Patent Application
20150157990
The present invention is directed toward thin film composite membranes along with methods for making and using the same.
Composite polyamide membranes are used in a variety of fluid separations. One class includes a porous support with a “thin film” polyamide layer. These membranes are commonly referred to as “thin film composite” (TFC) membranes. The thin film polyamide layer may be formed by an interfacial polycondensation reaction between polyfunctional amine (e.g. m-phenylenediamine) and polyfunctional acyl halide (e.g. trimesoyl chloride) monomers which are sequentially coated upon the support from immiscible solutions, see for example U.S. Pat. No. 4,277,344 to Cadotte. WO 2010/120326 describes the use of trimesoyl chloride in combination with its mono and di-hydrolyzed counterparts. Additional constituents have also been added to the coating solution to improve membrane performance. For example, U.S. Pat. No. 6,878,278 to Mickols describes the addition of a tri-hydrocarbyl phosphate compound to the acyl halide coating solution.
The incorporation of carboxylic acid functional groups into the polyamide layer imparts the layer with a more negative charge and this is believed to improve the rejection of certain solutes (e.g. NaCl, nitrate) along with providing the membrane with improved resistance to certain foulants (e.g. humic acid). A portion of the acyl halide groups of the polyfunctional acyl halide monomer inevitably become hydrolyzed during interfacial polymerization (e.g. via reaction with water present in the atmosphere or with water present in the amine coating solution). The overall carboxylic acid content (“dissociated carboxylate content”) of the polyamide layer can be further increased by utilizing acyl halide monomers including carboxylic acid functional groups. See for example WO 2010/120326. Other approaches which may increase the carboxylic acid content of the polyamide layer involve post-treatment with a chlorinating agent (see for example: U.S. Pat. No. 4,277,344, U.S. Pat. No. 4,761,234, U.S. Pat. No. 5,051,178 and U.S. Pat. No. 5,876,602) or the post-application of charged coatings. Unfortunately, post-treatment steps add to the cost and complexity of membrane preparation. Moreover, post-chlorination can compromise long term membrane performance.
In one embodiment, the invention includes a thin film composite polyamide membrane including a porous support and a thin film polyamide layer which is a reaction product of m-phenylene diamine (mPD) and trimesoyl chloride (TMC), wherein the membrane is characterized by the thin film polyamide layer having a dissociated carboxylic acid content of at least 0.18 moles/kg at pH 9.5, and wherein pyrolysis of the thin film polyamide layer at 650° C. results in a ratio of responses from a flame ionization detector for fragments produced at 212 m/z and 237 m/z of less than 2.8. In another embodiment, the invention includes a method for making a thin film composite membrane by applying a polar solution of m-phenylene diamine and a non-polar solution comprising trimesoyl chloride to a surface of a porous support to form a thin film polyamide layer, wherein the non-polar solution further comprises: i) an acyl halide reactant comprising at least one acyl halide moiety and at least one carboxylic acid moiety, and ii) tri-hydrocarbyl phosphate compound. Many additional embodiments are described.
The invention includes thin film composite membranes and methods for making and using the same. The invention is not particularly limited to a specific type, construction or shape of composite membrane or application. For example, the present invention is applicable to flat sheet, tubular and hollow fiber polyamide membranes useful in a variety of applications including forward osmosis (FO), reverse osmosis (RO) and nano filtration (NF). RO composite membranes are relatively impermeable to virtually all dissolved salts and typically reject more than about 95% of salts having monovalent ions such as sodium chloride. RO composite membranes also typically reject more than about 95% of inorganic molecules as well as organic molecules with molecular weights greater than approximately 100 Daltons. NF composite membranes are more permeable than RO composite membranes and typically reject less than about 95% of salts having monovalent ions while rejecting more than about 50% (and often more than 90%) of salts having divalent ions—depending upon the species of divalent ion. NF composite membranes also typically reject particles in the nanometer range as well as organic molecules having molecular weights greater than approximately 200 to 500 Daltons.
