sol precursor onto 25-mm×75-mm glass microscope slides. The slides were soaked in piranha solution for 24 hours, rinsed with copious quantities of deionized water then soaked in isopropanol for 10 minutes, were air dried and stored at ambient temperature. A model P6700 spincoater was used at 100 rpm for 10 seconds to deliver the sol and at 3000 rpm for 30 seconds to coat. All coated surfaces were dried at ambient temperature for at least 7 days prior to analysis.
Atomic Force Microscopy (AFM) Imaging Measurements.
In order to determine surface roughness, xerogel samples were imaged 1 hour after immersion in artificial seawater (ASW) using the Molecular Force Probe AFM (Asylum Research, Santa Barbara, Calif.). V-shaped, silicon nitride cantilevers (Asylum Research, Santa Barbara, Calif.) with a nominal spring constant of 100 pN/nm were used. AFM images in ASW were obtained in a mode where the cantilever is actuated magnetically (iDrive). A small oscillating current flows through the cantilever legs in the presence of a magnetic field causing it to vibrate. The iDrive mode simplifies in-fluid imaging by eliminating the multitude of resonance peaks mechanically coupled from the holder and fluid.
Barnacle Removal Assays.
A 0.4 mL drop of seawater containing 20 to 40 barnacle cypris larvae was placed on the xerogel surface coated on glass microscope slides. The larvae are allowed to settle for 72 hours, or until the settlement rate on glass control surfaces reaches 50%. At this time, the number of barnacles that settle on each surface were counted, and compare the settlement rates on the experimental coatings with the settlement rates of controls.
Laboratory rearing of juvenile barnacles.
Newly metamorphosed juveniles on their respective coatings were transferred to growth chambers where they were fed the unicellular green alga Dunaliella sp. and the diatom Skeletonema costratum for two weeks and then a mixture of Dunaliella sp., S. costratum, and naupliar larvae of Artemia sp. Juveniles were maintained in a constant temperature incubator at 25° C. on a 12 hours:12 hours light:dark cycle for 6-8 weeks, which is the time it took the juvenile barnacles to reach a basal plate diameter of 3-5 mm, the minimum size necessary to conduct force gauge tests according to ASTM D 5618.
Critical removal stress for barnacles in shear. The procedures for critical removal stress followed from ASTM D 5618 with the following modifications: (i) The force measuring device was operated by a motorized stand, thus insuring a constant application of force during dislodgement; and (ii) Barnacle dislodgement studies from coatings were performed under water. The apparatus consists of an IMADA AXT 70 digital force gauge (4.4 lb) mounted on an IMADA SV-5 motorized stand. The slides are clamped into a custom-built Plexiglas chamber that allows their complete submersion during dislodgement tests.
Juvenile barnacles were selected for testing based on healthy appearance and minimum size requirements. Only barnacles occurring at least 5 mm from the edges of the slide were tested. Other barnacles in close proximity to the test subject were removed if they could potentially interfere with measurements. The barnacle base dimensions were measured with calipers. The base diameter was measured in two directions perpendicular to one another. Basal area (A) was then estimated using the formula A=π(0.5d1)(0.5d2), which is the area of an ellipse. After size measurements were taken, the slide was clamped into the Plexiglas chamber. The force gauge mounted on the motorized stand was used to apply a shear force to the base of the barnacles at a rate of approximately 4.5 N s−1 (1 lb s−1) until the organism was detached. Force was applied parallel to the film surface. The force required for detachment was noted and observations were made as to the mode of failure. If any portion of the base of the organism was left attached to the substratum, the test was deemed void for removal. The surfaces were examined visually for damage to the xerogel film caused by barnacle removal and by stereomicroscope if there were any ambiguity. The critical removal stress was calculated by dividing the force (F, Newtons) required to remove the test subject by the area of attachment (A, mm2).
For barnacles where a portion of the base of the organism was left attached to the substratum, the remaining basal plate is photographed and the exact percentage remaining after testing (% BPR) is calculated with digital image analysis.
Biofouling Assays.
Coatings applied to glass slides were equilibrated in circulating deionized water for 24 hours prior to the start of assays with algae. One hour prior to the assay the slides were transferred to ASW. Silastic® T2 (PDMSE) coated slides c. 500 μm in thickness, provided by Dr AB Brennan, University of Florida (Schumacher et al. 2007), were included in the assays to provide a standard fouling-release coating.
Settlement of Zoospores of Ulva.
Fronds of Ulva linza were collected from Llantwit Major, Wales (51840'N; 3848'W) and a spore suspension of 1.5×106 spores mL−1. Three replicate slides of each treatment were placed in individual wells of ‘quadriperm’ polystyrene culture dishes and 10 mL of spore suspension were added. Dishes were incubated in the dark for 1 hour at ˜20° C. After incubation the slides were gently washed in ASW to remove unattached (swimming) spores.
After washing away unattached spores, spores that had attached to the test surfaces were cultured in dishes containing supplemented seawater medium that was changed every 2 days. The dishes were placed in an illuminated incubator (75 mmol m−2 s−1 incident irradiation) for 7 days during which time the spores germinated and developed into sporelings (young plants).
The biomass produced was quantified by measuring the fluorescence of chlorophyll in a Tecan fluorescence plate reader (excitation=430 nm, emission=670 nm). Fluorescence was measured as relative fluorescence units (RFU) and was directly proportional to the quantity of biomass present. The RFU value for each slide was the mean of 70 point fluorescence readings taken from the central region.
The strength of adhesion of the sporelings was determined by exposing the slides to a range of impact pressures from an automated water jet, which traversed the central region of each slide. One replicate slide of each coating was exposed to one of five impact pressures. Pressures were selected to provide the widest range of biomass removal possible. The biomass that remained in the sprayed area after exposure to the water jet was quantified as described above. Percentage removal of sporelings was determined by comparison of the biomass (RFU) before exposure with that remaining attached to the coatings after exposure to the water jet. The critical impact pressure to remove 50% of the biomass was determined from plots of percentage removal vs water impact pressure.
This is an example of the preparation and characterization of xerogel films of the present invention.
This example provides preparation and characterization of four-component xerogel films of 1 mole % n-octadecyltrimethoxysilane (C18) and 50 mole % tetraethoxysilane (TEOS) in combination with 1-24 mole % tridecafluoro-1,1,2,2-tetrahydrooctyltriethoxysilane (TDF) and 25 to 48 mole % n-octyltriethoxysilane (C8) and a 1:49:50 mole % C18/TDF/TEOS. Settlement of barnacle cyprids and removal of juvenile barnacles, settlement of zoospores of the alga Ulva linza, and strength of attachment of 7-day sporelings (young plants) of Ulva were compared amongst the xerogel formulations. Several of the xerogel formulations were comparable to poly(dimethylsiloxane) elastomer with respect to removal of juvenile barnacles and removal of sporeling biomass. The 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogels displayed some phase segregation by atomic force microscopy (AFM) pre- and post-immersion in water. Imaging reflectance infrared microscopy showed the formation of islands of alkane-rich and perfluoroalkane-rich regions in these same xerogels both pre- and post-immersion in water. Surface energies were unchanged upon immersion in water for 48 hours amongst the TDF-containing xerogel coatings. AFM measurements demonstrated that surface roughness on the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogel coatings decreased upon immersion in water.
Materials and Methods. Chemical Reagents and Materials.
Deionized water was prepared to a specific resistivity of at least 18 MO using a Barnstead NANOpure Diamond UV ultrapure water system. Tetraethoxysilane (TEOS), n-octadecyl-trimethoxysilane (C18), tridecafluoro-1,1,2,2-tetrahydrooctyltriethoxysilane (TDF), and n-octyltriethoxysilane (C8) were purchased from Gelest, Inc. and were used as received. Ethanol was purchased from Quantum Chemical Corp. Hydrochloric acid and isopropanol were obtained from Fisher Scientific Co. Borosilicate glass microscope slides were obtained from Fisher Scientific, Inc. Silastic® T2 (Dow Corning) coated slides ca 500 μm in thickness, were provided by Dr. AB Brennan, University of Florida.
Sol Preparation.
The sol/xerogel composition is designated in terms of the molar ratio of Si-containing precursors. Thus, a 50:50 C8/TEOS composition contains 50 mole % C8 and 50 mole % TEOS. In all of the sol preparations described below, the aqueous HCl was added last. Unless noted otherwise, all sols were capped and stirred at ambient temperature.
50:50 C8/TEOS.
A mixture of TEOS (2.09 g, 2.24 mL, 10 mmol), C8 (2.78 g, 3.16 mL, 10 mmol), isopropanol (4.0 mL), and 0.100 M HCl (1.23 mL, 0.123 mmol) was capped and stirred for 24 hours. This sample, which did not contain TDF, served as a control xerogel surface.
1:1:48:50 C18/TDF/C8/TEOS.
A mixture of C18 (0.135 g, 0.36 mmol), TDF (0.184 g, 0.36 mmol), C8 (4.78 g, 17.3 mmol), TEOS (3.75 g, 18.0 mmol), ethanol (8.47 mL), and 0.1 M HCl (2.27 mL), was stirred for 24 hours.
1:4:45:50 C18/TDF/C8/TEOS.
A mixture of C18 (0.135 g, 0.36 mmol), TDF (0.735 g, 1.44 mmol), C8 (4.48 g, 16.2 mmol), TEOS (3.75 g, 18.0 mmol), ethanol (11.9 mL), and 0.1 M (2.27 mL) was stirred for 24 hours.
1:9:40:50 C18/TDF/C8/TEOS.
A mixture of C18 (0.135 g, 0.36 mmol), TDF (1.65 g, 3.24 mmol), C8 (3.98 g, 14.4 mmol), TEOS (3.75 g, 18.0 mmol), ethanol (11.9 mL), and 0.1 M HCl (2.27 mL) was stirred for 24 hours.
1:14:35:50 C18/TDF/C8/TEOS.
A mixture of C18 (0.135 g, 0.36 mmol), TDF (2.57 g, 5.04 mmol), C8 (3.48 g, 12.6 mmol), TEOS (3.75 g, 18.0 mmol), ethanol (11.5 mL) and 0.1 M HCl (2.27 mL) was stirred for 24 hours.
1:19:30:50 C18/TDF/C8/TEOS.
A mixture of C18 (0.135 g, 0.36 mmol), TDF (3.49 g, 6.84 mmol), C8 (2.99 g, 10.8 mmol), TEOS (3.75 g, 18.0 mmol), ethanol (11.5 mL), and 0.1 M HCl (2.27 mL) was stirred for 24 hours.
1:24:25:50 C18/TDF/C8/TEOS.
A mixture of C18 (0.135 g, 0.36 mmol), TDF (4.41 g, 8.64 mmol), C8 (2.49 g, 9.0 mmol), TEOS (3.75 g, 18.0 mmol), ethanol (11.5 mL), and 0.1 M HCl (2.27 mL) was stirred for 24 hours.
Xerogel Film Formation.
Prior to use, glass microscope slides (25-mm×75-mm) were soaked in piranha solution for 24 hours, rinsed with copious quantities of deionized water, soaked in isopropanol for 10 min, air dried and stored at ambient temperature. Xerogel films were formed by spin casting 400 μL of the sol precursor onto the microscope slides. A model P6700 spincoater (Specialty Coatings Systems, Inc.) was used at 100 rpm for 10 seconds to deliver the sol and at 3000 rpm for 30 seconds to coat. Profilometry indicated that the xerogel films cast in this manner were 1 to 2 μm thick.
