Dispersants for metal nanoparticles and dispersions of metal nanoparticles

Abstract

The present invention provides a dispersant for metal nanoparticles that can impart dispersion stability to metal nanoparticles, and a dispersion of metal nanoparticles that exhibits excellent dispersion stability and storage stability. [Solution] A dispersant for metal nanoparticles containing (A) a fatty acid having 16 carbon atoms and a cis double bond at the 9th position, and (B) a fatty acid having 16 carbon atoms and a trans double bond at the 9th position, wherein the content ratio of component (A) to component (B) [(A):(B)] is 99.99:0.01 to 60:40 by mass ratio. Alternatively, a dispersion of metal nanoparticles surface-modified with the dispersant.

Description

Technical Field

[0001] The present invention relates to a dispersant for metal nanoparticles containing a fatty acid composition, and a dispersion of metal nanoparticles surface-modified with the dispersant.

Background Art

[0002] Due to their excellent physical and chemical properties, metal nanoparticles are used in various fields such as catalysts, pharmaceuticals, and electronic materials. However, since metal nanoparticles have a very high surface energy, they tend to aggregate with each other, and ensuring dispersion stability has been a problem. In order to maintain dispersion stability, it has been necessary to modify the surface of metal nanoparticles with an appropriate compound to prevent aggregation. Conventionally, organic amines such as fatty acids and aliphatic amines have been widely used as dispersants for metal nanoparticles. In particular, oleic acid and oleylamine are cited as representative examples.

[0003] Oleic acid is an unsaturated fatty acid with 18 carbon atoms. It easily adsorbs on the surface of metal nanoparticles, and its hydrophobic tail helps the electrostatic repulsion between particles, thereby exerting a dispersion effect. In addition, since oleylamine has an amino group, it easily chemically bonds with metal nanoparticles, and thus it becomes possible to maintain a stable dispersion state. For example, in the method for producing nanoparticles described in Patent Document 1, oleic acid and oleylamine are used for surface modification of nanoparticles of metal compounds such as zinc sulfide. However, when the particle size and particle size distribution of the produced nanoparticles become small, the dispersion stability may not be sufficiently exhibited in the produced nanoparticles. In the dispersion with oleic acid or oleylamine, a sufficient dispersion stabilization effect may not be obtained in a high-concentration metal nanoparticle solution. In particular, the smaller the size of the metal nanoparticles, the easier aggregation occurs, and a significant decrease in dispersion stability is observed. In addition, unsaturated fatty acids such as oleic acid and unsaturated aliphatic amines such as oleylamine are unstable with respect to, for example, light and heat, and are likely to deteriorate under long-term storage or high-temperature environments, so it is difficult to maintain a stable dispersion state.

[0004] Furthermore, Patent Document 2 describes a method for producing an aqueous dispersion of metal nanoparticles by treating metal nanoparticles containing hydrophobic ligands on their surface with a surface modification solution containing a hydrophobic ligand such as a fatty acid such as oleic acid, an aliphatic amine such as oleylamine, a surfactant, a wetting and dispersing agent, and an aqueous solvent, or with a surface modification solution further containing an antioxidant. Furthermore, Patent Document 3 describes a metal particle dispersion containing metal particles coated with an aliphatic carboxylic acid or an aliphatic aldehyde, a dispersant having an aliphatic group and a polar group, and a polar solvent having a polar group at one end of the backbone of a main chain with three or more carbon atoms. However, these technologies may have adverse effects on the surface properties of metal nanoparticles due to the addition of a third component, and the expected improvement in dispersion stability due to the addition of antioxidants or hydrophobic ligands, or the addition of aliphatic carboxylic acids, aliphatic aldehydes, and dispersants having aliphatic and polar groups may be reduced. In particular, the structure and properties of antioxidants, hydrophobic ligands, aliphatic carboxylic acids, and aliphatic aldehydes may compete with other dispersion stabilizers present on the surface of metal nanoparticles, or chemical interactions may cause unexpected aggregation or recrystallization, potentially impairing the uniformity and stability of the dispersion.