The subject method involves forming a thin film polyamide layer upon a porous support. The porous support is not particularly limited and preferably includes a polymeric material having pore sizes which are of sufficient size to permit essentially unrestricted passage of permeate but not large enough so as to interfere with the bridging over of a thin film polyamide layer formed thereon. For example, the pore size of the support preferably ranges from about 0.001 to 0.5 μm. Non-limiting examples of porous supports include those made of: polysulfone, polyether sulfone, polyimide, polyamide, polyetherimide, polyacrylonitrile, poly(methyl methacrylate), polyethylene, polypropylene, and various halogenated polymers such as polyvinylidene fluoride.
The thin film polyamide layer is prepared by an interfacial polycondensation reaction between m-phenylene diamine (mPD) and trimesoyl chloride (TMC) on the surface of the porous support as described in U.S. Pat. No. 4,277,344 and U.S. Pat. No. 6,878,278. The term “polyamide” refers to a polymer in which amide linkages (—C(O)NH—) occur along the molecular chain. mPD and TMC may be applied to the porous support by way of a coating step from solution, wherein mPD is preferably coated from an aqueous-based or polar coating solution and TMC is coated from an organic-based or non-polar coating solution. The polar coating solution preferably contains from about 0.1 to about 20 weight percent and more preferably from about 0.5 to about 6 weight percent of mPD. The non-polar solution preferably contains from about 0.1 to 10 weight percent and more preferably from 0.3 to 3 weight percent of TMC. Once brought into contact with one another, the monomers react at their surface interface to form a polyamide layer or film. This layer, often referred to as a polyamide “discriminating layer” or “thin film layer,” provides the composite membrane with its principal means for separating solute (e.g. salts) from solvent (e.g. aqueous feed). The reaction time of the monomers may be less than one second but contact times typically range from about 1 to 60 seconds, after which excess liquid may be optionally removed by way of an air knife, water bath(s), dryer or the like. The removal of the excess solution can be achieved by drying at elevated temperatures, e.g. from about 40° C. to about 120° C., although air drying at ambient temperatures may be used. Although the coating steps need not follow a specific order, mPD is preferably coated first followed by TMC. Coating can be accomplished by spraying, film coating, rolling or through the use of a dip tank among other coating techniques.
The interfacial polymerization is conducted in the presence of an acyl halide reactant. The reactant may be coated from a separate coating solution or in some embodiments, combined and coated from the aforementioned non-polar coating solution. The acyl halide reactant includes at least one acyl halide moiety and at least one carboxylic acid moiety (or salt thereof). Non-limiting examples of acyl halide reactants include mono and di-hydrolyzed counterparts of the polyfunctional acyl halide monomers described below along with the mono, di and tri-hydrolyzed counterparts of the tetraacyl halide monomers described below. Preferred acyl halide reactants include mono-hydrolyzed trimesoyl chloride (mhTMC) i.e. 1-carboxy-3,5-dichloroformyl benzene and the mono-hydrolyzed isophthaloyl chloride (mhIPC). Additional examples include that represented by Formula (I):
wherein X is a halogen (preferably chlorine) and n is an integer from 1 to 10. Representative species include: 4-(chlorocarbonyl) butanoic acid; 5-(chlorocarbonyl) pentanoic acid; 6-(chlorocarbonyl) hexanoic acid; 7-(chlorocarbonyl) heptanoic acid; 8-(chlorocarbonyl) octanoic acid; 9-(chlorocarbonyl) nonanoic acid and 10-(chlorocarbonyl) decanoic acid. While the acyl halide and carboxylic acid groups are shown in terminal positions, one or both may be located at alternative positions along the aliphatic chain. While not shown in Formula (I), the aliphatic reactant may include additional carboxylic acid and acyl halide groups. Additionally, corresponding aromatic species may used including 3-carboxybenzoyl chloride and 4-carboxybenzoyl chloride.
