Cold storage medium

BACKGROUND
1. Technical Field

The present invention relates to a cold storage medium.

2. Description of the Related Art

Patent Literature 1 discloses a heat storage device and a heat storage method. In more detail, in the storage device disclosed in Patent Literature 1, overcooling of a heat storage material capable of forming a clathrate hydrate by cooling can be released with a smaller amount of energy.

CITATION LIST
Patent Literature

Patent Literature 1

Japanese Patent Laid-open Publication No. 2017-003182A

Non-Patent Literature

Non-Patent Literature 1: Seungmin Lee et. al., “Phase Equilibria of Semiclathrate Hydrate for Nitrogen in the Presence of Tetra-n-butylammonium Bromide and Fluoride”, Journal of Chemical & Engineering Data, Vol. 55, No. 12, 2010, pp 5883-5886

Non-Patent Literature 2: Kai Sato. Hiroki Tokutomi, Ryo Ohmura, “Phase equilibrium of ionic semiclathrate hydrates formed with tetrabutylammonium bromide and tetrabutylammonium chloride”, Fluid Phase Equilibria 337 (2013) 115-118

SUMMARY

An object of the present invention is to provide a cold storage medium which satisfies the following two requirements (I) and (II).

Requirement (I): In order to use suitably for a refrigerator, the cold storage medium has a melting point of not less than 3 degrees Celsius and not more than 10 degrees Celsius.

Requirement (II): The cold storage medium has a small supercooling degree ΔT.

The cold storage medium according to the present invention contains:

tetra-n-butylammonium ions;

silver ions;

at least one kind of halide ions selected from the group consisting of fluoride ions, chloride ions, and bromide ions;

water; and

at least one kind of carboxylic acid ions each represented by the following chemical formula (I),

R—COO⁻ (I)

- where R represents a hydrocarbon group.

The present invention provides a cold storage medium which satisfies the following two requirements (I) and (II).

Requirement (I): In order to use suitably for a refrigerator, the cold storage medium has a melting point of not less than 3 degrees Celsius and not more than 10 degrees Celsius.

Requirement (II): The cold storage medium has a small supercooling degree ΔT.

The cold storage medium according to the present invention can be used suitably for a refrigerator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a characteristic of a cold storage medium during a period of cooling.

FIG. 2 is a graph showing a characteristic of the cold storage medium during a period of warming.

DETAILED DESCRIPTION OF THE EMBODIMENT

Hereinafter, the embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a graph showing a characteristic of a cold storage medium during a period of cooling. In FIG. 1, a horizontal axis and a vertical axis indicate time and temperature, respectively.

The cold storage medium according to the present embodiment is cooled. See a section A included in FIG. 1. Unlike a case of a common liquid, as well known in the technical field of the cold storage medium, even when the temperature of the cold storage medium reaches the melting point thereof due to cooling of the cold storage medium, the cold storage medium is not be solidified but is in a supercooling state. See a section B included in FIG. 1. In the supercooling state, the cold storage medium is liquid.

Then, the cold storage medium begins to crystallize spontaneously. With the crystallization, the cold storage medium releases crystallization heat which is substantially equal to its latent heat. As a result, the temperature of the cold storage medium begins to be raised. See a section C included in FIG. 1. In the instant specification, the temperature at which the cold storage medium begins to crystallize spontaneously is referred to as “crystallization temperature”.

ΔT represents difference between the melting point and the crystallization temperature of the cold storage medium. ΔT may be referred to as “supercooling degree”. Due to the crystallization of the cold storage medium in the supercooling state, the cold storage medium becomes a semiclathrate hydrate (for example, see Patent Literature 1). A cage-shaped crystal is formed by hydrogen-bonding, and then, a material other than water is held in the cage-shaped crystal to form a crystal, which is referred to as a clathrate hydrate. A guest molecule joins in a hydrogen-bonding network of water molecules to form a crystal, which is referred to as a semiclathrate hydrate. A concentration at which hydrates are formed with water molecules and guest molecules without deficiency and excess is referred to as a congruent concentration. Generally, hydrates are often used near the congruent concentration.

After the release of the crystallization heat of the cold storage medium was completed with the completion of the crystallization, the temperature of the cold storage medium gradually falls so as to be equal to ambient temperature. See a section D included in FIG. 1.

The crystallization temperature is lower than the melting point of the cold storage medium. The melting point of the cold storage medium can be measured with a differential scanning calorimeter (which may be referred to as “DSC”), as well known in the technical field of the cold storage medium.

FIG. 2 is a graph showing a characteristic of the cold storage medium during a period of warming. In FIG. 2, a horizontal axis and a vertical axis indicate time and temperature, respectively. During a section E, the temperature of the cold storage medium is maintained at a temperature which is equal to or lower than the crystallization temperature. For example, while a door of a refrigerator is closed, the temperature in the refrigerator is maintained at a temperature which is equal to or lower than the crystallization temperature in such a manner that the temperature of the cold storage medium disposed in the refrigerator is maintained at a temperature which is equal to or lower than the crystallization temperature.