[0005] Therefore, there is a need for a dispersant that can effectively disperse metal nanoparticles and prepare a metal nanoparticle dispersion with excellent dispersion stability and storage stability. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2009-233845 [Patent Document 2] Special Publication No. 2013-504692 [Patent Document 3] Japanese Patent Publication No. 2020-100867 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Therefore, an object of the present invention is to provide a dispersant for metal nanoparticles that can impart dispersion stability to metal nanoparticles, and a dispersion of metal nanoparticles that is excellent in dispersion stability and storage stability. [Means for solving the problem]

[0008] The inventors of the present invention have diligently conducted research to solve the above problems and have found that by using a fatty acid composition containing (A) a fatty acid having 16 carbon atoms and a cis double bond at the 9th position, and (B) a fatty acid having 16 carbon atoms and a trans double bond at the 9th position, with a content ratio of component (A) to component (B) [(A):(B)] being 99.99:0.01 to 60:40 by mass ratio, as a dispersant to surface modification of metal nanoparticles, it is possible to impart excellent dispersion stability to metal nanoparticles and obtain a dispersion of metal nanoparticles with excellent storage stability. Further investigation has led to the completion of the present invention.

[0009] In other words, the present invention relates to the following: [1] A dispersant for metal nanoparticles, comprising (A) a fatty acid having 16 carbon atoms and a cis double bond at the 9th position, and (B) a fatty acid having 16 carbon atoms and a trans double bond at the 9th position, wherein the content ratio of component (A) to component (B) [(A):(B)] is 99.99:0.01 to 60:40 by mass ratio. [2] A dispersion of metal nanoparticles, wherein the metal nanoparticles are metal nanoparticles whose surface has been modified with the dispersant described in [1]. [3] The dispersion according to [2], wherein the metal nanoparticles are surface-modified with the dispersant described in [1] in an amount of 0.5 to 5 parts by mass of fatty acid per 1 part by mass of the metal nanoparticles. [Effects of the Invention]

[0010] The present invention provides a dispersant for metal nanoparticles that can impart excellent dispersion stability to metal nanoparticles and also exhibits excellent storage stability. Furthermore, the present invention provides a dispersion of metal nanoparticles that is surface-modified with the above-mentioned dispersant and exhibits excellent dispersion stability and storage stability. In particular, the present invention effectively suppresses aggregation between metal nanoparticles, which is evaluated by zeta potential measurement, and provides a dispersion of metal nanoparticles with excellent dispersion stability. Moreover, the metal nanoparticle dispersant provided by the present invention does not require synthetic dispersants or antioxidants used in conventional technologies, and imparts excellent dispersion stability to metal nanoparticles and has excellent storage stability, thus offering high safety and reducing environmental impact. This invention is not only expected to have applications in high-performance catalysts and electronic materials in the field of nanotechnology, but it can also be developed for cosmetics, pharmaceuticals, and other applications. [Modes for carrying out the invention]

[0011] The present invention provides a dispersant for metal nanoparticles (hereinafter also referred to as "the dispersant of the present invention" in this specification). The dispersant of the present invention contains (A) a fatty acid having 16 carbon atoms and a cis double bond at the 9th position, and (B) a fatty acid having 16 carbon atoms and a trans double bond at the 9th position, in the following content ratios.

[0012] In the dispersant of the present invention, an example of a fatty acid containing component (A) that has 16 carbon atoms and a cis double bond at the 9th position is (Z)-9-hexadecenoic acid (palmitoleic acid). The content of component (A) in the dispersant of the present invention is 99.99% by mass or less of the total amount of the dispersant of the present invention. Furthermore, the content of component (A) in the dispersant of the present invention is preferably 18% by mass or more, more preferably 30% by mass or more, and even more preferably 54% by mass or more of the total amount of the dispersant of the present invention.