For barnacle cyprid assays, glass 20-mm×60-mm Petri dish bottoms (VWR Scientific, Inc.) were soaked in piranha solution for 24 hours, rinsed with copious quantities of deionized water, and stored in an oven at 110° C. until use. The Petri dish bottoms were cooled to ambient temperature and 600 μL of the appropriate sol precursor was added. The Petri dish was manipulated until the bottom surface and approximately 5 mm of the side surface were covered. The excess sol precursor was removed via pipette. All coated surfaces (glass slides and Petri dishes) were dried at ambient temperature and humidity for at least 7 days prior to analysis.
Imaging Reflectance Infrared (IR) Microscopy of Xerogel Samples. Imaging reflectance IR microscopy was carried out using a Bruker Vertex 70 IR coupled with a Hyperion 3000 IR microscope (4 cm−1, 64 scans, 15× objective, 64×64 focal plane array). IR scans were collected in reflectance mode utilizing an FPA (focal plane array) detector with a detection area of 200 μm×200 um. Samples of the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogel on aluminum-coated glass slides were prepared by spin casting 400 μL of the sol precursor onto 25-mm×75-mm×1.1-mm borosilicate float glass microscope slides coated with 50±1 nm aluminum (Deposition Research Laboratories, Inc.) and air drying the films at ambient temperature for at least seven days. One set of xerogel films was analyzed following air-drying while a second set of xerogel films was soaked in deionized water at 25° C. for 24 hours and then dried at ambient temperature and humidity for 2 hours. The IR data collected for the 200×200 micron area was baseline corrected relative to the aluminum-coated slide as a blank and then integrated over the C—H stretching region (2800-3000 cm−1) and the C—F stretching region (1223-1275 cm−1). The 2D color images of relative intensity were then converted to 32 bit black and white images using Image-J software, where the “color” intensity was converted to gray-scale intensity. The ratio of the two images (C—F/C—H) was then calculated using the Ratio Plus plugin, resulting in a single image in which black areas pertain to an enhancement of fluorocarbon signal or reduced signal pertaining to hydrocarbon species and white areas pertain to enhanced signal from hydrocarbon species or reduced signal from fluorocarbon species.
Atomic Force Microscopy (AFM) Imaging Measurements.
The samples were imaged by AFM using a Nanoscope® Dimension 3100 scanning probe microscope (Bruker AXS, Santa Barbara, Calif.) in an environmentally controlled laboratory with the relative humidity set at 25%. Photomicrographs were acquired using TappingMode™ Atomic Force Microscopy (TM-AFM) under ambient conditions. With a TM-AFM, the tip is driven at a known amplitude and frequency of oscillation which is typically near the cantilever resonance. The oscillatory motion is reduced as the tip is brought closer to the surface. The changes in the amplitude allow the AFM to track the surface, providing topographical information. Single crystal silicon Nanoprobe™ with a spring constant of ca. 17 to 43 N/m and resonance frequencies in the 262 to 359 kHz range to examine the xerogel film surfaces was used. TappingMode™ AFM images were acquired at a 1-μm and 5 μm scan size with the z-scale set to 100-nm.
Phase mode AFM imaging can distinguish surface features that are related to surface composition differences. Phase shifts are registered as bright and dark regions in the phase AFM image. For the phase mode images of this example, brighter regions indicate stiffer material whereas darker regions indicate a softer material.
Comprehensive Contact Angle Analysis.
The xerogel films were stored in air prior to characterization. Comprehensive contact angle analyses were performed in air. The approximate sampling depth of the contact angle technique is 5 Å. Up to thirteen different diagnostic liquids were utilized for the analysis of each sample: water, glycerol, formamide, thiodiglycol, methylene iodide, 1-bromonaphthalene, 1-methylnaphthalene, dicyclohexyl, n-hexadecane, n-tridecane, n-decane, n-octane, and n-heptane. Liquid/vapor surface tensions of these liquids were determined directly; reference values for the liquid/vapor surface tensions are not used. The technique of “advanced angle” analysis was used, wherein a sessile drop of liquid (8-15 μL depending on the viscosity of the liquid) is placed on the sample surface and the angle of contact (θ) between the liquid and the solid is measured with a contact angle goniometer (Raine-Hart, Model NRL 100); both sides of the droplet profile are measured.
Another droplet of the same fluid is placed on top of the first droplet (the fluid is advanced across the surface), and the measurements are repeated. If the contact angles for the first droplet are ≦20°, no further measurements are taken for that liquid on the sample; fluids having contact angles of ≦20° use a relatively large amount of the limited sample surface area. Zisman plots were constructed by plotting the cosine of the average angle measured for each liquid against the liquid/vapor surface tension of the diagnostic liquid. A linear least squares analysis is performed to determine the sample's critical surface tension (γC) at the cos θ=1 axis. In cases of large data scatter (non linearity), the data for the spreading liquid (θ=0) with the greatest liquid/vapor surface tension and for those liquids closest to, but greater than, in surface tension to the first spreading liquid are used to determine γC. The data were also treated to give the surface free energy (γS), as well as its polar (γP) and dispersion (γD) components, after the xerogel films were aged in air or soaked in deionized water for 48 hours and then air-dried for 1 hour.
Static water contact angles (θWs) were measured by the sessile drop technique where the angle between a 15-μL drop of water and the xerogel surface was measured with a contact angle goniometer (Rame-Hart, Model NRL 100); both sides of the droplet profile were measured.
Biofouling Assays with Barnacles.
Barnacle cypris larvae were obtained from Duke University Marine Lab. Glass standards were acid washed in 10% HCl for 2 hours, rinsed well with deionized water, and dried completely prior to cyprid settlement. Silastic® T2 (T2) coated slides were included in the assays to provide a standard fouling-release (FR) coating.
Cyprid Settlement Assays.
Approximately 10 mL of seawater was added to each xerogel-coated Petri dish. This volume covered the bottom of the dish and allowed the cyprids free range of movement across the surface. A 400-μl drop of seawater containing between 30 and 60 2 to 4-day-old barnacle cypris larvae was then added to each of the dishes. After 48 hours the percentage of cyprids that had settled in each dish was counted. The average percent settlement for each of the experimental coatings was compared to the controls. Glass and T2 coated dishes were used as standard settlement substrates.
Barnacle Removal Assays.
A 400-μl drop of seawater containing between 20 and 40 2 to 4-day-old cypris larvae was placed on the surface of the xerogel film-coated glass microscope slides. The surfaces with larvae were placed in a constant temperature incubator at 25° C. on a 12 hours:12 hours light:dark cycle and the larvae were allowed to settle for 48 hours. Newly metamorphosed juveniles on their respective coatings were transferred to growth chambers where they were fed the unicellular green alga Dunaliella tertiolecta and the diatom Skeletonema costatum for two weeks, and then a mixture of D. tertiolecta, S. costatum, and naupliar larvae of Artemia sp. for an additional week. Juveniles were then transferred to a 16-L aquarium tank in an automated rack system with temperature, salinity, and pH monitors and programmed for a 10% daily water change. Barnacles in the tank were fed a 500-mL flask of Artemia sp. 3 times a week for 4 to 6 weeks, which is the time it took the juvenile barnacles to reach a basal plate diameter of 3-5 mm—the minimum size necessary to conduct force gauge tests according to ASTM D 5618.
The procedures for critical removal stress were followed from ASTM D 5618 with the following modifications: (i) the force measuring device was operated by a motorized stand, ensuring a constant application of force during dislodgement; and (ii) barnacle dislodgement studies from coatings were performed under water. The apparatus consists of an IMADA ZP-11 digital force gauge mounted on an IMADA SV-5 motorized stand. The slides are clamped into a custom-built Plexiglas chamber that allowed their complete submersion during dislodgement tests.
Juvenile barnacles were selected for testing based on healthy appearance and minimum size requirements. Only barnacles positioned at least 5 mm from the edges of the slide were tested. Other barnacles in close proximity to the test subject were removed if they could potentially interfere with measurements. Prior to removal of barnacles each basal plate was photographed using a Canon™ EOS 10D camera attached to an Olympus™ SZX12 dissecting microscope and images were later used to calculate basal plate areas using NIH's ImageJ software. After photographs were taken, the slide was clamped into the Plexiglas chamber. The force gauge mounted on the motorized stand was used to apply a shear force to the base of the barnacles at a rate of approximately 4.5 N s−1 until the organism was detached. Force was applied parallel to the film surface. The force required for detachment was noted and observations were made as to the mode of failure. If any portion of the base of the organism was left attached to the substratum, the test was deemed void for removal. The surfaces were examined visually for damage to the xerogel film caused by barnacle removal and by stereomicroscope if there were any ambiguity. The critical removal stress was calculated by dividing the force (F, Newtons) required to remove the test subject by the area of attachment (A, mm2). For barnacles where a portion of the base of the organism was left attached to the substratum, the remaining basal plate was photographed and the area was calculated as described above and used to calculate the exact fraction remaining after testing (fraction BPR).
Biofouling Assays with Ulva.
Coatings applied to glass slides were equilibrated in circulating deionized water for 48 hours prior to the start of assays with algae. One hour prior to the assay, the slides were transferred to artificial seawater (ASW). Silastic® T2-coated slides were included in the assays to provide a standard fouling-release coating.
Settlement of Zoospores of Ulva.
Fronds of Ulva linza were collected from Llantwit Major, Wales (51840′N; 3848′W) and a spore suspension of 1.0×106 spores mL−1 was prepared. Three replicate slides of each treatment were placed in individual wells of ‘quadriperm’ polystyrene culture dishes and 10 mL of spore suspension were added. Dishes were incubated in the dark for 1 hour at ˜20° C. After incubation, the slides were gently washed in ASW to remove unattached (swimming) spores. Slides were fixed in 2.5% glutaraldehyde. The density of spores attached to the surfaces was counted using an image analysis system attached to a fluorescence microscope. Spores were visualized by autofluorescence of chlorophyll. Counts were made for thirty (30) fields of view (each 0.17 mm2), 1 mm apart over the central region of each slide, using image analysis software (Axiovision 4.8.1, Carl Zeiss imaging systems) attached to a Zeiss epifluorescence microscope. Spore settlement data are expressed as the mean number of spores adhered per mm2 with 95% confidence limits (n=90).
Adhesion Strength of Sporelings of Ulva.
Spores were allowed to settle as described above. After washing away unattached spores, spores that had attached to the test surfaces were cultured in dishes containing supplemented seawater medium that was changed every 2 days. The dishes were placed in an illuminated incubator (75 mW m−2 s−1 incident irradiation) for 7 days during which time the spores germinated and developed into sporelings (young plants).
The biomass produced was quantified by measuring the fluorescence of chlorophyll in a Tecan fluorescence plate reader (excitation=430 nm, emission=670 nm). Fluorescence was measured as relative fluorescence units (RFU) and was directly proportional to the quantity of biomass present. The RFU value for each slide was the mean of 70 point fluorescence readings taken from the central region (middle third of the slide over a 1 in×1 in region).
The strength of adhesion of the sporelings was determined by exposing the slides to a range of impact pressures from an automated water jet, which traversed the central region (middle third of the slide over a 1 in×1 in region) of each slide. One replicate slide of each coating was exposed to one of five impact pressures. Pressures were selected to provide the widest range of biomass removal possible. The biomass that remained in the sprayed area after exposure to the water jet was quantified as described above. Percentage removal of sporelings was determined by comparison of the biomass (RFU) before exposure with that remaining attached to the coatings after exposure to the water jet. The critical impact pressure to remove 50% of the biomass (CP50) was determined from plots of percentage removal vs. water impact pressure.