Then, the cold storage medium is gradually warmed. See a section F included in FIG. 2. For example, after the door of the refrigerator is opened (to put food into the refrigerator) at the end of the section E (namely, at the beginning of the section F), the temperature in the refrigerator is raised gradually.

After the temperature of the cold storage medium reaches the melting point of the cold storage medium, the temperature of the cold storage medium is maintained near the melting point of the cold storage medium. See a section G included in FIG. 2. In case where the cold storage medium is absent, the temperature in the refrigerator rises continuously as shown in a section Z included in FIG. 2. On the other hand, if the cold storage medium is present, the temperature in the refrigerator is maintained near the melting point of the cold storage medium during a certain period of the section G. In this way, the cold storage medium exerts its cold storage effect. At the end of the section G, the crystal of the cold storage medium melts to disappear. As a result, the cold storage medium is liquefied.

Afterwards, the temperature of the liquefied cold storage medium rises to be equal to ambient temperature. See a section H included in FIG. 2.

The cold storage medium is cooled to be reused. For example, after the door of the refrigerator is closed, as shown in the section A included in FIG. 1, the cold storage medium is cooled again to be reused.

The following two requirements (I) and (II) must be satisfied for a cold storage medium used suitably for a refrigerator.

Requirement (I): The cold storage medium has a melting point of not less than 3 degrees Celsius and not more than 10 degrees Celsius. As one example, the cold storage medium has a melting point of not less than 3 degrees Celsius and not more than 7 degrees Celsius.

Requirement (II): The cold storage medium has a small supercooling degree ΔT.

The reason for the requirement (I) is that the temperature in the refrigerator should be maintained at a temperature of not less than approximately 3 degrees Celsius and not more than 10 degrees Celsius. In other words, if temperature in a cooler is maintained at a temperature of less than 0 degrees Celsius, such a cooler is not a “refrigerator” but a “freezer”. On the other hand, from a viewpoint of food preservation, if temperature in a cooler is maintained at a temperature of more than 10 degrees Celsius, such a cooler would be practically meaningless as a refrigerator.

The reason for the requirement (II) will be described below. The crystallization temperature is lowered with an increase in the supercooling degree ΔT. For this reason, excessive cooling is necessary to crystallize the cold storage medium in the supercooling state. As one example, crystallization temperature of ice (i.e., pure water) used as a cold storage medium is approximately minus 12 degrees Celsius. In other words, the melting point of the ice used as a cold storage medium is 0 degrees Celsius and its supercooling degree ΔT is equal to 12 degrees Kelvin. For this reason, a cooler needs an ability to cool the inside thereof to minus 12 degrees Celsius or less (e.g., to minus 18 degrees Celsius), if the ice is used as a cold storage medium. This increases an electrical power consumed by the refrigerator. Needless to say, it is desirable that power consumption is small. Therefore, the supercooling degree ΔT of the cold storage medium is required to be small.

To avoid confusing, “Kelvin” is used for the supercooling degree ΔT in the instant specification. For example, the present inventors describe “the supercooling degreeΔT is not more than n degree(s) Kelvin”. Needless to say n is a real number. The description “the supercooling degree ΔT≤5 degrees Kelvin” means that the difference between the crystallization temperature and the melting point of the cooling storage medium is not more than 5 degrees Kelvin. On the other hand, in the present specification, “Celsius” is used for temperature. For example, the present inventors describe “the crystallization temperature is 5 degrees Celsius (namely, 5° C.).”.

The cold storage medium according to the present embodiment contains (i) tetra-n-butylammonium ions, (ii) silver ions, (iii) at least one kind of halide ions selected from the group consisting of fluoride ions, chloride ions, and bromide ions, (iv) water, and (v) at least one kind of carboxylic acid ions each represented by the following chemical formula (I) to satisfy the above-mentioned two requirements (I) and (II).

R—COO⁻ (I)

- where R represents a hydrocarbon group.

(Tetra-n-Butylammonium Ion)

A tetra-n-butylammonium ion is represented by the chemical formula ((CH₃(CH₂)₃)₄N⁺). As well known in the technical field of the cold storage medium, the tetra-n-butylammonium ion is used as a main component of the cold storage medium, namely, as a cold storage main component. It is desirable that a molar ratio between the tetra-n-butylammonium ions and the water is from 1:20 to 1:40. In other words, it is desirable that a molar ratio of the tetra-n-butylammonium ions to the water is not less than 1/40 and not more than 1/20.

As is clear from a fabrication method of the cold storage medium which will be described later, tetra-n-butylammonium ions may be derived from a tetra-n-butylammonium carboxylate, a tetra-n-butylammonium fluoride, a tetra-n-butylammonium chloride, or a tetra-n-butylammonium bromide.

(Carboxylic Acid Ion)

The carboxylic acid ion is represented by the following chemical formula (II).

R—COO⁻ (II)

- where R is a hydrocarbon group.

Desirably, the carboxylic acid ion is a monocarboxylic acid ion.