Non-limiting polyfunctional acyl halide monomers that may be hydrolyzed to form the subject reactant include: terephthalic acid chloride, isophthalic acid chloride, biphenyl dicarboxylic acid chloride, and naphthalene dicarboxylic acid dichloride. Non-limiting examples of aliphatics include adipoyl chloride, malonyl chloride, glutaryl chloride, and sebacoyl chloride. Non-limiting examples of alicyclic polyfunctional acyl halides include: cyclopropane tri carboxylic acid chloride, cyclopentane tri carboxylic acid chloride, cyclohexane tri carboxylic acid chloride, cyclopentane dicarboxylic acid chloride, cyclobutane dicarboxylic acid chloride, cyclohexane dicarboxylic acid chloride, and tetrahydrofuran dicarboxylic acid chloride. Representative polyfunctional acyl halide monomers also include a tetraacyl halide monomers as represented by Formula (II):
wherein A is selected from: oxygen (—O—), carbon (—C—), silicon (—Si—), sulfur (—S—) and nitrogen (—N—) which may be unsubstituted or substituted, e.g. with alkyl groups of 1-4 carbon atoms; or a carbonyl group (—C(O)—). X is the same or different and is selected from a halogen. In a preferred embodiment, each X is chlorine. In another preferred embodiment, A is an unsubstituted carbon, i.e. the subject monomer is 5,5′-methylene diisophthaloyl dichloride.
The acyl halide reactant and TMC are preferably coated from a common non-polar coating solution having a total acyl halide content of at least 0.10 wt % (e.g. 0.10 to 5 wt %). As used herein, the term “acyl halide content” refers to the concentration of compounds including at least one acyl halide moiety, as measured by weight and common dilution practices. The concentration of the acyl halide reactant in the coating solution is preferably at least 0.001 wt % and in some embodiments at least 0.005, 0.01, 0.02, 0.04 or 0.07 wt %. A preferred upper limit for this reactant is 0.25 wt %, 0.15 wt %, 0.010 wt % and in some embodiments, equal to or less than 0.08 wt %. The coating solution may be applied to the porous support as part of a continuous or batch coating operation. Suitable non-polar solvents are those which are capable of dissolving the acyl halide reactants and which are immiscible with water, e.g. hexane, cyclohexane, heptane and halogenated hydrocarbons such as the FREON series. Preferred solvents include those which pose little threat to the ozone layer and which are sufficiently safe in terms of flashpoints and flammability to undergo routine processing without taking special precautions. A preferred solvent is ISOPAR™ available from Exxon Chemical Company. The coating solution may optionally include additional materials including co-solvents, phase transfer agents, solubilizing agents and complexing agents wherein individual additives may serve multiple functions. Representative co-solvents include: benzene, toluene, xylene, mesitylene, ethyl benzene, diethylene glycol dimethyl ether, cyclohexanone, ethyl acetate, butyl carbitol™ acetate, methyl laurate and acetone. In one embodiment, the non-polar coating solution utilized to coat the polyfunctional acyl halide includes from 4 to 100 wt % mesitylene as a co-solvent along with a solvent such as an isoparaffin.
In another embodiment, the subject method includes the step of conducting the interfacial polymerization in the presence of a tri-hydrocarbyl phosphate compound. The means of applying the tri-hydrocarbyl phosphate compound to the porous support are not particularly limited, e.g. the tri-hydrocarbyl phosphate compound may be included in one or both of the aforementioned coating solutions or may be coated from a separate coating solution before or during the interfacial polymerization. In a preferred embodiment, the tri-hydrocarbyl phosphate compound is added to the non-polar coating solution used to apply TMC to the porous support.
In preferred embodiments, the tri-hydrocarbyl phosphate compound is present during the interfacial polymerization in a molar ratio with TMC of at least 0.1:1, 0.5:1, 1:1, 1.1:1, 1.2:1, 1.5:1 or 2:1. Preferred ranges of molar ratios (tri-hydrocarbyl phosphate compound to TMC) include: 1:1 to 5:1, 1.2:1 to 4:1, 1.5: to 3:1 and 2:1 to 3:1.