Results. Xerogel Surfaces.
A series of xerogel surfaces containing C18, TDF, C8 and TEOS were prepared from sols with the following mole % ratios: 1:1:48:50, 1:4:45:50, 1:9:40:50, 1:14:35:50, 1:19:30:50 and 1:24:25:50 C18/TDF/C8/TEOS, respectively. A 1:49:50 C18/TDF/TEOS xerogel surface (C18/TDF/TEOS xerogel in the remainder of the manuscript) was also prepared and a 50:50 C8/TEOS xerogel surface (C8/TEOS xerogel in the remainder of the manuscript) was prepared as a xerogel control. The xerogel films prepared by spin coating were 1 to 2 μm thick as measured by profilometry. All of the xerogel films of this example were optically transparent.
The xerogel surfaces were aged in air at ambient temperature for 7 days and were then examined by comprehensive advanced contact angle analyses to give values of the critical surface tension (γC) and the surface free energy (γS) (Table 3). Static water contact angles, θWs, were measured for all xerogel surfaces described in this example and are compiled in Table 3. For the TDF-containing xerogels, values of γC varied between 11.5 and 19.8 mN m−1, values of γs varied between 16.1 and 21.8 mN m−1 and values of θWs varied between 97.0° and 110.3°.
To evaluate the impact of water on surface properties, values of θWs and γS were measured before and after the xerogel surfaces were immersed in deionized water for 48 hours and air-dried for 1 hour. The values of θWs and γS, pre- and post-immersion in deionized water, are compared graphically in
The 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogels were examined by AFM prior to immersion in water and after 24 hours immersion in deionized water. Immersed surfaces were air-dried for 1 hour prior to imaging in air.
Values of the root-mean-square roughness (Rrms) for the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogel surfaces pre- (1.87±0.20 and 2.31±0.21 nm, respectively, where error limits are ±one standard deviation) and post-immersion (0.93±0.05 and 0.95±0.03 nm, respectively) in deionized water were calculated on six 5-μm×5-μm images for each sample, where Rrms is defined as the root mean square average of the topographic deviations (t) as shown in eq 1:
Values of Rrms pre- and post-immersion in deionized water as well as changes in the peak-to-valley heights are shown graphically in
Samples of the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogel on aluminum-coated glass slides were also examined by imaging reflectance IR microscopy. The sol-gel process produces xerogels with residual silanol functionality (3200 to 3700 cm−1), which is uniformly distributed across the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogels. In contrast, IR microscopy images of the integrated C—F stretching region (1223 to 1275 cm−1) and the integrated C—H stretching region (2800 to 3000 cm−1) show some segregation into higher C—F/lower C—H-containing features and lower C—F/higher C—H-containing features across the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogel surfaces. These features are illustrated in
Settlement of Cypris Larvae and Removal of Juvenile Barnacles of B. Amphitrite.
The settlement of 2 to 4-day-old barnacle cypris larvae that were placed on the xerogel coatings and the glass and T2 standard surfaces was compared (
The strength of attachment of juvenile barnacles to the seven TDF-containing xerogel surfaces, the C8/TEOS xerogel and glass and T2 standards was measured via force-gauge measurements with forces applied in shear. All barnacles on both the C8/TEOS xerogel and the glass standard broke when force was applied to them in shear, and left a complete or partial basal plate attached to the surface. For the glass standard, the fraction of the barnacle basal plate remaining was 1.00, i.e., essentially all of the barnacle basal plate remained on the glass surface. For the C8/TEOS xerogel, the fraction of the barnacle basal plate remaining was 0.80±0.04. All of the TDF-containing xerogel surfaces as well as the T2 standard performed as fouling-release surfaces as shown in
There was a significant difference in critical removal stress (CRS) between test coatings (ANOVA p<0.001). The value of CRS for the 1:1:48:50 C18/TDF/C8/TEOS xerogel surface (0.24±0.01 MPa,
For barnacles not completely removed, the percentage of the basal plate remaining (BPR) was calculated with digital image analysis. These results were combined with data for barnacles completely removed (fraction BPR=0.0) and are shown in
Settlement and Removal of Zoospores of Ulva.
Settlement of zoospores, the growth of sporeling biomass and the impact pressure needed to remove 50% from the TDF-containing xerogel surfaces, the C8/TEOS xerogel control, and the T2 standards were examined. Spore settlement densities on the C18/TDF/C8/TEOS coatings and the C8/TEOS xerogel did not follow a trend in terms of composition of the C18/TDF/C8/TEOS xerogels (
Strength of Attachment of Sporelings of Ulva.
Sporelings grew well and after 7 days, a green covering was visible on all surfaces. The TDF-containing xerogels and the C8/TEOS control and glass and T2 standards were exposed to a range of water pressures (20 to 54 kPa) to determine the critical water pressure (CP50) required to remove 50% of 7-day sporeling biomass. These values are shown graphically in
A value of CP50 could not be determined for the glass standard. At the highest pressure examined (54 kPa), the fraction of sporelings removed was less than 0.2. In previous studies, CP50 for glass has been estimated at >200 kPa and, in the current example, would be estimated to be at least 100 kPa.
Scanning electron microscopy (SEM) studies of several xerogel surfaces indicate that these surfaces are uniform, uncracked, and topographically smooth when dry. Atomic force microscopy (AFM) measurements on the same series of xerogels submerged in ASW show very low surface roughness (≦0.8 nm) and no phase segregation. Time-of-flight, secondary-ion mass spectrometry (ToF-SIMS) studies show that there is no phase segregation of fluorocarbon and hydrocarbon groups on the μm scale in a 25:25:50 trifluoropropyl-trimethoxysilane/C8/TEOS xerogel.
Chemical segregation in the bulk xerogel is possible and can lead to topographical features over multiple scales. The C18- and TDF-containing xerogel coatings of this example showed decreased surface roughness upon exposure to water. Both AFM (
The nature of the cross-linking and functional group distribution in the xerogels differs from that of fluorinated block copolymers that undergo surface reorganization upon exposure to water. Immersion in water did not change the relative intensity of the silanol bands in the surface regions shown in
The IR microscope showed some segregation of chemical functionality in the bulk xerogel on roughly the μm scale, which is the spatial resolution of the IR microscopy images (
The experimental values of γS (Table 3) for the TDF-containing xerogels of this example are lower in comparison to values of γS for the C8/TEOS xerogel control surface and T2 standard surface and are also either below or at the low end of the 20-25 mN m−1 range of the “Baier curve” where minimal bioadhesion has been reported. Materials with lower values of γS have shown increased bioadhesion.
One might expect less than optimal performance for the lower surface energy materials as either AF or FR surfaces relative to surfaces with values of γs in the 20-25 mN m−1 range of the “Baier curve” if surface energy alone were the sole determining factor. The T2 standard, for example, has γS of 23.0±0.4 mN m−1, which is in the middle of the Baier minimum. In particular, the 1:4:45:50 through 1:24:25:50 C18/TDF/C8/TEOS surfaces had values of γC in the range 11.5 to 19.8 mN m−1 and values of γS in the range 16.1 to 21.8 mN m−1 and gave release of 68 to 100% of juvenile barnacles. In contrast, the 50:50 C8/TEOS xerogel with γC of 21.3 mN m−1 and γS of 27.1 mN m−1 in the middle of the Baier minimum gave 0% release of juvenile barnacles.
Settlement studies of cypris larvae of the barnacle B. amphitrite showed that there were no significant differences between the TDF-containing xerogel test coatings and the C8/TEOS xerogel control surface or glass and T2 standards (
Settlement studies of zoospores of Ulva linza indicated significant differences among the C8/TEOS and TDF-containing coatings, but these differences did not correlate with either θWs or with γS. Settlement densities on the 1:1:48:50 and 1:24:25:50 C18/TDF/C8/TEOS xerogels were significantly lower in comparison to settlement on the C8/TEOS xerogel standard while settlement on the 1:4:45:50 C18/TDF/C8/TEOS xerogel and the C18/TDF/TEOS xerogel was not significantly different in comparison to settlement on the C8/TEOS xerogel standard. All four of these surfaces had values of γS (17.2 to 21.8 mN m−1) well below that of the C8/TEOS xerogel (27.1 mN m−1). In contrast, settlement densities were highest on the 1:4:45:50, 1:9:40:50, 1:14:35:50, and 1:19:30:50 C18/TDF/C8/TEOS xerogels with values of γS in the same range. With the exception of the 1:1:48:50 C18/TDF/C8/TEOS xerogel (θWs=110°), all of the other xerogel surfaces including the C8/TEOS xerogel had a value of θWs of ˜100° (within experimental error). On these surfaces, settlement of zoospores does not appear to be correlated with either total surface energy or hydrophobicity.
The TDF-containing xerogel surfaces acted as FR surfaces with several comparable to the T2 standard with respect to release of juvenile barnacles and 7-day Ulva sporeling growth. The 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogel surfaces and T2 standard gave essentially complete release of juvenile barnacles (
The data for 7-day sporeling removal taken with the performance of the TDF-containing xerogel surfaces for removal of juvenile barnacles suggest that the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogels perform similarly to the T2 standard as FR surfaces. The thinner, harder xerogel surfaces may release the macrofoulers via shear rather than by peeling as one might expect with T2 and related silicone elastomers.
The chemical segregation of the C18/TDF/C8/TEOS xerogels of this example into nm- and μm-domains of higher fluorocarbon and hydrocarbon content likely contributes to the FR performance observed in these systems in addition to other surface properties. The hydrophobic nature of C18/TDF/C8/TEOS xerogel coatings and low surface energy likely contribute to their FR behavior, as well.
Xerogel surfaces can be fine-tuned to provide surfaces with different wettability and values of γC or γS. The topography of the xerogel surfaces can also be fine-tuned by the incorporation of a long-chain alkyl component and varying amounts of the polyfluorinated TDF as shown by the xerogels of this example. The formulation and coating of these TDF-containing xerogel surfaces require no special attention or preparation (pre-patterning). Depositing the xerogel by spin coating leads to self-segregation of hydrocarbon and fluorocarbon domains.
Overall, xerogel surfaces have high potential as FR or easy-clean materials with the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogels of this example being perhaps the most promising leads yet in xerogel surface chemistry. These coatings can be useful as AF/FR surfaces, for example, in applications where thicker coatings are not optimal or practical. In particular, these coatings, as with other xerogel coatings, are optically transparent and have applications as AF/FR coatings where optical transparency is important (marine sensors, underwater cameras, submersible solar panels).
aMean of five (5) independent measurements for coatings stored in air prior to measurement. ± one standard deviation.
bMean of two (2) independent measurements for coatings stored in air for 7 days prior to measurement.
cMean of three (3) independent measurements for coatings stored in air for 7 days prior to measurement.
dFrom Tang et al. (2005).
eFrom Feinberg et al. (2003).
f From Gunari et al. (2011).
The first three entries in Table 3 are comparative examples.
This is an example of the preparation and characterization of xerogel films of the present invention.