Desirably, the carboxylic acid ion has a carbon number not less than 1 and not more than 10. More desirably, the carboxylic acid ion has a carbon number of not less than 1 and not more than 7. The carbon number of the carboxylic acid ion may be not less than 2.

R may be a linear hydrocarbon group. For example, the carboxylic acid ion is a 1-pentanoic acid ion represented by chemical formula ⁻OCO(CH₂)₃CH₃or a 1-heptanoic acid ion represented by the chemical formula ⁻OCO(CH₂)CH₃.

Alternatively, R may be a hydrocarbon group having a branched chain. For example, the carboxylic acid ion is a 2-methylbutanoic acid ion represented by the chemical formula ⁻OCOCH(CH₃)CH₂CH₃or a 2-ethylbutanoic acid ion represented by the chemical formula ⁻OCOCH(CH₂CH₃)CH₂CH₃. Another example of the carboxylic acid ion is a methylpentanoic acid ion, an ethylpentanoic acid ion, a propylpentanoic acid ion, a methylhexanoic acid ion, an ethylhexanoic acid ion, a propylhexanoic acid ion, or a butylhexanoic acid ion

The cold storage medium according to the present embodiment may contain two or more kinds of the carboxylic acid ions. The combination of the two or more kinds of the carboxylic acid ions is not limited. An example of the combination of two kinds of the carboxylic acid ions is:

(i) 1-pentanoic acid ions and 2-ethylbutanoic acid ions,

(ii) 1-heptanoic acid ions and 2-ethylbutanoic acid ions, or

(iii) 2-methylbutanoic acid ions and 2-ethylbutanoic acid ions.

Alternatively, the cold storage medium according to the present embodiment may contain one kind of the carboxylic acid ion. It is desirable that a molar ratio between the carboxylic acid ions and the water is from 1:20 to 1:40. In other words, it is desirable that the molar ratio of the carboxylic acid ions to the water is not less than 1/40 and not more than 1/20.

As is clear from a fabrication method of the cold storage medium which will be described later, the carboxylic acid ions may be derived from tetra-n-butylammonium carboxylate or silver carboxylate.

(Silver Ion and Halide Ion)

The cold storage medium according to the present embodiment contains silver ions and at least one kind of halide ions selected from the group consisting of fluoride ions, chloride ions, and bromide ions. As is clear from the fabrication method of the cold storage medium which will be described later, the silver ions may be derived from the silver carboxylate. Similarly, the halide ions may be derived from the tetra-n-butylammonium halide.

The term “halide ions” used in the present specification includes fluoride ions, chloride ions, and bromide ions. However, the term “halide ions” used in the present specification does not include iodide ions. Similarly, the term “halide” used in the present specification includes fluoride, chloride, and bromide. However, the “halide” used in the present specification does not include iodide.

On the condition that the cold storage medium according to the present embodiment contains the tetrabutylammonium ions, the cold storage medium according to the present embodiment is characterized by containing three components of the silver ions, the at least one kind of halide ions selected from the group consisting of fluoride ions, chloride ions, and bromide ions, and the carboxylic acid ions.

As is demonstrated in the inventive examples which will be described later, the cold storage medium according to the present embodiment has a melting point of not less than 3 degrees Celsius and not more than 10 degrees Celsius (desirably, not less than 4 degrees Celsius and not more than 7 degrees Celsius) and a small supercooling degree ΔT of not more than 5 degrees Kelvin (desirably, not more than 2 degrees Kelvin), since the cold storage medium according to the present embodiment contains the tetrabutylammonium ions as the cold storage main component and further contains three components of the silver ions, the at least one kind of halide ions selected from the group consisting of fluoride ions, chloride ions, and bromide ions, and the carboxylic acid ions. For this reason, the cold storage medium according to the present embodiment can be used suitably for a refrigerator.

In case where the cold storage medium does not contain silver ions, the cold storage medium fails to crystallize, as is clear from the comparative examples F1-F11, the comparative examples CI1-CI11, and the comparative examples Br1-Br3. For this reason, the cold storage effect is not exerted.

In case where the cold storage medium does not contain the at least one kind of halide ions selected from the group consisting of fluoride ions, chloride ions, and bromide ions, the cold storage medium fails to crystallize, as is clear from the comparative example I. For this reason, the cold storage effect is not exerted.

In case where the cold storage medium does not contain the carboxylic acid ions, the cold storage medium fails to crystallize, as is clear from the comparative examples X1 and X2. For this reason, the cold storage effect is not exerted.

The cold storage medium according to the present embodiment contains tetrabutylammonium ions as the cold storage main component.

As one example, a molar ratio between the silver ions and the halide ions is from 1:5 to 1:20. In other words, a molar ratio of the halide ions to the silver ions is not less than 5 and not more than 20. Desirably, the molar ratio is not less than 7.5 and not more than 15.

(Mechanism of Crystallization)

The present inventors do not like to be bound to a theory; however, the mechanism of the present invention which the present inventors believe will be described below.

The silver ions contained in the cold storage medium according to the present embodiment forms a metallic silver core due to Brownian movement or intermolecular interaction. Then, the metallic silver core is integrated with the silver carboxylate. In this way, a silver nanoparticle which serves as a microcation is formed.