[0013] Furthermore, in the dispersant of the present invention, an example of a fatty acid containing 16 carbon atoms and having a trans double bond at the 9th position is (E)-9-hexadecenoic acid (palmitoelaidic acid, trans-palmitoleic acid). The content of component (B) in the dispersant of the present invention is preferably 0.003% by mass or more relative to the total amount of the dispersant of the present invention. Furthermore, the content of component (B) in the dispersant of the present invention is preferably 40% by mass or less, preferably 20% by mass or less, and more preferably 5% by mass or less, relative to the total amount of the dispersant of the present invention.

[0014] The content ratio of component (A) and component (B) in the dispersant of the present invention [(A):(B)] is 99.99:0.01 to 60:40 by mass ratio. From the viewpoint of the dispersibility of metal nanoparticles, the content ratio of component (A) to component (B) [(A):(B)] is preferably 99.99:0.01 to 80:20 by mass ratio, more preferably 99.99:0.01 to 95:5, and particularly preferably 99.99:0.01 to 99.5:0.5.

[0015] From the viewpoint of the effect of imparting dispersibility to metal nanoparticles, the total content of components (A) and (B) in the dispersant of the present invention is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 90% by mass or more, based on the total amount of the dispersant of the present invention.

[0016] From the viewpoint of the effect of imparting dispersibility to metal nanoparticles, the dispersant of the present invention preferably further contains a monounsaturated fatty acid other than a fatty acid in which component (A) has 16 carbon atoms and a cis double bond at the 9th position. In the dispersant of the present invention, as the monounsaturated fatty acid that can be contained as component (C), a cis-type monounsaturated fatty acid other than component (A) with 14 to 18 carbon atoms is preferred. Examples of such cis - monounsaturated fatty acids include (Z)-5 - tetradecenoic acid (physeteric acid), (Z)-8 - tetradecenoic acid, (Z)-7 - tetradecenoic acid, (Z)-4 - tetradecenoic acid, (Z)-6 - tetradecenoic acid, (Z)-9 - tetradecenoic acid (myristoleic acid), (Z)-5 - hexadecenoic acid, (Z)-6 - hexadecenoic acid (sapienic acid), (Z)-7 - hexadecenoic acid, (Z)-10 - hexadecenoic acid, (Z)-11 - hexadecenoic acid, (Z)-7 - heptadecenoic acid, (Z)-8 - heptadecenoic acid, (Z)-9 - heptadecenoic acid, (Z)-10 - heptadecenoic acid, (Z)-9 - octadecenoic acid (oleic acid), (Z)-11 - octadecenoic acid (cis - vaccenic acid), (Z)-12 - octadecenoic acid, (Z)-13 - octadecenoic acid, (Z)-14 - octadecenoic acid, (Z)-15 - octadecenoic acid, and the like. These monounsaturated fatty acids may be selected individually and used alone, or two or more of them may be selected and used in combination. From the perspective of the oxidation stability of the dispersant of the present invention, the content of component (C) in the dispersant of the present invention is preferably 50% by mass or less, and more preferably 30% by mass or less, based on the total amount of the dispersant of the present invention.

[0017] The dispersant of the present invention may further contain (D) saturated fatty acids as long as the characteristics of the present invention are not impaired. Examples of component (D) include linear or branched saturated fatty acids having 8 to 22 carbon atoms, such as octanoic acid (caprylic acid), 2 - ethylhexanoic acid, decanoic acid (capric acid), dodecanoic acid (lauric acid), tetradecanoic acid (myristic acid), 12 - methyltridecanoic acid (isomyristic acid), pentadecanoic acid (pentadecyl acid), hexadecanoic acid (palmitic acid), 14 - methylpentadecanoic acid (isopalmitic acid), heptadecanoic acid (margaric acid), octadecanoic acid (stearic acid), 16 - methylheptadecanoic acid (isostearic acid), eicosanoic acid (arachidic acid), docosanoic acid (behenic acid), and the like.

[0018] In addition, the dispersant of the present invention may further contain common lipophilic additives, such as fatty acid miscible solvents like ethanol and propanol, etc., as long as the effects of the present invention are not impaired. The common lipophilic additives can be used according to the general usage amounts according to their usage purposes.