Five non-biocidal xerogel coatings were compared to two commercial non-biocidal coatings and a silicone standard with respect to antifouling (AF)/fouling-release (FR) characteristics. The formation and release of bacterial biofilm of Cellulophaga lytica, the attachment and release of the microalga Navicula incerta, and the fraction removal and critical removal stress of reattached adult barnacles of Amphibalanus amphitrite were evaluated in laboratory assays. Correlations of AF/FR performance with surface characteristics such as wettability, surface energy, elastic modulus, and surface roughness were examined. Several of the xerogel coating compositions performed well against both microfouling organisms while the commercial coatings performed more poorly toward the removal of microalgae. Reattached barnacle adhesion as measured by critical removal stress was significantly lower on the commercial coatings when compared to the xerogel coatings. However, two xerogel compositions showed release of 89 to 100% of reattached barnacles. These two formulations were also tested in the field and showed similar results.
The AF and FR performance of several xerogel surfaces with the AF and FR performance of the commercial coatings IS700 and IS900 and a Silastic® T2 (T2) poly(dimethylsiloxane)elastomer standard with respect to micro- and macrofoulers in laboratory assays was compared. Specifically, the biofilm growth and removal of the marine bacterium Cellulophaga lytica, attachment and removal of the marine microalga (diatom) Navicula incerta, and the removal of reattached adult barnacles of Amphibalanus amphitrite were compared. Two of the xerogel coatings showed desirable FR behavior toward all three fouling organisms. The same two coatings were also tested at a static immersion test site in Morro Bay, Calif. In the field, both coatings exhibited good FR performance (i.e., removal of the barnacle Balanus crenatus) and cleaned back relatively easily using a standard water jet apparatus.
Materials and Methods. Chemical Reagents and Materials.
Deionized water was prepared to a specific resistivity of at least 18 MΩ using a Barnstead NANOpure Diamond UV ultrapure water system. 3-N,N-Dimethylaminopropyltriethoxysilane (DMAP), TEOS, C18, C8 and TDF were purchased from Gelest, Inc. and were used as received. Ethanol was purchased from Quantum Chemical Corp. Hydrochloric acid and borosilicate glass microscope slides were obtained from Fisher Scientific Co.
Sol Preparation.
The sol/xerogel composition is designated in terms of the molar ratio of Si-containing precursors. Thus, a 50:50 C8/TEOS composition contains 50 mole % C8 and 50 mole % TEOS. In all of the sol preparations described below, the aqueous HCl was added last. Unless noted otherwise, all sols were stirred at ambient temperature.
Sol TEOS.
TEOS (11.9 g, 51.3 mmol, 10.05 ml), water (1.62 ml, 90 mmol), ethanol (10.2 ml), and HCl (0.10 M, 45 ml) were stirred for 6 hours.
Sol DMAP.
DMAP (3.16 g, 14.5 mmol) was added dropwise to a mixture of 6.67 M HCl (2.87 ml) and ethanol (14.0 ml). The resulting solution was stirred for 40 min.
1:9 DMAP/TEOS.
Sol DMAP (15.3 ml, 11.0 mmol) was added dropwise to sol TEOS (48.6 ml, 99.3 mmol). The mixture was stirred for 20 min.
50:50 C8/TEOS.
A mixture of TEOS (20.9 g, 22.4 mL, 0.10 mol), C8 (27.8 g, 31.6 mL, 0.10 mol), isopropanol (40.0 mL), and 0.100 N HCl (12.3 mL, 1.23 mmol) was stirred for 24 hours.
1:49:50 C18/C8/TEOS.
A mixture of C18 (0.54 g, 1.4 mmol, 0.61 mL), C8 (19.5 g, 70.6 mmol, 22.1 mL), TEOS (15.0 g, 72.0 mmol, 16.1 mL), 0.1 N HCl (9.1 mL, 0.91 mmol), and isopropanol (46.2 mL) was stirred at ambient temperature for 24 hours.
1:4:45:50 C18/TDF/C8/TEOS.
A mixture of C18 (0.405 g, 1.08 mmol), TDF (2.20 g, 4.3 mmol), C8 (13.4 g, 48.6 mmol), TEOS (11.25 g, 54.0 mmol), ethanol (35.7 mL), and 0.1 M HCl (6.8 mL, 0.68 mmol) was stirred for 24 hours.
1:14:35:50 C18/TDF/C8/TEOS.
A mixture of C18 (0.405 g, 1.08 mmol), TDF (7.71 g, 15.1 mmol), C8 (10.4 g, 38.1 mmol), TEOS (11.25 g, 54.0 mmol), ethanol (34.5 mL) and 0.1 M HCl (6.8 mL, 0.68 mmol) was stirred for 24 hours.
Preparation of Coatings.
Xerogel films were formed by spin casting 400 μL of the sol precursor onto 25-mm×75-mm glass microscope slides. Prior to use, the slides were soaked in piranha solution for 24 hours, rinsed with copious quantities of deionized water, soaked in isopropanol for 10 min, air dried and stored at ambient temperature. A model P6700 spincoater (Specialty Coatings Systems, Inc.) was used at 100 rpm for 10 seconds to deliver the sol and at 3000 rpm for 30 seconds to coat.
The xerogel coatings were applied to aluminum Q-panels® (102×204 mm, 0.6-mm thickness, type A, alloy 3003 H14, obtained from Q-lab) that had been primed via air-assisted spray with Intergard® 264 at a thickness of 70 to 80 μm. The sol solutions were applied to the surface via dip-coating and air dried for 7 days to give xerogel surfaces of a thickness of −10 μm as determined by profilometry. Discs of the xerogel-coated aluminum were adhered to the bottom of each well of 24-well polystyrene plates as described previously.
The xerogel coatings were also applied to G10 fiberglass panels (102×204×4.8 mm, Piedmont Plastics, Inc.) that had been primed via brushing with Interlux® Epoxy Primekote at a thickness of 70 to 80 μm. The sol solutions were applied to the surface via dip-coating and the resulting surfaces were air dried for 7 days to give xerogel surfaces of a thickness of ˜10 μm as determined by profilometry.
In addition to the experimental coatings, reference T2 poly(dimethylsiloxane) elastomer coatings and the commercial coatings IS700 and IS900 were deposited (0.25 mL) using an Eppendorf Repeater plus pipetter into wells of 24-well plates (6 columns and 4 rows) modified with primed aluminum discs in each well. The primer used was Intergard® 264. Deposition or adhesion of discs was done such that a given coating composition occupied three entire columns of the 24-well plate (12 replicate samples per plate). Coatings were allowed to cure for 24 hours at room temperature and then heated at 50° C. for 24 hours to ensure full cure. (While heating to 50° C. is not in the manufacturer's specifications, 8 years' experience with this procedure has always given good agreement between laboratory and field testing.) For the barnacle reattachment assay, 1.0 mL of each coating was deposited into 3 wells of a stamped aluminum panel.
Surface Energy Analysis.
Contact angles were. measured for all xerogel formulations with a contact angle goniometer (Rame-Hart, Model NRL 100) at room temperature using the sessile drop technique with water and diiodomethane. Surface free energies (γS) as well as dispersive (γD) and polar components (γP) were determined by using static contact angles for xerogel surfaces on glass slides that had been air-dried for 7 days.
Adult Barnacle Reattachment Assay.
An adult barnacle reattachment assay was utilized to gauge the fouling-release performance of the coatings with respect to macrofouling. Adult barnacles of A. amphitrite with a basal diameter of ˜5 mm were removed from a coating of T2 on glass and placed on the coating surfaces that had been pre-conditioned for 14 days in running tap water to remove leachates from the coatings prior to the biological assay. Nine barnacles were allowed to reattach to the coating surfaces by immersing the panels in artificial sea water (ASW) for 14 days with daily feedings of brine shrimp nauplii. The reattached barnacles were dislodged from the coating surfaces using a hand held digital force gauge in accordance with ASTM D5618-94. In this regard, the force gauge was placed at the barnacle base plate, parallel to the coating surface, and pushed laterally (i.e., in shear) until the barnacle became detached from the surface. Once detached, the area of the barnacle base plates were measured using digital image analysis (Sigma Scan Pro 5.0) and barnacle adhesion strengths (i.e., critical removal stress in shear (CRS) values) were calculated from the removal force and the area of the barnacle basal plates. The adhesion values for each coating were reported as the mean of the total number of barnacles exhibiting a measurable removal force. The number of reattached barnacles that exhibited any detectable shell breakage or baseplate damage during force gauge removal was recorded for each coating (i.e., fraction removal) and excluded from the CRS calculations.
C. lytica Biofilm Retention and Adhesion.
The high-throughput assessment of bacterial biofilm retention on coatings was prepared. After 14 days of pre-conditioning in running tap water to remove leachates, the coating plates were inoculated with 1.0 mL of a 107 cells mL−1 suspension of C. lytica in biofilm growth medium (0.1 g yeast extract and 0.5 g of peptone per IL of ASW) and incubated at 28° C. for 24 hours. Following the incubation, the planktonic growth was discarded and the plates were rinsed three times with ASW to remove unattached cells or biofilm that was only weakly attached to the coating surfaces. The retained biofilms were dried at ambient laboratory conditions for ˜1 hour and stained with crystal violet (0.3% w/v in deionized water). The crystal violet stain was subsequently extracted from the biofilm retained on the coating surfaces with 0.5 mL of 33% glacial acetic acid and the resulting eluates were transferred to a 96-well plate and measured for absorbance at 600 nm using a multi-well plate spectrophotometer. The absorbance values (λ=600 nm) were reported as the mean of three replicate wells and were considered to be directly proportional to the amount of biofilm retained on the coating surfaces.
C. lytica biofilm adhesion was determined after 24 hours of incubation using an automated water jet apparatus. The first column of each coating was not treated with the water jet and served as the measure of the biofilm retained on the coating surface before water jetting. The second and third column of each coating was jetted for 5 seconds at a pressure of 69 and 138 kPa, respectively. The percent removal was recorded as the difference in absorbance values between the coating replicates that were exposed to the water-jet and those which were not.
Attachment and Adhesion of N. incerta.
The assay for attachment and adhesion of the diatom N. incerta was carried out in a similar manner to the assay for retention and adhesion of C. lytica biofilm. After 14 days of pre-conditioning in running tap water to remove leachates, the coating plates were inoculated with 1.0 mL of a 105 cells mL−1 suspension of N. incerta in F/2 medium and incubated at ambient laboratory conditions for 2 hours. Water jetting followed the incubation of the plates. The first column of each coating was not treated with the water jet and served as the measure of the N. incerta cell attachment on the coating surface before water jetting. The second and third column of each coating was jetted for 5 seconds at a pressure of 69 and 138 kPa, respectively. One milliliter of DMSO was immediately added to each well and the plates were incubated in darkness for 30 min. A homogeneous solution was obtained by gentle shaking and 0.2 mL of the solution was transferred to a 96-well plate for fluorescence measurements of chlorophyll using a multi-well plate spectrophotometer (excitation wavelength: 360 nm, emission wavelength: 670 nm). The percent removal was recorded as the difference in relative fluorescence units (RFU) between the coating replicates that were exposed to the water-jet and those which were not.
Field testing.
The 1:4:45:50 C18/TDF/C8/TEOS and 1:14:35:50 C18/TDF/C8/TEOS xerogel coatings were applied to fiberglass panels as described above and deployed at a static immersion test site in Morro Bay, Calif. for a series of field tests. Panels were submerged approximately 1 m below the surface from a floating dock. Panels were deployed at the beginning of the heaviest fouling season and were observed monthly over approximately six months. Panels were subjected to tests quantifying antifouling and fouling-release performance. Antifouling performance was quantified in accordance with ASTM D6990-05. Fouling-release performance was estimated using two separate tests: removal of adult barnacles in accordance with ASTM D5618-94 and an adaptation of water jet testing.