A halide ion contained in the cold storage medium is adsorbed on the silver ion included in the silver carboxylate disposed on the surface of the microcation. Furthermore, a tetra-n-butylammonium ion is adsorbed on the halide ion.

The halide ions are drawn to the surface of the silver nanoparticle which serves as the microcation. Tetra-n-butylammonium ions which serve as counter cations of the halide ions are drawn electrically to the halide ions. A plurality of the halide ions and the tetra-n-butylammonium ions which have been dispersed in the cold storage medium come close to each other. As a result, increased is the probability that molecules of these ions are disposed suitably for formation of hydrate composed of the tetra-n-butylammonium ions and the halide ions. The melting point of the hydrate composed of the tetra-n-butylammonium ions and the halide ions is approximately 15 degrees Celsius. See Non-Patent Literature 1. For this reason, when the cold storage medium is disposed in a refrigerator having an inside temperature of not more than 10 degrees Celsius, the cold storage medium crystallizes in a rapid manner, since the cold storage medium has a supercooling degree ΔT of not less than 5 Kelvin. Furthermore, the hydrate composed of the tetra-n-butylammonium ions and the halide ions (hereinafter, the hydrate is referred to as “TX hydrate”) has a crystallization structure similar to that of a hydrate composed of tetra-n-butylammonium ions and 2-ethylbutanoic acid ions (hereinafter, the hydrate is referred to as “TE hydrate”). For this reason, when the TX hydrate is formed, the crystallization of the TE hydrate proceeds in a rapid manner using the TX hydrate as a seed crystal.

EXAMPLES

Hereinafter, the present invention will be described in more detail with reference to the following examples.

Inventive Example F1

(Synthesis of TBA-2-EBA)

Tetra-n-butylammonium-2-ethylbutylate (hereinafter, referred to as “TBA-2-EBA”) represented by the following chemical formula (F-I) was synthesized as below.

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First, 2-ethylbutanoic acid (chemical formula: HOOC—CH(CH₂CH₃)CH₂CH₃, 0.1 mol, purchased from Tokyo Chemical Industry Co. Ltd.) was added to a tetra-n-butylammonium hydroxide aqueous solution (0.1 mol, purchased from Tokyo Chemical Industry Co. Ltd., 40% aqueous solution, hereinafter referred to as “TBA-OH”) represented by the following chemical formula (F-II). In this way, an aqueous solution was provided.

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The provided aqueous solution was dried with an evaporator under a reduced pressure at a temperature of 40 degrees Celsius. In this way, the provided aqueous solution was dehydrated to provide TBA-2-EBA. In this way, TBA-2-EBA (0.1 mol) was provided.

Then, the provided TBA-2-EBA (0.1 mol) was dissolved in pure water (3.85 mol) to provide a TBA-2-EBA aqueous solution (34% by weight).

(Synthesis of Silver Pentanoate)

Silver pentanoate represented by the following chemical formula (F-III) was synthesized as below.

[Chem. 3]
Ag⁺⁻OOC—(CH₂)₃CH₃ (F-III)

In a darkroom, silver nitrate (0.11 mol, purchased from Wako Pure Chemical Industries, Ltd.) and pentanoic acid (0.10 mol, purchased from Tokyo Chemical Industry Co. Ltd.) were added to water to provide an aqueous solution in which the silver nitrate and the pentanoic acid were mixed with each other. Then, while the aqueous solution was stirred, the aqueous solution was neutralized with a sodium hydroxide aqueous solution (0.09 mol) having a concentration of 1 mol/liter to provide silver pentanoate represented by the chemical formula (F-III) as a precipitation. The provided precipitation (i.e., the silver pentanoate) was filtered with a filter. The residue was washed with distilled water and subsequently was dried. In this way, the silver pentanoate (0.10 mol) was provided.

(Purchase of TBA-F)

Tetra-n-butylammonium fluoride aqueous solution (0.287 mol, 75% aqueous solution, hereinafter, referred to as “TBA-F”) represented by the following chemical formula (F-IV) was purchased from Sigma Aldrich.

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(Preparation of Cold Storage Medium)

The cold storage medium according to the inventive example F1 was prepared as below. First, the silver pentanoate (2.0 micromol) was dissolved in the TBA-F aqueous solution (20 micromol) to provide a mixture. Note that silver carboxylate such as silver pentanoate is insoluble in both a hydrophilic solvent (e.g., water) and a hydrophobic solvent (e.g., oil); however, the silver carboxylate is soluble in a quaternary ammonium salt aqueous solution.

Then, the mixture was added to the TBA-2-EBA aqueous solution (TBA-2-EBA: 476 micromol). Just after the addition, the aqueous solution got cloudy; however, after several hours, the aqueous solution became transparent. Precipitation was slightly observed in the aqueous solution. The aqueous solution was left at a room temperature over 24 hours. In this way, the cold storage medium according to the inventive example F1 was prepared. As is clear from above, the cold storage medium according to the inventive example F1 has a composition shown in the following Table 1.