Representative examples of applicable tri-hydrocarbyl phosphate compounds are described in U.S. Pat. No. 6,878,278. A preferred class of such compounds includes those represented by Formula (III):
wherein “P” is phosphorous, “O” is oxygen and R1, R2 and R3 are independently selected from hydrogen and hydrocarbyl groups comprising from 1 to 10 carbon atoms, with the proviso that no more than one of R1, R2 and R3 are hydrogen. R1, R2 and R3 are preferably independently selected from aliphatic and aromatic groups. Applicable aliphatic groups include both branched and unbranched species, e.g. methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-pentyl, 3-pentyl. Applicable cyclic groups include cyclopentyl and cyclohexyl. Applicable aromatic groups include phenyl and naphthyl groups. Cyclo and aromatic groups may be linked to the phosphorous atom by way of an aliphatic linking group, e.g., methyl, ethyl, etc. The aforementioned aliphatic and aromatic groups may be unsubstituted or substituted (e.g., substituted with methyl, ethyl, propyl, hydroxyl, amide, ether, sulfone, carbonyl, ester, cyanide, nitrile, isocyanate, urethane, beta-hydroxy ester, etc); however, unsubstituted alkyl groups having from 3 to 10 carbon atoms are preferred. Specific examples of tri-hydrocarbyl phosphate compounds include: tripropyl phosphate, tributyl phosphate, tripentyl phosphate, trihexyl phosphate, triphenyl phosphate, propyl biphenyl phosphate, dibutyl phenyl phosphate, butyl diethyl phosphate, dibutyl hydrogen phosphate, butyl heptyl hydrogen phosphate and butyl heptyl hexyl phosphate.
In a preferred embodiment, the thin film polyamide layer is characterized by having a dissociated carboxylate content of at least 0.18, 0.20, 0.22, 0.3, and in some embodiments at least 0.4 moles/kg of polyamide at pH 9.5 as measured by the Rutherford Backscattering measurement technique described below in the Example section. While not used in the present experiments, the carboxylic acid content of TFC polyamide membranes (moles/kg of polyamide) can also be determined using the measured polyamide mass per unit area and the moles of carboxylic acids groups per unit area, such as by complexation and quantification methods also described in the Example section.
In another preferred embodiment, pyrolysis of the thin film polyamide layer at 650° C. results in a ratio of responses from a flame ionization detector for fragments produced at 212 m/z and 237 m/z of less than 2.8, and more preferably less than 2.6. The fragments produced at 212 and 237 m/z are represented by Formula IV and V, respectively.
This ratio of fragments is believed to be indicative of polymer structures that provide improved flux, salt passage or integrity (particularly for membranes having relatively high carboxylic acid content, e.g. a dissociated carboxylate content of at least 0.18, 0.20, 0.22, 0.3, and in some embodiments at least 0.4 moles/kg of polyamide at pH 9.5). With reference to
A preferred pyrolysis methodology is conducted using gas chromatography mass spectrometry with mass spectral detection, e.g. a Frontier Lab 2020iD pyrolyzer mounted on an Agilent 7890 GC with detection using a LECO time of flight (TruTOF) mass spectrometer. Peak area detection is made using a flame ionization detector (FID). Pyrolysis is conducted by dropping the polyamide sample cup into pyrolysis oven set at 650° C. for 6 seconds in single shot mode. Separation is performed using a 30 M×0 25mm id column from Varian (FactorFour VF-5MS CP8946) with a 1 um 5% phenyl methyl silicone internal phase. Component identification is made by matching the relative retention times of the fragment peaks to that of the same analysis performed with a LECO time of flight mass spectrometer (or optionally by matching mass spectra to a NIST database or references from literature). Membrane samples are weighed into Frontier Labs silica lined