Adult barnacles (Balanus crenatus) recruited directly onto panels in the field and were then removed from test panels in accordance with ASTM standard D5618-94. Barnacle removal testing was conducted 126 days later once all individuals met the minimum size requirements for testing according to the ASTM standard. Values of CRS were calculated for all barnacles that could be removed from test panels with less than an estimated 10% basal plate remaining on the coating surface as per the ASTM standard. All barnacles that were isolated and met the minimum size requirements were removed from each panel replicate so sample sizes were not equal. Few barnacles settled and grew on the IS900 control panel so this sample size was much smaller than for the other two test coatings; however, CRS was consistent with what was seen in other field trials.
One replicate panel of each formulation was reserved for water-jet testing to provide another measure of FR performance. A pressurized stream of water was applied perpendicular to and ˜25 mm away from the surface as evenly as possible across the entire surface of the panel at a series a pressures (40, 80, 120, 180 and 240 psi). Panels were observed after each water pressure was applied and assessed for complete removal of organisms or estimated percentage cover remaining. These estimates were then used to rank coatings based on how easily biofouling could be removed. All water jet testing was done on about one months' accumulated fouling.
Antifouling and physical performance of coatings were evaluated as outlined in ASTM standard D6990-05. Each panel was given a fouling rating with 100 being completely free of fouling. Each panel was also given a physical deterioration rating (PDR) with 100 being completely free of deterioration. Fouling rating is only reported for the 92nd day because after that time all panels were altered in order to remove barnacles to measure CRS values.
RESULTS. Characterization of Xerogel Surfaces.
Static water contact angles, θWs, are compiled in Table 4 along with values of surface free energy (γS) and the corresponding dispersive (γD) and polar (γP) components determined for the 10:90 DMAP/TEOS (referred to as DMAP/TEOS in the remainder of the manuscript), 50:50 C8/TEOS (referred to as C8/TEOS in the remainder of the manuscript), 1:1:48:50 C18/C8/TEOS (referred to as C18/C8/TEOS in the remainder of the manuscript), 1:4:45:50 C18/TDF/C8/TEOS, and 1:14:35:50 C18/TDF/C8/TEOS xerogel coatings. Literature values of θWs and γS for T2 and IS700 and IS900 are compiled in Table 4 as are values of Young's modulus for the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogels. Literature values of Young's modulus for the remaining xerogels, T2. and the commercial standards IS700 and IS900 are also compiled in Table 4 as are literature values of root-mean-square roughness for the five comparative examples of xerogel coatings and the IS700 and IS900 coatings.
Values of the root-mean-square roughness for the xerogel surfaces (˜10−9 to 10−10 m) were calculated from AFM measurements. AFM images of the 1:14:35:50 C18/TDF/C8/TEOS xerogel prior to and after immersion in water are shown in
AF/FR Properties in Laboratory Testing.
Three organisms were used in laboratory testing to characterize AF and FR properties for the xerogel, T2, IS700, and IS900 coatings. Biofilm growth and removal of the marine bacterium C. lytica and the attachment and removal of the marine microalga N. incerta were measured in this example to determine AF and FR properties towards microfoulers while the adhesion strength of reattached barnacles (A. amphitrite) was used to evaluate the FR properties of the coatings toward a macrofouler.
The significance level, α, for the statistical tests described below was α=0.05. There were significant differences among the surfaces in the quantity of biofilm attaching to the material (ANOVA p<0.0001,
There were significant differences among the surfaces with respect to attachment of the microalgae (ANOVA p<0.0001,
The strength of attachment of reattached adult barnacles was measured via force-gauge measurements with forces applied in shear. Nine barnacles were removed from each surface and the fraction removed completely is shown in
The T2, IS700, IS900, 1:4:45:50 C18/TDF/C8/TEOS xerogel, and 1:14:35:50 C18/TDF/C8/TEOS xerogel surfaces all released >89% of the reattached barnacles completely (
One-way analysis of variance and Tukey tests indicate that there are significant differences among the surfaces of this example with respect to the CRS (ANOVA p<0.0001). Values of CRS were significantly lower on the IS700 (0.090±0.025 MPa) and IS900 surfaces (0.043±0.015 MPa) relative to all the other surfaces (
AF/FR Properties in Field Testing.
The 1:4:45:50 C18/TDF/C8/TEOS and 1:14:35:50 C18/TDF/C8/TEOS xerogel coatings were also assessed for AF and FR properties under static immersion conditions in the field. Analysis using a one-way ANOVA and Tukey tests indicate that there were significant differences in CRS of barnacles among coating surfaces exposed at the static field site (ANOVA p<0.00001). In the field tests, only those barnacles with <10% basal plate remaining following removal were used in the calculation of CRS. Values of CRS were significantly lower on the IS900 surfaces (0.046±0.012 MPa) in comparison to the 1:4:45:50 C18/TDF/C8/TEOS (0.322±0.018 MPa) or 1:14:35:50 C18/TDF/C8/TEOS (0.299±0.014 MPa) xerogel coatings, which were not statistically different from one another (
The results for water-jet testing of the 1:4:45:50 C18/TDF/C8/TEOS and 1:14:35:50 C18/TDF/C8/TEOS xerogel surfaces showed that the coatings cleaned back relatively well with only small amounts of hard foulers (in this case encrusting bryozoans) and slime remaining on the surfaces after the highest pressure was applied (Table 5). The IS900 control panel cleaned back completely at low pressures and had much less fouling accumulation initially.
Table 6 shows fouling ratings and physical deterioration rating (PDR) for the 1:4:45:50 C18/TDF/C8/TEOS and 1:14:35:50 C18/TDF/C8/TEOS xerogel coatings and the IS900 surface. The fouling ratings of the 1:4:45:50 C18/TDF/C8/TEOS and 1:14:35:50 C18/TDF/C8/TEOS xerogel surfaces were significantly lower in comparison to the IS900 control surfaces (ANOVA, p<0.001) but were not significantly different from each other. No defects were observed on any of the coatings for the duration of testing giving a PDR of 100 for all coatings.
Surface energy, elastic modulus, wettability, and roughness are important surface properties for determining the settlement and the ease of removal of fouling. The elastic modulus in particular influences the fracture mechanism of fouling organisms. As shown in Table 4, T2, IS700, and IS900 (all with elastic modulus of ˜1 MPa) are low modulus materials. They are typically applied as “thick” coatings (thickness≧150 μm) and macrofoulers often are released by a peeling mechanism involving interfacial slippage with failure at the bioadhesive-surface interface. The interfacial slippage involves both extensional deformation of the adhesive and shrinkage at the base area of. In contrast, the harder (elastic modulus>60 MPa, Table 4), thinner xerogel coatings (˜1 to 10 μm by profilometry) cannot undergo similar deformation, and release of fouling will be more dependent upon shear.
The coatings of this example provide an interesting contrast between high-(C8/TEOS, C18/C8/TEOS, and 1:4:45:50 C18/TDF/C8/TEOS xerogels) and low-modulus coatings (T2) with surface energies near the Baier minimum (21.3 to 23.1 mN m−1, Table 4), high- (DMAP/TEOS xerogel) and low-modulus coatings (IS700 and IS900) with surface energies higher in comparison to the Baier minimum (34 to 54.7 mN m−1, Table 4), and a high-modulus coating (1:14:35:50 C18/TDF/C8/TEOS xerogel) with surface energy below the Baier minimum (17.6 mN m−1, Table 4). The removal of barnacles from these surfaces offers a comparison of the importance of these two parameters—modulus and surface energy. All three low-modulus coatings (T2, IS700, IS900) displayed excellent FR behavior with respect to reattached adult barnacles in the laboratory assay as did the high-modulus C18/TDF/C8/TEOS and 1:14:35:50 C18/TDF/C8/TEOS xerogel coatings (
While both low- and high-modulus coatings released barnacles, the energy required in the removal process as measured by CRS both in the laboratory assay and field testing indicate the importance of modulus to the removal process. Values of CRS for the 1:4:45:50 C18/TDF/C8/TEOS xerogel (0.35 MPa) and 1:14:35:50 C18/TDF/C8/TEOS xerogel (0.42±0.08 MPa) coatings are 4- to 5-fold greater in comparison to CRS for the IS700 coating (0.090 MPa) and 8- to 10-fold greater in comparison to CRS for the IS900 coating (0.043 MPa). The field testing gave similar results with values of CRS for the 1:4:45:50 C18/TDF/C8/TEOS xerogel (0.32 MPa) and 1:14:35:50 C18/TDF/C8/TEOS xerogel (0.30 MPa) coatings roughly 7-fold greater than CRS for the IS900 coating (0.046 MPa). These results show a similar relationship to the CRS values reported for the low surface energy Teflon® and UHM polyethylene with high modulus (0.588 MPa and 0.776 MPa, respectively) when compared to polydimethylsiloxane coatings of lower modulus (0.09 to 0.29 MPa).
The FR characteristics are contrary to those predicted by surface energy and the Baier curve where the IS700 and IS900 coatings have significantly higher surface energies (p<0.01, Student t-test) than either the 1:4:45:50 C18/TDF/C8/TEOS or 1:14:35:50 C18/TDF/C8/TEOS xerogel and would be expected to be poorer performers based on surface energy alone (Baler et al. 1968; Baier 1984). The T2 standard has a value of surface energy similar to those of the two C18/TDF/C8/TEOS xerogels and within the Baier minimum. However, CRS for the T2 surface (0.25 MPa) is significantly lower (p<0.03, Student t-test) in comparison to CRS for either C18/TDF/C8/TEOS xerogel surface. It must be remembered, however, that the Baier curve was developed from observations of microfouling and not barnacle attachment. The results emphasize the importance of a thick, low-modulus surface to provide lower values of CRS for the removal of barnacles.
Since the xerogels are all harder, high-modulus surfaces, one can ask the reasonable question of whether release of barnacles follows a trend based on surface energies? The strength of protein adhesion to a series of xerogel coatings reproduced the Baier curve with respect to either surface tension (γS) or critical surface tension (γC). However, the strength of attachment of Ulva sporelings as measured by water-jet pressure for removal did not show similar agreement with the Baier curve for the same series of xerogels. In the Ulva sporeling study, it was postulated that the amine-containing xerogels might interact chemically with the sporeling adhesive. When the amine-containing xerogels were excluded from the correlation, linear correlations of adhesion strength with either water contact angle or polar contributions to the surface energy (γP) were then observed.
The removal of reattached barnacles from the xerogel surfaces roughly correlated with surface energy. The DMAP/TEOS xerogel coating has a value of γS (54.7 mN m−1) that is well above the Baier zone of minimal bioadhesion and this surface did not release barnacles at all. The basal plate remained affixed to the surface. The 1:4:45:50 C18/TDF/C8/TEOS, C8/TEOS, and C18/C8/TEOS xerogels have values of γS within the Baier zone of minimal bioadhesion (21.3-23.1 mN m−1) and all three surfaces released some barnacles completely (2/9 for C8/TEOS, 6/9 for C18/C8/TEOS, and 8/9 for 1:4:45:50 C18/TDF/C8/TEOS). The value of γS for the 1:14:35:50 C18/TDF/C8/TEOS xerogel (17.6 mN m−1) was well outside the Baier zone of minimal bioadhesion, but on the low energy side, yet released 9/9 reattached barnacles completely. All of the values of CRS for the xerogel surfaces that released barnacles were statistically equivalent (0.35 to 0.51 mN m−1, p>0.20, Student t-test).