TABLE 1

Name of Material
Amount

TBA quaternary ammonium ions
496
micromol

TBA-2-EBA ions
476
micromol

Pentanoic acid ions
2
micromol

Silver ions
2
micromol

Fluoride ions
20
micromol

Water (derived from
Approximately

TBA-2-EBA aqueous solution)
18,300 micromol

(Measurement of Melting Point and Latent Heat Amount)

The melting point and the latent heat amount of the cold storage medium according to the inventive example F1 were measured as below with a differential scanning calorimeter. First, the cold storage medium according to the inventive example F1 (10 milligrams) was put into a container formed of aluminum. Then, the container was sealed with a lid.

The container was set in a differential scanning calorimeter (purchased from PerkinElmer, trade name: DSC-8500). The cold storage medium put in the container was cooled from a room temperature to minus 20 degrees Celsius at a rate of 2 degrees Celsius/minute. Then, the cold storage medium was left at rest at a temperature of minus 20 degrees Celsius for five minutes. In this way, the present inventors believed that the cold storage medium crystallized to be a semiclathrate hydrate. Finally, the cold storage medium was warmed from minus 20 degrees Celsius to a room temperature at a rate of 2 degrees Celsius/minute. In this way, the crystallized cold storage medium was melted.

On the basis of an endothermic peak output from the differential scanning calorimeter in a period during which the cold storage medium (namely, the semiclathrate hydrate cold storage medium) was melted, the melting point of the cold storage medium according to the inventive example F1 was identified and the latent heat amount of the cold storage medium according to the inventive example F1 was calculated. As a result, the melting point of the cold storage medium according to the inventive example F1 was 6 degrees Celsius. The latent heat amount of the melting point of the cold storage medium according to the inventive example F1 was 178 joules/gram.

(Measurement of Supercooling Degree ΔT)

The supercooling degree ΔT of the cold storage medium according to the inventive example F1 was measured as below. First, the cold storage medium according to the inventive example F1 (0.5 grams) was put into a glass bottle having a volume of 9 milliliters. Then, the glass bottle was sealed with a lid.

The glass bottle was put into a thermostat bath (purchased from ESPEC Corp., trade name: SU-241). The temperature in the thermostat bath was 20 degrees Celsius.

After the temperature in the thermostat bath was lowered to 5 degrees Celsius at a rate of minus 1 degree Celsius/minute, the temperature in the thermostat bath was maintained at 5 degrees Celsius.

When six minutes elapsed from the beginning of the maintenance of the temperature in the thermostat bath at 5 degrees Celsius, the cold storage medium according to the inventive example F1 crystallized spontaneously. With the crystallization, the cold storage medium according to the inventive example F1 released crystallization heat. Due to the release of the crystallization heat, the temperature of the cold storage medium according to the inventive example F1 was raised. Finally, the present inventors visually confirmed the completion of the crystallization of the cold storage medium according to the inventive example F1. The period between the beginning and the completion of the crystallization was 36 minutes.

As described above, since the cold storage medium according to the inventive example F1 crystallized spontaneously at a temperature of 5 degrees Celsius, the cold storage medium according to the inventive example F1 had a supercooling degree ΔT of 1 Kelvin (=(the melting point of 6 degrees Celsius)−(the crystallization point of 5 degrees Celsius).

Inventive Example F2

In the inventive example F2, an experiment similar to the inventive example F1 was performed except that silver heptanoate represented by the following chemical formula (F-V) was used in place of the silver pentanoate.

[Chem. 5]
Ag⁺⁻OOC—(CH₂)₅CH₃ (F-V)

(Synthesis of Silver Heptanoate)

The silver heptanoate was synthesized as below. In the darkroom, silver nitrate (0.11 mol, purchased from Wako Pure Chemical Industries, Ltd.) and heptanoic acid (0.10 mol, purchased from Tokyo Chemical Industry Co. Ltd.) were added to water to provide an aqueous solution in which the silver nitrate and the heptanoic acid were mixed. Then, while the aqueous solution was stirred, the aqueous solution was neutralized with a sodium hydroxide aqueous solution (0.09 mol) having a concentration of 1 mol/liter to provide silver heptanoate represented by the chemical formula (F-V) as a precipitation. The provided precipitation (i.e., the silver heptanoate) was filtered with a filter. The residue was washed with distilled water and subsequently was dried. In this way, the silver heptanoate (0.10 mol) was provided.

Inventive Example F3

In the inventive example F3, an experiment similar to the inventive example F1 was performed except that silver 2-methylbutanoate represented by the following chemical formula (F-VI) (hereinafter, referred to as “2-MBA silver salt”) was used in place of the silver pentanoate.