stainless steel cups using a Mettler E20 micro-balance capable of measuring to 0.001 mg. Sample weight targets were 200 ug +/−50 ug. Gas chromatograph conditions are as follows: Agilent 6890 GC (SN: CN10605069), with a 30 M×0.25 mm, 1 μm 5% dimethyl polysiloxane phase (Varian FactorFour VF-5MS CP8946); injection port 320° C., Detector port: 320° C., Split injector flow ratio of 50:1, GC Oven conditions: 40° C. to 100° C. at 6° C. per min , 100° C. to 320° C. at 30° C./min, 320° C. for 8 min; Helium carrier gas with constant flow of 0.6 mL/min providing a back pressure of 5.0 psi. LECO TruTOF Mass Spectrometer Parameters are as follows: electron ionization source (positive EI mode), Scan Rate of 20 scans per second, Scan range: 14-400 m/z; Detector voltage=3200 (400V above tune voltage); MS acquisition delay=1 min; Emission Voltage—70V. The peak area of the fragment 212 m/z and fragment 237 m/z are normalized to the sample weight. The normalized peak areas are used to determine the ratio of fragments 212 m/z to 237 m/z. Further the normalize peak area of fragment 212 m/z is divided by the sum of the normalized peak areas for all other fragments providing a fraction of the m/z 212 fragment relative to the polyamide and is commonly noted as a percent composition by multiplying by 100. Preferably this value is less than 12%.
In another preferred embodiment, the thin film polyamide layer is characterized by having an ATR IR carboxylic acid carbonyl stretch/amide carbonyl stretch (here in referred to as COOH/amide) of less than 0.1 and more preferably 0.08. The measurement is believed to be indicative of polymer structures that provide improved flux, salt passage or integrity (particularly for membranes having relatively high carboxylic acid content). A preferred ATR IR methodology is conducted by first isolating the polyamide layer from the porous support by first delaminating from the backing sheet. The delaminated membrane is immersed in a solvent suitable to dissolve the porous support (e.g. dimethyl formamide). After dissolving the porous support, the insoluble polyamide is collected by filtration, washed 2 times dimethylformamide, 2 times with DI water, and 2 times with methanol then dried in a vacuum oven at 50 ° C. for 20 hours. Infrared spectra of delaminated polyamide layer was acquired with a Perkin Elmer Spectrum One FT-IR and Universal ATR Sampling Accessory at a nominal resolution of 4 cm−1 and 16 scans (approximate acquisition time of 90 seconds). The Universal ATR Sampling Accessory was equipped with a single bounce diamond/ZnSe crystal. The carboxylic acid peak height was measured at 1706 cm− with a single-point baseline at 1765 cm−1. The amide peak height was measured at 1656 cm− with a single-point baseline at 1765 cm−.
In yet another preferred embodiment, the subject membranes have an isoelectric point (IEP) of less than or equal to 4.3, 4.2, 4.1, 4, 3.8, 3.6 or in some embodiments 3.5, as measured using a standard zeta-potential measurement method as described below.
In another preferred embodiment, the subject membranes have improved flux as compared with comparable membranes prepared without the subject acyl halide reactants, e.g. when tested with an aqueous salt solution (2000 ppm NaCl) at 150 psi, 25° C. and pH 8. In another embodiment, the subject membranes also maintain comparable NaCl passage values, e.g. less than 1% when tested using an aqueous solution of 2000ppm NaCl at 150 psi, 25° C. and pH 8. In another embodiment, the subject membranes have a NaCl passage value less than 5% when tested such conditions.
In another preferred embodiment, the subject composite membrane is not subject to post-treatment with a chlorinating agent.
While post application coatings may be used in combination with the present invention, in many preferred embodiments, no such coating is used, i.e. no coating of polyacrylic acid or polyvinyl acetate.