With respect to the xerogel surfaces, one can also consider other factors influencing adhesion strength other than modulus and surface energy. Surface roughness is one parameter with statistically significant differences in all pair-wise comparisons of the xerogel coatings (Table 4, p<0.005, Student t-test). Among xerogel coatings that released reattached barnacles, the percentage released completely increased with surface roughness. The C8/TEOS xerogel had the lowest value of root-mean-square roughness and released the fewest barnacles, while the 1:14:35:50 C18/TDF/C8/TEOS xerogel, whose topography as measured by AFM is shown in
In comparisons of the C8/TEOS, C18/C8/TEOS, and the two C18/TDF/C8/TEOS xerogels, the increase in root-mean-square roughness is also accompanied by chemical segregation of functionality. The C8/TEOS xerogel is “smooth” topographically by AFM imaging of topography and chemically by IR microscopy of bulk chemical composition. The C18/C8/TEOS xerogel in contrast shows a network of nanoscale pores by AFM imaging of topography and regions of higher and lower hydrocarbon content in the bulk surface by IR microscopy. In the two C18/TDF/C8/TEOS xerogels, AFM phase images show islands of harder and softer material, which also appear as chemical inhomogeneities in hydrocarbon and fluorocarbon content in the IR microscopy images. Perhaps the chemical “mosaic” presented by the C18/C8/TEOS and the two C18/TDF/C8/TEOS xerogels contributes to the increased fraction of reattached barnacles removed via shear.
The idea of a chemical “mosaic” also comes into play in comparisons of comparative examples, commercial IS700 and IS900 coatings. With respect to biofilm growth and removal of the C. lytica biofilm, there is no apparent correlation of surface energy (γS), water contact angle (θWs), elastic modulus or surface roughness with either AF or FR character. The IS700 and IS900 coatings, with nearly identical values of elastic modulus, surface energy, and roughness were the “best” and “worst” surfaces, respectively, for FR behavior with C. lytica biofilm among the seven coatings of this example. However, the fluoropolymer-modified silicone composition of 15900 provides a chemical “mosaic” (enforced by the block co-polymer structure) not present in the silicone-based IS700, and chemical interactions of the surface with the biofilm may be quite different. It is also interesting that the C18/C8/TEOS and the two C18/TDF/C8/TEOS xerogels—all with demonstrated chemical segregation of functionality—also outperformed the other chemically homogenous xerogel surfaces, T2 silicone, and IS700 silicone with respect to C. lytica removal and were comparable to IS900.
The attachment of diatoms to a surface involves the initial secretion of an adhesive consisting of various proteoglycans. Attachment of diatoms appears to be stronger on hydrophobic surfaces relative to hydrophilic surfaces. However, the seven coatings of this example suggest that factors other than surface energy and wettability may be involved. The low-modulus and “rough” commercial T2, IS700, and IS900 coatings all gave poorer FR performance than all of the xerogel coatings with respect to percentage removal of microalgae biomass (
The tunability of the xerogel surfaces can show improved release of barnacles and other hard foulers through control of surface energy, chemical homogeneity/heterogeneity, and roughness. The xerogel coatings may be particularly useful as AF/FR surfaces in applications where, for example, thicker coatings are not optimal or practical. The xerogel coatings of this example, are optically transparent and have applications as AF/FR coatings where optical transparency is important (e.g., marine sensors, underwater cameras, submersible solar panels). Typically, coatings for these applications are thinner and the xerogels are appropriate materials.
The performance of 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogel coatings in the field with respect to barnacle removal and, after six months in the field, physical condition (PDR=100 for both) indicates that these materials are quite robust. The xerogel coatings may be excellent candidates for deployment where grooming in the field will be used to control the buildup of biofouling and coatings will need to survive the physical demands of grooming.
The control of chemical heterogeneities in the xerogel surfaces, surface energies, and roughness as well as the control of the physical characteristics of the coatings (thickness, modulus) all provide means for improving the AF/FR performance of the xerogel surfaces. The path forward is characterized by defining the surface properties and physical characteristics necessary to minimize the settlement and adhesion of both microfoulers and macrofoulers under a variety of field conditions.
9.1 ± 0.5f
aMean of five (5) independent measurements for coatings stored in air prior to measurement. ± one standard deviation.
bMean of three (3) independent measurements for coatings stored in air for 7 days prior to measurement.
cFrom Tang et al. 2005.
dFrom Feinberg et al. 2003.
eFrom Dobretsov and Thomason 2011.
fFrom Bennett et al. 2010.
gFrom Gunari et al. 2011.
hFrom Sokolova et al. 2012.
The first five entries are comparative examples.
The first entry is a comparative example.
aMean of three panels. Error limits are ±1 SEM.
bMean of barnacles removed completely (<10% basal plate remaining). Error limits are ±1 standard deviation of the mean.
The first entry is a comparative example.
Atomic Force Microscopy (AFM) Measurements.
The samples were imaged by atomic force microscopy (AFM) using a Nanoscope Dimension 3100 scanning probe microscope (Bruker AXS) in an environmentally controlled laboratory with the relative humidity set at 25%. Photomicrographs were acquired using TappingMode™ Atomic Force Microscopy (TM-AFM) under ambient conditions with a single crystal silicon Nanoprobe™ with a spring constant of ca. 17 to 43 N/m and resonance frequencies in the 262 to 359 kHz range. TappingMode™ AFM images were acquired at a 1-μm scan size with the z-scale set to 100-nm. Nanoindentation experiments yielded force-indentation curves, which were analyzed with custom-programmed analysis software (Igor Pro, Wavemetrics) allowing the calculation of the Young's modulus for the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogels.
The Young's modulus (Table 4) was determined by considering load-indentation dependence for a paraboloidal tip shape given by equation (1):
Here, F is the loading force in nN, E is Young's modulus in Pa, R is the radius of curvature of the tip in nm, δ is the indentation in nm, and ν is the Poisson's ratio (0.3).
Mixtures of n-octadecyltrimethoxysilane (C18, 1 to 5 mole %), n-octyltriethoxysilane (C8) and tetraethoxysilane (TEOS) gave xerogel surfaces of varying topography. The 1:49:50 C18/C8/TEOS xerogel formed 100 to 400-nm-wide, 2 to 7-nm deep pores by AFM while coatings with ≧3% C18 were free of such features. Segregation of the coating into alkane-rich and alkane-deficient regions in the 1:49:50 C18/C8/TEOS xerogel was observed by IR microscopy. 48-hour immersion in ASW gave no statistical difference in surface energy for the 1:49:50 C18/C8/TEOS xerogel and a significant increase for the 50:50 C8/TEOS xerogel. Settlement of barnacle cyprids and removal of juvenile barnacles, settlement of zoospores of the alga Ulva linza, and strength of attachment of 7-day sporelings were compared amongst the xerogel formulations. Settlement of barnacle cyprids was significantly lower in comparison to glass and polystyrene standards. The 1:49:50 and 3:47:50 C18/C8/TEOS xerogels were comparable to PDMSE with respect to removal of juvenile barnacles and sporeling biomass, respectively.
Materials and Methods. Chemical Reagents.
Deionized water was prepared to a specific resistivity of at least 18 MΩ using a Barnstead NANOpure Diamond UV ultrapure water system. Tetraethoxysilane (TEOS), n-octadecyltrimethoxysilane (C18), and n-octyltriethoxy-silane (C8) were purchased from Gelest, Inc. and were used as received. Ethanol was purchased from Quantum Chemical Corp. Hydrochloric acid was obtained from Fisher Scientific Co. Borosilicate glass microscope slides were obtained from Fisher Scientific, Inc.
Sol Preparation.
The sol/xerogel composition is designated in terms of the molar ratio of Si-containing precursors. Thus, a 50:50 C8/TEOS composition contains 50 mole % C8 and 50 mole % TEOS. In all of the sol preparations described below, the aqueous HCl was added last.
50:50 C8/TEOS.
A mixture of TEOS (2.09 g, 2.24 mL, 10 mmol), C8 (2.78 g, 3.16 mL, 10 mmol), isopropanol (4.0 mL), and 0.100 N HCl (1.23 mL, 0.123 mmol) was capped and stirred at ambient temperature for 24 hours.
5:45:50 C18/C8/TEOS.
A mixture of C18 (0.269 g, 0.720 mmol, 0.305 mL), C8 (1.79 g, 6.48 mmol, 2.03 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL) was stirred at ambient temperature for 24 hours.
4:46:50 C18/C8/TEOS. A mixture of C18 (0.215 g, 0.580 mmol, 0.244 mL), C8 (1.83 g, 6.62 mmol, 2:08 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL) was stirred at ambient temperature for 24 hours.
3:47:50 C18/C8/TEOS.
A mixture of C18 (0.161 g, 0.430 mmol, 0.183 mL), C8 (1.87 g, 6.77 mmol, 2.12 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL) was stirred at ambient temperature for 24 hours.
2:48:50 C18/C8/TEOS.
A mixture of C18 (0.108 g, 0.29 mmol, 0.122 mL), C8 (1.91 g, 6.91 mmol, 2.17 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL) was stirred at ambient temperature for 24 hours.
1:49:50 C18/C8/TEOS.
A mixture of C18 (0.054 g, 0.14 mmol, 0.061 mL), C8 (1.95 g, 7.06 mmol, 2.21 mL), TEOS (1.50 g, 7.20 mmol, 1.61 mL), 0.1 N HCl (0.91 mL, 0.09 mmol), and isopropanol (4.62 mL) was stirred at ambient temperature for 24 hours.
Xerogel Film Formation.
Xerogel films were formed by spin casting 400 μL of the sol precursor onto 25-mm×75-mm glass microscope slides. Prior to use, the slides were soaked in piranha solution for 24 hours, rinsed with copious quantities of deionized water, soaked in isopropanol for 10 min, air dried and stored at ambient temperature. A model P6700 spincoater (Specialty Coatings Systems, Inc.) was used at 100 rpm for 10 seconds to deliver the sol and at 3000 rpm for 30 seconds to coat.
For barnacle cyprid assays, glass 20-mm×60-mm Petri dish bottoms (VWR Scientific, Inc.) were soaked in piranha solution for 24 hours, rinsed with copious quantities of deionized water, and stored in an oven at 110° C. until use. The Petri dish bottoms were cooled to ambient temperature and 600 μL of the appropriate sol precursor was added and the Petri dish was manipulated until the bottom surface and approximately 5 mm of the side surface were covered. The excess sol precursor was removed via pipette. All coated surfaces (glass slides and Petri dishes) were dried at ambient temperature for at least 7 days prior to analysis.
Imaging Transmission Infrared (IR) Microscopy of Xerogel Samples.
Imaging transmission IR microscopy was carried out by using a Bruker Vertex 70 and Hyperion 3000 IR microscope (4 cm−1, 64 scans, 15× objective, 64×64 focal plane array). Samples were prepared by spin casting 400 μL of the sol precursor onto 25-mm×75-mm glass microscope slides and air drying the films at ambient temperature for at least seven days. Prior to IR imaging, xerogel films were soaked in doubly deionized water at 25° C. for 24 hours and then dried at 60° C. for 2 hours.
Atomic Force Microscopy (AFM) Imaging Measurements.