[Chem. 6]
Ag⁺⁻OOC—CH(CH₃)CH₂CH₃ (F-VI)

(Synthesis of 2-MBA Silver Salt)

The 2-MBA silver salt was synthesized as below. In the darkroom, silver nitrate (0.11 mol, purchased from Wako Pure Chemical Industries, Ltd.) and 2-methylbutanoic acid (0.1 mol, purchased from Tokyo Chemical Industry Co. Ltd.) were added to water to provide an aqueous solution in which the silver nitrate and the 2-methylbutanoic acid were mixed. Then, while the aqueous solution was stirred, the aqueous solution was neutralized with a sodium hydroxide aqueous solution (0.09 mol) having a concentration of 1 mol/liter to provide a 2-MBA silver salt represented by the chemical formula (F-VI) as a precipitation. The provided precipitation (i.e., the 2-MBA silver salt) was filtered with a filter. The residue was washed with distilled water and subsequently was dried. In this way, the 2-MBA silver salt (0.10 mol) was provided.

Inventive Example F4

In the inventive example F4, an experiment similar to the inventive example F1 was performed except that silver 2-ethylbutanoate represented by the following chemical formula (F-VII) (hereinafter, referred to as “2-EBA silver salt”) was used in place of the silver pentanoate.

[Chem. 7]
Ag⁺⁻OOC—CH(CH₂CH₃)CH₂CH₃ (F-VII)

(Synthesis of 2-EBA Silver Salt)

The 2-EBA silver salt was synthesized as below. In the darkroom, silver nitrate (0.11 mol, purchased from Wako Pure Chemical Industries, Ltd.) and 2-ethylbutanoic acid (0.10 mol, purchased from Tokyo Chemical Industry Co. Ltd.) were added to water to provide an aqueous solution in which the silver nitrate and the 2-ethylbutanoic acid were mixed. Then, while the aqueous solution was stirred, the aqueous solution was neutralized with a sodium hydroxide aqueous solution (0.09 mol) having a concentration of 1 mol/liter to provide a 2-EBA silver salt represented by the chemical formula (F-VII) as a precipitation. The provided precipitation (i.e., the 2-EBA silver salt) was filtered with a filter. The residue was washed with distilled water and subsequently was dried. In this way, the 2-EBA silver salt (0.10 mol) was provided.

Note that each of the cold storage media according to the inventive examples F1-F3 contain two kinds of organic anions, whereas the cold storage media according to the inventive example F4 contains one kind of organic anion (namely, 2-methylbutanoic acid anion).

The following Tables 2-3 show the results of the inventive example F1-F4.

TABLE 2

Ag:F
Latent heat
Melting

Molar
amount
point

Composition
ratio
(J/gram)
(° C.)

Inventive
TBA-2-EBA: 476 micromol,
1:10
178
6

example F1
Silver pentanoate: 2 micromol,

TBA-F 20: micromol, and

Water: approximately 18,300 micromol

Inventive
TBA-2-EBA: 476 micromol,
1:10
175
6

example F2
Silver heptanoate: 2 micromol,

TBA-F: 20 micromol, and

Water: approximately 18,300 micromol

Inventive
TBA-2-EBA: 476 micromol,
1:10
180
6

example F3
2-MBA silver salt: 2 micromol,

TBA-F: 20 micromol, and

Water: approximately 18,300 micromol

Inventive
TBA-2-EBA: 476 micromol,
1:10
181
6

example F4
2-EBA silver salt: 2 micromol,

TBA-F: 20 micromol, and

Water: approximately 18,300 micromol

TABLE 3

Period between

the crystallization

Crystallization
start time and

Crystallization
Completion
the crystallization

ΔT
Start time
time
completion time

(Kelvin)
(minute)
(minute)
(minute)

Inventive
1
6
36
30

example

F1

Inventive
1
4
31
27

example

F2

Inventive
1
8
31
23

example

F3

Inventive
1
7
31
24

example

F4

Inventive Examples F5-F8

In the inventive examples F5-F8, experiments similar to the inventive examples F1-F4 were performed respectively, except that the amount of the TBA-F aqueous solution was not 20 micromol but 40 micromol.

The following Tables 4-5 show the results of the inventive examples F5-F8.

TABLE 4

Latent

Ag:F
heat
Melting

Molar
amount
point

Composition
ratio
(J/gram)
(° C.)

Inventive
TBA-2-EBA: 476 micromol,
1:20
174
7

example
Silver pentanoate: 2 micromol,

F5
TBA-F: 40 micromol, and

Water: approximately 18,300

micromol

Inventive
TBA-2-EBA: 476 micromol,
1:20
171
7

example
Silver heptanoate: 2 micromol,

F6
TBA-F: 40 micromol, and

Water: approximately 18,300

micromol

Inventive
TBA-2-EBA: 476 micromol,
1:20
178
7

example
MBA silver salt: 2 micromol,

F7
TBA-F: 40 micromol, and

Water: approximately 18,300

micromol

Inventive
TBA-2-EBA: 476 micromol,
1:20
170
7

example
2-EBA silver salt: 2 micromol,

F8
TBA-F: 40 micromol, and

Water: approximately 18,300

micromol

TABLE 5

Period between

the crystallization

Crystallization
start time and

Crystallization
Completion
the crystallization

ΔT
Start time
time
completion time

(Kelvin)
(minute)
(minute)
(minute)

Inventive
2
6
32
26

example

F5

Inventive
2
10
38
28

example

F6

Inventive
2
5
31
26

example

F7

Inventive
2
3
30
27

example

F8

Inventive Examples F9-F12

In the inventive examples F9-F12, experiments similar to the inventive examples F1-F4 were performed respectively, except that the amount of the organic silver salt (namely, the silver pentanoate, the silver heptanoate, the 2-MBA silver salt, or the 2-EBA silver salt) was not 2 micromol but 1 micromol, and that the amount of the TBA-F was not 20 micromol but 10 micromol.