Sample membranes were prepared using a pilot scale membrane manufacturing line. Polysulfone supports were casts from 16.5 wt % solutions in dimethylformamide (DMF) and subsequently soaked in an aqueous solution of 3.5 wt % meta-phenylene diamine (mPD). The resulting support was then pulled through a reaction table at constant speed while a thin, uniform layer of a non-polar coating solution was applied. The non-polar coating solution included a isoparaffinic solvent (ISOPAR L), a combination of trimesoyl acid chloride (TMC) and 1-carboxy-3,5-dichloroformyl benzene (mhTMC) in varying ratios while maintaining the total acid chloride content at 0.26 wt %, and tributyl phosphate (TBP) in a constant stoichiometric molar ratio of 1.1:1 with TMC. Excess non-polar solution was removed and the resulting composite membrane was passed through water rinse tanks and drying ovens. Coupons of the sample membranes were then subject to standard testing using an aqueous salt solution (2000 ppm NaCl) at 150 psi, 25° C. and pH 8. The dissociated carboxylate content, isoelectric point (IEP), dimer fragment ratio (212 m/z:237 m/z) and carboxylic acid/amide (COOH:Amide) ratios for the sample membranes were also determined (as per the methodologies described herein). The results are summarized in Table 1. A second set of similar membranes were prepared without TBP and the results are summarized in Table 2. Standard deviations for flux and NaCl passage are provided in parenthesis. A third set of membranes were prepared in a manner the same as the first set with the exception that 1-carboxy-3,5-dichloroformyl benzene (mhTMC) was replaced with 3-(chlorocarboynyl)benzoic acid, i.e. “mono hydrolyzed isophthaloyl chloride” or “mhIPC”. The dissociated carboxylate content and dimer fragment ratio (212 m/z : 237 m/z) ratios for the sample membranes were also determined (as per the methodologies described herein). The results are summarized in Table 3.
The “dissociated carboxylate content” of the polyamide layer of the sample membranes was determined by silver titration and a Rutherford Backscattering measurement as described below.
(i) Samples membranes (1 inch×6 inch) were boiled for 30 minutes in deionized water (800 mL), then placed in a 50/50 w/w solution of methanol and water (800 mL) to soak overnight. Next, 1 inch×1 inch size sample of these membranes were immersed in a 20 mL 1×10−4 M AgNO3 solution with pH adjusted to 9.5 for 30 minutes. Vessels containing silver ions were wrapped in tape and to limit light exposure. After soaking with the silver ion solution, the unbound silver was removed by soaking the membranes in 2 clean 20 mL aliquots of dry methanol for 5 minutes each. Finally, the membranes were allowed to dry in a nitrogen atmosphere for a minimum of 30 minutes.
(ii) Rutherford Backscattering (RBS) Measurement: Membrane samples were mounted on a thermally and electrically conductive double sided tape, which was in turn mounted to a silicon wafer acting as a heat sink The tape used was Chromerics Thermattach T410 or a 3M copper tape. RBS measurements were obtained with a Van de Graff accelerator (High Voltage Engineering Corp., Burlington, Mass.); A 2 MeV He room temperature beam with a diameter of 3 mm was used at an incident angle of 22.5°, exit angle of 52.5°, scattering angle of 150°, and 40 nanoamps (nAmps) beam current. Membrane samples were mounted onto a movable sample stage which is continually moved during measurements. This movement allows ion fluence to remain under 3×1014 He+/cm2.