In order to determine surface roughness, xerogel samples were imaged using an atomic force microscope (AFM). In initial experiments, xerogel samples were immersed in artificial seawater (ASW) for 1 hour, rinsed with deionized water, and then air dried for 2 hours. Following immersion for 1 hour in ASW, no significant changes in water contact angle were observed over a 2-hour time period for samples dried in the air at ambient temperature. AFM images were obtained using the Molecular Force Probe AFM (Asylum Research, Santa Barbara, Calif.). Rectangular shaped silicon tips with a radius of curvature of <8 nm (NCH_W, NanoWorld, Switzerland) and with a nominal spring constant of 42 N/m were used. Intermittent contact mode imaging was used to obtain the AFM images.
The 1:49:50 C18/C8/TEOS xerogel was allowed to equilibrate for 48 hours in ASW and was then imaged in ASW using contact mode AFM imaging. Images were obtained using silicon nitride AFM tips (Veeco, MLCT) with a nominal spring constant of 100 pn/nm.
Surface Energy Analysis.
Contact angles were measured for all xerogel formulations with a contact angle goniometer (Raine-Hart, Model NRL 100) at room temperature using the sessile drop technique. The xerogel films were stored in air prior to characterization and measurements were performed in air. Up to 13 different diagnostic liquids were used including: water, glycerol, formamide, thiodiglycol, methylene iodide, 1-bromonaphthalene, 1-methylnaphthalene, dicyclohexyl, n-hexadecane, n-tridecane, n-decane, n-octane, and n-heptane. Liquid/vapor surface tensions of these liquids were determined using data obtained with a ring tensiometer (Cenco-duNuoy). The technique of “advanced angle” analysis was used. Zisman plots were constructed by plotting the cosine of the average angle measured for each liquid against the liquid/vapor surface tension of the diagnostic liquid. A linear least squares analysis was performed to determine the sample's critical surface tension (γC) at the cos θ=1 axis. The data were also treated to give the surface free energy (γS). For the 2:48:50 C18/C8/TEOS and 4:46:50 C18/C8/TEOS xerogels, values of γS were estimated using methylene iodide and water.
Biofouling Assays with Barnacles.
Barnacle cypris larvae were obtained from Duke University Marine Lab. Glass controls were acid washed in 10% HCl for 2 hours, rinsed well with deionized water, and allowed to dry completely prior to cyprid settlement. Silastic® T2 coated slides c. 500 nm in thickness, provided by Dr. AB Brennan, University of Florida, were included in the assays to provide a standard fouling-release coating.
Cyprid Settlement Assays.
Approximately 5 mL of seawater was added to each xerogel-coated Petri dish. This volume covered the bottom of the dish and allowed the cyprids free range of movement across the surface. A 400-μl drop of seawater containing between 30 and 60 2 to 4-day-old barnacle cypris larvae was then added to each of the dishes. The PDMSE (Silastic® T2) standard on glass microscope slides could not be completely immersed. A 400-μl drop of seawater containing between 20 and 40 2 to 4-day-old cypris larvae was added to the PDMSE surfaces. The larvae were allowed to settle for 48 hours at which time the percentage of barnacles that settled in each dish were counted. The average percent settlement for each of the experimental coatings was then compared to the glass and polystyrene standards. The concurrent settlement on the PDMSE (Silastic® T2) standard was used as a positive control for larval metamorphic competence.
Barnacle Removal Assays.
A 400-μl drop of seawater containing between 20 and 40 2 to 4-day-old cypris larvae was placed on the xerogel surface coated on glass microscope slides. The surfaces with larvae were placed in a constant temperature incubator at 25° C. on a 12 hours:12 hours light:dark cycle and larvae were allowed to settle for 48 hours. Newly metamorphosed juveniles on their respective coatings were transferred to growth chambers and kept in the incubator where they were fed the unicellular green alga Dunaliella tertiolecta and the diatom Skeletonema costatum for two weeks, and then a mixture of D. tertiolecta., S. costatum, and naupliar larvae of Artemia sp. for an additional week. Juveniles were then transferred to a 16-L aquarium tank in an automated rack system with temperature, salinity, and pH monitors and programmed for a 10% daily water change. Barnacles in the tank were fed a 500-mL flask of Artemia sp. 3 times a week for 4 to 6 weeks, which is the time it took the juvenile barnacles to reach a basal plate diameter of 3-5 mm, the minimum size necessary to conduct force gauge tests according to ASTM D 5618.
The procedures for critical removal stress were followed from ASTM D 5618 with the following modifications: (i) The force measuring device was operated by a motorized stand, thus insuring a constant application of force during dislodgement; and (ii) Barnacle dislodgement studies from coatings were performed under water. The apparatus consists of an IMADA ZP-11 digital force gauge (11.2 lb) mounted on an IMADA SV-5 motorized stand. The slides are clamped into a custom-built Plexiglas chamber that allows their complete submersion during dislodgement tests.
Juvenile barnacles were selected for testing based on healthy appearance and minimum size requirements. Only barnacles occurring at least 5 mm from the edges of the slide were tested. Other barnacles in close proximity to the test subject were removed if they could potentially interfere with measurements. Prior to removal of barnacles each basal plate was photographed using a Canon™ EOS 10D camera attached to an Olympus™ SZX12 dissecting microscope, and images were calculated using NIH's ImageJ. After photographs were taken, the slide was clamped into the Plexiglas chamber. The force gauge mounted on the motorized stand was used to apply a shear force to the base of the barnacles at a rate of approximately 4.5 N s−1 until the organism was detached. Force was applied parallel to the film surface. The force required for detachment was noted and observations were made as to the mode of failure. If any portion of the base of the organism was left attached to the substratum, the test was deemed void for removal. The surfaces were examined visually for damage to the xerogel film caused by barnacle removal and by stereomicroscope if there were any ambiguity. The critical removal stress was calculated by dividing the force (F, Newtons) required to remove the test subject by the area of attachment (A, mm2). For barnacles where a portion of the base of the organism was left attached to the substratum, the remaining basal plate is photographed and the exact percentage remaining after testing (% BPR) is calculated with digital image analysis.
Biofouling Assays with Ulva.
Coatings applied to glass slides were equilibrated in circulating deionized water for 24 hours prior to the start of assays with algae. One hour prior to the assay, the slides were transferred to ASW. Silastic® T2 coated slides c. 500 μm in thickness, provided by Dr. AB Brennan, University of Florida, were included in the assays to provide a standard fouling-release coating.
Settlement of Zoospores of Ulva.
Fronds of Ulva linza were collected from Llantwit Major, Wales (51840′N; 3848′W) and a spore suspension of 1.5×106 spores mL−1 prepared. Three replicate slides of each treatment were placed in individual wells of ‘quadriperm’ polystyrene culture dishes and 10 mL of spore suspension were added. Dishes were incubated in the dark for 1 hour at ˜20° C. After incubation the slides were gently washed in ASW to remove unattached (swimming) spores. Slides were fixed in 2.5% glutaraldehyde. The density of spores attached to the surfaces was counted using an image analysis system attached to a fluorescence microscope. Spores were visualized by autofluorescence of chlorophyll. Counts were made for thirty (30) fields of view (each 0.17 mm2), 1 mm apart over the central region of each slide, using image analysis software (Imaging Associates Ltd.) attached to a Zeiss epifluorescence microscope. Spore settlement data are expressed as the mean number of spores adhered per mm2 with 95% confidence limits (n=90).
Adhesion strength of Sporelings of Ulva.
Spores were allowed to settle as described above. After washing away unattached spores, spores that had attached to the test surfaces were cultured in dishes containing supplemented seawater medium that was changed every 2 days. The dishes were placed in an illuminated incubator (75 mW m−2 s−1 incident irradiation) for 7 days during which time the spores germinated and developed into sporelings (young plants).
The biomass produced was quantified by measuring the fluorescence of chlorophyll in a Tecan fluorescence plate reader (excitation=430 nm, emission=670 nm). Fluorescence was measured as relative fluorescence units (RFU) and was directly proportional to the quantity of biomass present. The RFU value for each slide was the mean of 70 point fluorescence readings taken from the central region.
Sporeling adhesion strength was determined by exposing the slides to an impact pressure of 54 kPa from an automated water jet, which traversed the central region of each slide. Three replicate slides of each coating were used in the analysis. The biomass that remained in the sprayed area after exposure to the water jet was quantified as described above. Percentage removal of sporelings was determined by comparison of the biomass (RFU) before exposure with that remaining attached to the coatings after exposure to the water jet.
Characterization of Xerogel Surfaces.
A 5 μm×5 μm AFM image of the 1:49:50 C18/C8/TEOS xerogel surface after immersion in ASW for 48 hours and imaging in ASW is shown in
The roughness of the surfaces shown in
Values of Rrms are compiled in Table 7. The 50:50 C8/TEOS, 4:46:50 C18/C8/TEOS, and 3:47:50 C18/C8/TEOS surfaces have a very low surface roughness (<0.25 nm) when the surfaces have been immersed in ASW for 1 hour, rinsed with distilled water, and air dried for 2 hours (Table 7). The 2:48:50 C18/C8/TEOS and 1:49:50 C18/C8/TEOS surfaces display increased values of Rrms (0.67±0.03 and 1.15±0.04 nm, respectively, Table 7).
Values of γC and γS were determined for surfaces stored in air by comprehensive contact angle analysis. The 50:50 C8/TEOS and the C18/C8/TEOS xerogel surfaces have similar values of γC (21.3-22.4 mN m−1) and γS (22.8-27.1 mN m−1) as shown in Table 7. Measured values of γC for the xerogel surfaces are quite similar to γC for PDMSE (23 mN m−1).
Prior to the comprehensive contact angle analysis, the xerogel films were stored in air for several weeks until values of θWs were no longer increasing and measurements were also performed in air. The static water contact angles, θWs, for all of the C18-containing xerogel coatings were higher in comparison to θWs for the 50:50 C8/TEOS coating ((100±1)°, Table 7) with the 1:49:50 C18/C8/TEOS coating having the highest value of θWs ((111.2±0.2)°, Table 7).
The 50:50 C8/TEOS and 1:49:50 C18/C8/TEOS xerogel coatings were immersed in ASW for various periods of time to examine the impact of the aqueous environment on θWs. Following immersion in ASW, the xerogel films were rinsed with distilled water, dried with a stream of argon (filtered through DriRite®) at ambient temperature for 2 min prior to contact angle measurement. For the 50:50 C8/TEOS xerogel, θWs was unchanged after 4-hour immersion in ASW ((99 f)2)°, decreased to (93±2)° after 24 hours of immersion, to (87±1)° after 48 hours of immersion, and remained constant with longer immersion times (up to 1 week). For the 1:49:50 C18/C8/TEOS xerogel, the value of θWs decreased to (104±1)° after 4 hours of immersion in ASW and to (96±1)° after 24 hours of immersion in ASW where it remained constant.
The “recovery” of the 1:49:50 C18/C8/TEOS xerogel surface following immersion in an aqueous environment was examined with surfaces that had been soaked in deionized water. The value of θWs decreased from (110±1)° prior to immersion in deionized water to (87±2)° after 1 week of immersion. Upon standing in air at ambient temperature, values of θWs increased with time returning to pre-immersion values (>107°) after 4 weeks.
Values of γS were estimated for the 50:50 C8/TEOS and 1:49:50 C18/C8/TEOS xerogels following immersion in ASW with water and methylene iodide contact angles. Prior to immersion in ASW, values of γS were 27.1±0.3 mN m−1 for the 50:50 C8/TEOS xerogel surface and 24.8±1.1 mN m−1 for the 1:49:50 C18/C8/TEOS xerogel surface (Table 7). Following immersion in ASW, γS increased to 35.2±0.7 mN m−1 for the 50:50 C8/TEOS xerogel surface over the 48-hour to 96-hour immersion window and 25.6±1.1 mN m−1 for the 1:49:50 C18/C8/TEOS xerogel surface over the 24-hour to 96-hour immersion window.