The following Tables 6-7 show the results of the inventive examples F9-F12.

TABLE 6

Latent

Ag:F
heat
Melting

Molar
amount
point

Composition
ratio
(J/gram)
(° C.)

Inventive
TBA-2-EBA: 476 micromol,
1:10
180
6

example
Silver pentanoate: 1 micromol,

F9
TBA-F: 10 micromol, and

Water: approximately 18,300

micromol

Inventive
TBA-2-EBA: 476 micromol,
1:10
171
6

example
Silver heptanoate: 1 micromol,

F10
TBA-F: 10 micromol, and

Water: approximately 18,300

micromol

Inventive
TBA-2-EBA: 476 micromol,
1:10
170
6

example
2-MBA silver salt: 1 micromol,

F11
TBA-F: 10 micromol, and

Water: approximately 18,300

micromol

Inventive
TBA-2-EBA: 476 micromol,
1:10
175
6

example
2-EBA silver salt: 1 micromol,

F12
TBA-F: 10 micromol, and

Water: approximately 18,300

micromol

TABLE 7

Period between

the crystallization

Crystallization
start time and

Crystallization
Completion
the crystallization

ΔT
Start time
time
completion time

(Kelvin)
(minute)
(minute)
(minute)

Inventive
1
18
100
82

example

F9

Inventive
1
30
120
90

example

F10

Inventive
1
16
110
94

example

F11

Inventive
1
105
Immeasurable
—

example

F12

Inventive Example F13

(Preparation of TBA-2-EBA Aqueous Solution)

The TBA-2-EBA provided in the inventive example F1 (0.1 mol) was dissolved in pure water (3.85 mol). In this way, a TBA-2-EBA aqueous solution (34% by weight) was provided.

(Preparation of Cold Storage Medium)

The cold storage medium according to the inventive example F13 was prepared as below. First, the silver pentanoate (1.2 micromol) was dissolved in the TBA-F aqueous solution (24 micromol) to provide a mixture. Then, the mixture was added to the TBA-2-EBA aqueous solution (1,400 micromol). The aqueous solution was left at a room temperature over 24 hours. In this way, the cold storage medium according to the inventive example F13 was prepared.

(Measurement of Melting Point and Latent Heat Amount)

The melting point and the latent heat amount of the cold storage medium according to the inventive example F13 were measured as below with the differential scanning calorimeter. First, the cold storage medium according to the inventive example F13 (10 milligrams) was put into a container formed of aluminum. Then, the container was sealed with a lid.

The container was set in the differential scanning calorimeter similarly to the case of the inventive example F1. The cold storage medium put in the container was cooled from a room temperature to minus 20 degrees Celsius at a rate of 2 degrees Celsius/minute. Then, the cold storage medium was left at rest at a temperature of minus 20 degrees Celsius for five minutes. In this way, the present inventors believed that the cold storage medium crystallized to be a semiclathrate hydrate. Finally, the cold storage medium was warmed from minus 20 degrees Celsius to a room temperature at a rate of 2 degrees Celsius/minute. In this way, the crystallized cold storage medium was melted. The crystallization beginning time and the crystallization completion time were 1 minute and 23 minutes, respectively. According to the differential scanning calorimeter, the melting point and the heat latent amount of the cold storage medium according to the inventive example F13 were 6 degrees Celsius and 175 joules/gram, respectively. Therefore, the cold storage medium according to the inventive example F13 had a supercooling degree ΔT of 1 degree Kelvin.

Inventive Example F14

In the inventive example F14, an experiment similar to the inventive example F13 was performed, except that the amounts of the silver pentanoate and the TBA-F aqueous solution were 2.4 micromol and 12 micromol, respectively.

Inventive Example F15

In the inventive example F15, an experiment similar to the inventive example F13 was performed, except that the amount of the silver pentanoate was 2.4 micromol.

Inventive Example F16

In the inventive example F16, an experiment similar to the inventive example F13 was performed, except that silver acetate (2.4 micromol) represented by the following chemical formula (F-VIII) was used in place of the silver pentanoate (1.2 micromol) and that the amount of the TBA-F aqueous solution was 12 micromol.