(iii) Data Analysis: Analysis of the spectra obtained from RBS was carried out using SIMNRA®, a commercially available simulation program. A description of its use to derive the elemental composition from RBS analysis of RO/NF membranes has been described by.; Coronell, et. al. J. of Membrane Sci. 2006, 282, 71-81 and Environmental Science & Technology 2008, 42(14), 5260-5266. Data in tables were obtained using the SIMNRA® simulation program to fit a two layer system, a thick polysulfone layer beneath a thin polyamide layer, and fitting a three-layer system (polysulfone, polyamide, and surface coating) can use the same approach. The atom fraction composition of the two layers (polysulfone before adding the polyamide layer, and the surface of final TFC polyamide layer) was measured first by XPS to provide bounds to the fit values. As XPS cannot measure hydrogen, an H/C ratio from the proposed molecular formulas of the polymers were used, 0.667 for polysulfone and a range of 0.60-0.67 was used for polyamide. Although the polyamides titrated with silver nitrate only introduces a small amount of silver, the scattering cross section for silver is substantially higher than the other low atomic number elements (C, H, N, O, S) and the size of the peak is disproportionately large to the others despite being present at much lower concentration thus providing good sensitivity. The concentration of silver was determined using the two layer modeling approach in SIMNRA® by fixing the composition of the polysulfone and fitting the silver peak while maintaining a narrow window of composition for the polyamide layer (layer 2, ranges predetermined using XPS). From the simulation, a molar concentration for the elements in the polyamide layer (carbon, hydrogen, nitrogen, oxygen and silver) was determined The silver concentration is a direct reflection of the carboxylate molar concentration available for binding silver at the pH of the testing conditions.
The moles of carboxylic acids groups per unit area of membrane is indicative of the number of interactions seen by a species passing through the membrane, and a larger number will thus favorably impact salt passage. This value may be calculated by multiplying the measured carboxylate content by a measured thickness and by the polyamide density. Alternatively, the carboxylate number per unit area of membrane (moles/m2) may be determined more directly by methods that measure the total complexed metal within a known area. Approaches using both Uranyl acetate and toluidine blue O dye are described in: Tiraferri, et. al., Journal of Membrane Science, 2012, 389, 499-508. An approach to determine the complexed cation (sodium or potassium) content in membranes by polymer ashing is described in (Wei Xie, et al., Polymer, Volume 53, Issue 7, 22 March 2012, Pages 1581-1592).
A preferred method to determine the dissocated carboxylate number at pH 9.5 per unit area of membrane for a thin film polyamide membrane is as follows. A membrane sample is boiled for 30 minutes in deionized water, then placed in a 50 wt % solution of methanol in water to soak overnight. Next, the membrane sample is immersed in a 1−10−4 M AgNO3 solution with pH adjusted to 9.5 with NaOH for 30 minutes. After soaking in the silver ion solution, the unbound silver is removed by soaking the membranes twice in dry methanol for 30 minutes. The amount of silver per unit area is preferably determined by ashing, as described by Wei, and redissolving for measurement by ICP. Preferably, the dissocated carboxylate number at pH 9.5 per square meter of membrane is greater than 6×10−5, 8×10−5, 1×10−4, 1.2×10−4, 1.5×10−4, 2×10−4, or even 3×10−4 moles/m2.
The isoelectric point for the sample membranes was determined using a standard Zeta-Potential technique with a quartz cell by electrophoretic light scattering (ELS) using Desal Nano HS instrument. Membrane samples (2 inch×1 inch) were first boiled for 20 minutes in DI water, then rinsed well with room temperature DI water and stored at room temperature in a fresh DI solution overnight. The samples were then loading as per reference: 2008 “User's Manual for the Delsa™ Nano Submicron Particle Size and Zeta Potential,” and the “Pre-Course Reading” for the same instrument presented by Beckmann Coulter. The pH titration was done over a range from pH 10 to pH 2 and isoelectric point was determined at the pH where the zeta potential became zero.
Many embodiments of the invention have been described and in some instances certain embodiments, selections, ranges, constituents, or other features have been characterized as being “preferred.” Characterizations of “preferred” features should in no way be interpreted as deeming such features as being required, essential or critical to the invention. The entire subject matter of each of the aforementioned US patent documents is incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2013/049182 | 7/3/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61673462 | Jul 2012 | US | |
| 61673467 | Jul 2012 | US | |
| 61673466 | Jul 2012 | US | |
| 61673456 | Jul 2012 | US | |
| 61673453 | Jul 2012 | US | |
| 61674634 | Jul 2012 | US | |
| 61675412 | Jul 2012 | US | |
| 61775814 | Mar 2013 | US | |
| 61775777 | Mar 2013 | US |