[0019] The dispersant of the present invention can be prepared by adding component (A) and component (B), and optionally component (C), component (D), and further common lipophilic additives, and preferably mixing and stirring them uniformly under a nitrogen atmosphere. Alternatively, component (A) and component (B) can be preferably mixed and stirred uniformly under a nitrogen atmosphere, and then, optionally, component (C), component (D), and further common lipophilic additives can be added and mixed and stirred to be uniform. The above mixing and stirring are preferably carried out at 40°C to 60°C for about 5 minutes to 30 minutes.

[0020] The dispersant of the present invention is used for surface modification of metal nanoparticles and can impart excellent dispersion stability to the metal nanoparticles. The dispersion stability of the metal nanoparticles is confirmed by measuring the zeta potential. Here, in this specification, "dispersion stability" means that the aggregation of metal nanoparticles is effectively prevented and good dispersion is maintained. In addition, the dispersant of the present invention is excellent in that it can maintain storage stability without using conventional synthetic antioxidants or antioxidants, reduces the environmental load, and is also easily applicable to cosmetic uses and the like. Here, in this specification, "storage stability" means that the degree of unsaturation of the fatty acid composition does not decrease and no denaturation or deterioration due to oxidation or the like occurs.

[0021] The dispersant of the present invention, by containing component (A) and component (B) in a specific content ratio, is thought to adjust the degree of repulsion of hydrocarbon groups on the particle surface when coordinated to the surface of metal nanoparticles, and suppress the oxidation of unsaturated bonds, thereby achieving both dispersion stability and storage stability of metal nanoparticles. Therefore, it shows remarkable advantages in terms of safety, reduced environmental impact, and expandability of applications compared to existing technologies. Furthermore, it is expected to have applications not only in the field of nanotechnology but also in diverse fields such as cosmetics and pharmaceuticals.

[0022] The content of each fatty acid contained in the dispersant of the present invention can be determined by performing fatty acid composition analysis by gas chromatography. An example of the analytical conditions for fatty acid composition analysis is shown below. <Fatty acid purity analysis (fatty acid composition analysis)> Equipment: Gas chromatograph ("Nexis GC-2030", Shimadzu Corporation) Detector: Flame ionization detector (FID) Carrier gas: Nitrogen Column: Capillary column ("TC-70", 60m x 0.25mm x 0.25mm, GL Sciences Co., Ltd.) Temperature condition: 180℃ (120min Hold) Pretreatment: Methyl esterification (fatty acid methylation kit, GL Sciences Co., Ltd.)

[0023] The present invention also provides a dispersion of metal nanoparticles whose surfaces are modified with the dispersant of the present invention as described above (hereinafter sometimes abbreviated as "dispersion of the present invention" in this specification). The dispersion of the present invention is a dispersion in which metal nanoparticles whose surfaces have been modified with the dispersant of the present invention as described above are dispersed in a solvent.

[0024] In the dispersion of the present invention, the metal nanoparticles whose surfaces are modified by the dispersant of the present invention are metal particles with an average particle diameter of typically 100 nm or less, preferably 1 nm to 50 nm, and more preferably 1 nm to 10 nm. The average particle diameter of the metal nanoparticles can be determined from the particle size distribution obtained by dynamic light scattering. Furthermore, metals are a general term for substances that are highly malleable and plastic, can be machined, are good conductors of electricity and heat, and possess a characteristic luster known as metallic luster. In the present invention, there are no particular limitations on the metals that can be made into nanoparticles, and examples include alkali metals such as lithium, sodium, potassium, and rubidium; alkaline earth metals such as magnesium, calcium, strontium, and barium; zinc group elements such as zinc and cadmium; aluminum group elements such as aluminum, gallium, and indium; rare earth elements such as yttrium, lanthanum, and cerium; tin group elements such as titanium, zirconium, tin, and lead; iron group elements such as iron, cobalt, and nickel; earth element elements such as vanadium, niobium, and tantalum; chromium group elements such as chromium, molybdenum, and tungsten; manganese group elements such as manganese and rhenium; precious metals such as copper, silver, and gold; and platinum group elements such as ruthenium, rhodium, palladium, osmium, iridium, and platinum. For the purposes of this invention, metals belonging to the platinum group, precious metals, and metals belonging to the iron group are preferably used, platinum, rhodium, palladium, nickel, gold, silver, and copper are more preferably used, and platinum, gold, and silver are particularly preferably used.