[Chem. 8]
Ag⁺⁻OOC—CH₃ (F-VIII)

(Synthesis of Silver Acetate)

The silver acetate was synthesized as below. In the darkroom, silver nitrate (0.11 mol, purchased from Wako Pure Chemical Industries, Ltd.) and acetic acid (0.10 mol, purchased from Tokyo Chemical Industry Co. Ltd.) were added to water to provide an aqueous solution in which the silver nitrate and the acetic acid were mixed. Then, while the aqueous solution was stirred, the aqueous solution was neutralized with a sodium hydroxide aqueous solution (0.09 mol) having a concentration of 1 mol/liter to provide silver acetate represented by the chemical formula (F-VIII) as a precipitation. The provided precipitation (i.e., the silver acetate) was filtered with a filter. The residue was washed with distilled water and subsequently was dried. In this way, the silver acetate (0.10 mol) was provided.

Inventive Example F17

In the inventive example F17, an experiment similar to the inventive example F13 was performed, except that the silver acetate (2.4 micromol) represented by the chemical formula (F-VIII) was used in place of the silver pentanoate (1.2 micromol).

Inventive Example F18

In the inventive example F18, an experiment similar to the inventive example F13 was performed, except that silver propanoate (2.4 micromol) represented by the following chemical formula (F-IX) was used in place of the silver pentanoate (1.2 micromol) and that the amount of the TBA-F aqueous solution was 12 micromol.

[Chem. 9]
Ag⁺⁻OOC—CH₂CH₃ (F-IX)

(Synthesis of Silver Propanoate)

The silver propanoate was synthesized as below. In the darkroom, silver nitrate (0.11 mol, purchased from Wako Pure Chemical Industries, Ltd.) and propanoic acid (0.10 mol, purchased from Tokyo Chemical Industry Co. Ltd.) were added to water to provide an aqueous solution in which the silver nitrate and the propanoic acid were mixed. Then, while the aqueous solution was stirred, the aqueous solution was neutralized with a sodium hydroxide aqueous solution (0.09 mol) having a concentration of 1 mol/liter to provide silver propanoate represented by the chemical formula (F-IX) as a precipitation. The provided precipitation (i.e., the silver propanoate) was filtered with a filter. The residue was washed with distilled water and subsequently was dried. In this way, the silver propanoate (0.10 mol) was provided.

Inventive Example F19

In the inventive example F19, an experiment similar to the inventive example F13 was performed, except that the silver propanoate (2.4 micromol) represented by the chemical formula (F-IX) was used in place of the silver pentanoate (1.2 micromol).

The following Tables 8-9 show the results of the inventive examples F13-F19.

TABLE 8

Ag:F
Latent heat
Melting

Molar
amount
point

Composition
ratio
(J/gram)
(° C.)

Inventive
TBA-2-EBA: 1,400 micromol,
1:20
175
6

example F13
Silver pentanoate: 1.2 micromol,

TBA-F: 24 micromol, and

Water: approximately 18,300 micromol

Inventive
TBA-2-EBA: 1,400 micromol,
1:5
175
6

example F14
Silver pentanoate: 2.4 micromol,

TBA-F 12 micromol, and

Water approximately 18,300 micromol

Inventive
TBA-2-EBA: 1,400 micromol,
1:10
180
6

example F15
Silver pentanoate: 2.4 micromol,

TBA-F: 24 micromol, and

Water: approximately 18,300 micromol

Inventive
TBA-2-EBA: 1,400 micromol,
1:5
165
6

example F16
Silver acetate: 2.4 micromol,

TBA-F: 12 micromol, and

Water: approximately 18,300 micromol

Inventive
TBA-2-EBA: 1,400 micromol,
1:10
180
6

example F17
Silver acetate: 2.4 micromol,

TBA-F: 24 micromol, and

Water: approximately 18,300 micromol

Inventive
TBA-2-EBA: 1,400 micromol,
1:5
175
6

example F18
Silver propanoate: 2.4 micromol,

TBA-F: 12 micromol, and

Water: approximately 18,300 micromol

Inventive
TBA-2-EBA: 1,400 micromol,
1:10
173
6

example F19
Silver propanoate: 2.4 micromol,

TBA-F: 24 micromol, and

Water approximately 18,300 micromol

TABLE 9

Period between

the crystallization

Crystallization
start time and

Crystallization
Completion
the crystallization

ΔT
Start time
time
completion time

(Kelvin)
(minute)
(minute)
(minute)

Inventive
1
1
23
22

example

F13

Inventive
1
7
48
41

example

F14

Inventive
1
4
26
22

example

F15

Inventive
1
11
93
82

example

F16

Inventive
1
2
32
30

example

F17

Inventive
1
11
93
82

example

F18

Inventive
1
11
36
25

example

F19

Comparative Example F1

In the comparative example F1, an experiment similar to the inventive example F1 was performed, except that neither the silver pentanoate nor the TBA-F was used. In other words, the cold storage medium according to the comparative example F1 contained TBA-2-EBA only.

Comparative Example F2

In the comparative example F2, an experiment similar to the inventive example F1 was performed, except for the following two matters (I) and (II).

(I) Diacetoxyzirconium(IV) oxide (1.0 micromol, purchased from Tokyo Chemical Industry Co. Ltd.) represented by the chemical formula ZrO(C₂H₃O₂)₂was used in place of the silver pentanoate (2.0 micromol).