Settlement of Cypris Larvae and Removal of Juvenile Barnacles of B. Amphitrite.
The settlement of 2 to 4-day-old barnacle cypris larvae that were placed on the xerogel, glass, and polystyrene surfaces was compared as shown in
The strength of attachment of juvenile barnacles to the five C18/C8/TEOS xerogel surfaces, the 50:50 C8/TEOS xerogel and glass and PDMSE (Silastic® T2) standards was measured via force-gauge measurements with forces applied in shear. Only the 1:49:50 C18/C8/TEOS (15/19 barnacles removed completely), 2:48:50 C18/C8/TEOS (7/21 barnacles removed completely), and 4:46:50 C18/C8/TEOS (4/15 barnacles removed completely) xerogel surfaces and the PDMSE (Silastic® T2) standard (13/15 barnacles removed completely) performed as fouling-release surfaces (
The critical removal stress (CRS) for the 1:49:50 C18/C8/TEOS, 2:48:50 C18/C8/TEOS, and 4:46:50 C18/C8/TEOS xerogel surfaces was 0.20-0.21 N mm−2, which was somewhat higher in comparison to the removal force for the PDMSE (Silastic® T2) standard (0.15 N mm−2, Table 8). There were no significant differences in CRS amongst the 1:49:50 C18/C8/TEOS, 2:48:50 C18/C8/TEOS, and 4:46:50 C18/C8/TEOS xerogel surfaces. However, on all coatings except for the 4:46:50 C18/C8/TEOS xerogel, the average CRS values were significantly higher than the average CRS of the PDMSE (Silastic® T2) standard (ANOVA p=0.0371). Barnacles on all other coatings and the glass standard broke when force was applied to them in shear, and left a complete or partial basal plate attached to the surface.
The remaining fraction of the basal plate left on a surface may be a function of barnacle bioadhesive bond strength and could be used as a measure of the efficacy of foul-release coatings. For barnacles not completely removed, the percentage of the basal plate remaining was calculated with digital image analysis. These results were combined with data for barnacles completely removed (0% basal plate remaining) and are compiled in Table 8 and shown in
Settlement and Removal of Zoospores of Ulva.
Settlement of zoospores and growth of sporeling biomass and removal on the xerogel surfaces and glass and PDMSE (Silastic® T2) standards were examined (Table 8,
Strength of Attachment of Sporelings of Ulva.
Sporelings grew well and after 7 days, a green covering was visible on all surfaces. Sporeling growth tracked the density of settled zoospores and sporeling growth amongst all the C18/C8/TEOS xerogels was comparable (Table 8). The percentage removal of 7-day sporeling biomass from the test surfaces with a water pressure of 54 kPa is compiled in Table 8 and is shown in
Scanning electron microscopy (SEM) studies of the 50:50 C8/TEOS xerogel as well as several other xerogel surfaces indicate that these surfaces are uniform, uncracked, and topographically smooth when thy. Atomic force microscopy (AFM) measurements on the same series of xerogels submerged in ASW show very low surface roughness (≦0.8 nm). Time-of-flight, secondary-ion mass spectrometry (ToF-SIMS) studies show that there is no phase segregation of fluorocarbon and hydrocarbon groups on the μm scale in a 25:25:50 trifluoropropyltrimethoxysilane/C8/TEOS xerogel. The 50:50 C8/TEOS, 50:50 C3/TEOS and 5:45:50 C18/C8/TEOS xerogel surfaces have nearly identical values of γC and γS.
In the current example, the incorporation of 1 to 5 mole % C18 in the C8/TEOS sol (a relatively small change in formulation) dramatically impacted the distribution of chemical functionality on the xerogel surface (from transmission IR microscopy images) and the surface topography (AFM studies). The IR images of the 50:50 C8/TEOS and 1:49:50 C18/C8/TEOS xerogels films shown in
The dimensions of the nanopores in the AFM images are much smaller than the 2.7-μm resolution of the IR microscopy images, where spatial resolution is essentially limited by the diffraction limit, so it is difficult to compare directly (overlay) the structural features. However, it is clear that the small changes in formulation dramatically impact the distribution of chemical functionality, as well as the surface topography. In spite of these differences, values of γC (21.4-22.4 mN m−1) and γS (22.8-25.7 mN m−1) are identical within statistical error within the series of C18/C8/TEOS xerogels in this example for surfaces stored in the air (Table 7). Measured values of γC for the C18/C8/TEOS xerogel surfaces are quite similar to γC for comparative examples, 50:50 C8/TEOS xerogel (21.3 mN m−1, Table 7) and γC for PDMSE (23 mN m−1, and are all within the 20 to 25 mN m−1 range, where minimal bioadhesion has been reported due to the formation of weak boundary layers between the surface and the adhesives of fouling organisms.
The values of γC and γS described in the paragraph above are all for surfaces that had been stored in the air prior to contact angle measurement. For the 1:49:50 C18/C8/TEOS xerogel, immersion in ASW for 24 to 96 hours gave surfaces with no significant differences in γS (25.6±1.1 mN m−1) relative to surfaces stored in air (24.8±1.1 mN m−1). This is in contrast to the behavior of the 50:50 C8/TEOS where γS increased significantly from 27.1±0.3 mN m−1 for surfaces stored in the air to 35.2±0.7 mN m−1 for surfaces immersed 48 to 96 hours in ASW.
Although the 50:50 C8/TEOS and 1:49:50 C18/C8/TEOS xerogels undergo some surface reorganization upon immersion in an aqueous environment as indicated by the drop in θWs for both surfaces, the reorganization is reversible. Contact angles returned to pre-immersion values upon standing in the air indicating that no irreversible chemical or structural changes had occurred to either of the surfaces upon wetting.
One can speculate with respect to the differences observed in the distribution of surface functionality and topography in the 1:49:50 C18/C8/TEOS xerogel and the remaining C18/C8/TEOS xerogels and the 50:50 C8/TEOS xerogel. Through increased London forces (the “hydrophobic effect”), interactions among the C18 molecules during sol formation might lead to preferential formations of islands of higher C18 density. With 2 mole % or less C18 in the formulation, these islands are significantly discrete to impact the surface topography. At higher C18 concentrations, segregation in the sol is still possible, but interactions among the C18 groups lead to a more uniform surface following xerogel formation. Values of θWs increase over several weeks from the time of coating to their final value given in Table 7 suggesting that the initial surface is not in its final state and matures with time. “Untangling” of the C18 islands followed by formation of more uniform C18-C18 or C18-C8 interactions perhaps leads to less surface roughness at higher C18 concentrations.
The 1:49:50 C18/C8/TEOS xerogel surface segregates alkane functionality as indicated by transmission IR microscopy. Transmission IR microscopy, as a surface characterization technique, complements more commonly used methods such as SEM, AFM, ToF-SIMS, and x-ray photoelectron spectroscopy (XPS). Transmission IR microscopy images show the distribution of functional groups on a surface and the integrated spectra quantify the density of functional groups. The technique provides additional details of surface nanostructure that will inform the development of future antifouling and fouling release coatings.
Initial studies with the barnacle B. amphitrite suggest that the differences in topography and distribution of chemical functionality among the C18/C8/TEOS xerogels significantly impacts the settlement and/or release of this fouling organism in comparison to one another and in comparison to the 50:50 C8/TEOS xerogel as shown in
Biological evaluation is imperfect because of variability in the various organisms considered in this example. The reason why some barnacles are removed completely and why other barnacles are only partially removed is attributable to variability in coating, variability in barnacle adhesive, and variability in the structural integrity of the barnacle. All are involved to some degree, but there is no way to tease apart the relative contribution of the different factors resulting in basal plate breakage. In this example, the 1:49:50 C18/C8/TEOS xerogel and the PDMSE (Silastic® T2) standard are statistically equivalent for the removal of barnacles—completely or partially.
The equivalence of the 1:49:50 C18/C8/TEOS xerogel and the PDMSE (Silastic® T2) standard in this example is somewhat surprising since the xerogel surfaces are thin (<5 μm thickness) and have relative high modulus (60 MPa for a comparative example, 50:50 C8/TEOS xerogel) in comparison to the PDMSE (Silastic® T2) standard with a thickness of approximately 500 um and a low elastic modulus of 0.6 MPa. The 1:49:50 C18/C8/TEOS xerogel is unlikely to follow Brady-Singer fracture mechanics, which is thought to be the major mode of barnacle release for PDMSE coatings. The surface topography of the 1:49:50 C18/C8/TEOS xerogel on both the nm and the μm scales is unlike the surface topography of other xerogels examined to date. In the absence of features that favor Brady-Singer fracture mechanics, one may speculate on the relationship of surface topography and the adhesive strength of barnacle cements.
With Ulva, differences in topography and distribution of chemical functionality amongst the C18/C8/TEOS xerogels also impacted settlement of zoospores and removal of sporeling biomass, but the best performance was exhibited by the 3:47:50 C18/C8/TEOS xerogel—not the 1:49:50 C18/C8/TEOS xerogel that performed well with barnacles. Following exposure to a water pressure of 54 kPa, removal of sporeling biomass from the 3:47:50 C18/C8/TEOS xerogel was significantly greater in comparison to the glass standard and the remaining C18/C8/TEOS xerogels (Table 8,
While opposing trends in the settlement and release of different fouling organisms make it difficult to design a single surface to minimize settlement of all fouling organisms, the xerogels can be fine-tuned to provide surfaces with different water wettability and critical surface tension/surface energy. The topography of the xerogel surfaces can also be fine-tuned by the incorporation of small amounts of a long-chain alkyl component as shown by the C18/C8/TEOS xerogels of this example. The formulation and coating of these xerogel-based surfaces require no special attention or preparation (pre-patterning). Overall, xerogel surfaces have high potential as fouling-release or easy-clean materials.
23.0 ± 0.4e
22.8 ± 1.1f
22.8 ± 1.1f
aMean of five (5) independent measurements for coatings stored in air prior to measurement. ± one standard deviation.
bMean of two (2) independent measurements for coatings stored in air prior to measurement.
cAverage of five (5) replicate measurements. ± one standard deviation.
dFrom Tang et al. (2005).
eFrom Feinberg et al. (2003).
fMethod of Owens and Wendt (1969).
The first three entries are comparative examples.
aEach value is the mean from 90 counts on three (3) replicate slides. ±95% confidence limits.
bSporeling biomass data are expressed as the mean RFU of three (3) replicate slides; ± standard error of the mean.
cPercent removal of 7-day old sporelings with a surface water pressure 54 kPa from 50:50 C8/TEOS and C18/C8/TEOS hybrid xerogel coatings.
dNumber of juvenile barnacles completely removed with n as the total number of barnacles removed.
eAverage critical removal stress (CRS); ± standard error of the mean.
fAverage percentage of basal plate remaining after barnacle removal with n as the total number of barnacles removed; ± standard error of the mean.
The first three entries are comparative examples.
While the invention has been particularly shown and described with reference to specific embodiments (some of which are preferred embodiments), it should be understood by those having skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as disclosed herein.
This application claims priority to U.S. provisional patent application No. 61/494,924, filed Jun. 9, 2011, the disclosure of which is incorporated herein by reference.
This invention was made with government support under grant no. N00014-09-1-0217 awarded by the Office of Naval Research. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61494924 | Jun 2011 | US |