[0025] In the present invention, metal nanoparticles can be preferably produced by wet methods, such as reducing a metal salt in a liquid or thermally decomposing a metal complex, because their shape and particle size can be easily controlled and they can be easily mass-produced. They can be more preferably produced by a chemical reduction method of a metal salt. The chemical reduction of metal salts can be carried out by adding an alcohol, polyol, hydrazine, aliphatic compound, ascorbic acid, citric acid, borohydride, diborane, etc., as a reducing agent to the metal salt dissolved in a solvent and then reducing it. Suitable solvents include water; lower alcohols such as ethanol and propanol; diols such as ethylene glycol and diethylene glycol; and aprotic polar solvents such as N,N-dimethylformamide, dimethyl sulfoxide, and acetonitrile. The amount of reducing agent added is appropriately determined depending on the type of metal salt and the type of reducing agent, but can be in an amount equivalent to 0.1 to 10 times the mass of the metal. The reduction of metal salts can be carried out by stirring a metal salt solution to which a reducing agent has been added. The temperature used to reduce metal salts is set appropriately depending on the type of metal salt and reducing agent used, but it is usually around room temperature to 100°C. For example, when ascorbic acid is used as a reducing agent, the temperature is 20°C to 80°C; when hydrazine is used as a reducing agent, the temperature is 0°C to 60°C; and when polyol is used as a reducing agent, the temperature is 100°C to 200°C. Furthermore, the duration of the reduction reaction is set appropriately depending on the conditions of the reduction reaction, but it is usually between 10 and 60 minutes.

[0026] Surface modification of metal nanoparticles using the dispersant of the present invention can be performed by adding the dispersant of the present invention to a dispersion of metal nanoparticles produced as described above, stirring and mixing, and coordinating or adsorbing fatty acids onto the surface of the metal nanoparticles. Such stirring and mixing is preferably carried out at 20°C to 80°C for 10 to 120 minutes. Furthermore, surface modification of metal nanoparticles with the dispersant of the present invention is preferably carried out by adding 0.5 to 5 parts by mass of the dispersant of the present invention in terms of fatty acid content per 1 part by mass of metal nanoparticles, and more preferably by adding 0.5 to 3 parts by mass of the dispersant of the present invention in terms of fatty acid content.

[0027] The dispersion of the present invention can be used as is in its dispersion form or diluted with a solvent, depending on the intended use. Furthermore, metal nanoparticles can be recovered from the dispersion of the present invention by centrifugation, washed by repeating the redispersion in a solvent and centrifugation several times, and then dried using a vacuum drying apparatus or the like. To suppress aggregation of metal nanoparticles and changes in crystal structure, drying should preferably be performed at the lowest possible temperature. Considering the stability of the fatty acid composition surface-modified on the metal nanoparticles, the drying should be performed at a temperature lower than the decomposition temperature of the fatty acids contained in the fatty acid composition. For example, in the case of vacuum drying, it is preferable to perform the drying at 20°C to 60°C under a pressure of 10 Pa to 500 Pa.

[0028] The dispersion of the present invention exhibits excellent dispersion stability and storage stability of metal nanoparticles. Because the dispersion of the present invention provides good dispersion stability and storage stability of metal nanoparticles without requiring synthetic dispersants or antioxidants used in conventional techniques, it is useful in that it is highly safe and reduces environmental impact. The dispersions of the present invention are not only expected to have applications in high-performance catalysts and electronic materials in the field of nanotechnology, but can also be used in cosmetics, pharmaceuticals, and other applications.

[0029] Furthermore, the present invention provides a method for producing a dispersion of metal nanoparticles whose surfaces are modified with the dispersant of the present invention as described above (hereinafter referred to as "the method for producing the dispersion of the present invention" in this specification). The method for producing the dispersion of the present invention includes (a) adding a reducing agent to a solution of a metal salt to reduce the metal salt and generate metal nanoparticles, and (b) adding the dispersant of the present invention to a dispersion of metal nanoparticles to surface-modify the metal nanoparticles with the dispersant of the present invention.

[0030] The type of metal used to generate nanoparticles in step (a), the average particle size of the metal nanoparticles, the solvent used to dissolve the metal salt, the reducing agent that can be used and the amount added, and the conditions for the reduction reaction are as described above for the dispersion of the present invention. Furthermore, the dispersant of the present invention used to modify the surface of metal nanoparticles in step (b) is as described above, and the amount added is also as described above for the dispersion of the present invention. The conditions for modifying the surface of metal nanoparticles with the dispersant of the present invention are also as described above for the dispersion of the present invention.

[0031] The present invention provides a method for producing dispersions that yield metal nanoparticle dispersions with excellent dispersion stability and storage stability. [Examples]

[0032] The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto.

[0033] [Example 1] Dispersant for metal nanoparticles In a 50 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 0.095 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath under a nitrogen atmosphere while being heated to 40°C until homogenized. Then, 1.584 g of (Z)-9-octadecenoic acid (oleic acid) ("EXTRA OLEIN 99", NOF Corporation) was added and stirred for a further 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 1.

[0034] [Example 2] Dispersant for metal nanoparticles In a 50 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 0.067 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath under a nitrogen atmosphere while being heated to 40°C until homogenized. Then, 3.341 g of (Z)-9-octadecenoic acid (oleic acid) ("EXTRA OLEIN 99", NOF Corporation) was added and stirred for a further 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 2.

[0035] [Example 3] Dispersant for metal nanoparticles In a 100 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 0.106 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath under a nitrogen atmosphere while being heated to 40°C until homogenized. Then, 22.711 g of (Z)-9-octadecenoic acid (oleic acid) ("EXTRA OLEIN 99", NOF Corporation) was added and stirred for a further 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 3.

[0036] [Example 4] Dispersant for metal nanoparticles In a 100 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 0.03 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath at 40°C under a nitrogen atmosphere for 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 4.

[0037] [Example 5] Dispersant for metal nanoparticles In a 100 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 1.492 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath under a nitrogen atmosphere while being heated to 40°C until homogenized. Then, 1.657 g of (Z)-9-octadecenoic acid (oleic acid) ("EXTRA OLEIN 99", NOF Corporation) was added and stirred for a further 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 5.

[0038] [Example 6] Dispersant for metal nanoparticles In a 100 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 1.088 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath at 40°C under a nitrogen atmosphere for 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 6.

[0039] [Example 7] Dispersant for metal nanoparticles In a 100 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 5.728 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath under a nitrogen atmosphere while being heated to 40°C until homogenized. Then, 23.327 g of (Z)-9-octadecenoic acid (oleic acid) ("EXTRA OLEIN 99", NOF Corporation) was added and stirred for a further 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 7.

[0040] [Example 8] Dispersant for metal nanoparticles In a 100 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 14.571 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath under a nitrogen atmosphere while being heated to 40°C until homogenized. Then, 41.143 g of (Z)-9-octadecenoic acid (oleic acid) ("EXTRA OLEIN 99", NOF Corporation) was added and stirred for a further 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 8.

[0041] [Example 9] Dispersant for metal nanoparticles In a 100 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 17.619 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) were added and stirred in a water bath at 40°C under a nitrogen atmosphere for 10 minutes to prepare a fatty acid composition, which was used as the dispersant in Example 9.

[0042] [Comparative Example 1] Dispersant for metal nanoparticles (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid," Thermo Scientific Chemicals) was used as the dispersant in Comparative Example 1.

[0043] [Comparative Example 2] Dispersant for metal nanoparticles (E)-9-hexadecenoic acid (palmitoleic acid) ("trans-Palmitoleic acid," TargetMol) was used as the dispersant in Comparative Example 2.

[0044] [Comparative Example 3] Dispersant for metal nanoparticles A 100 mL four-necked flask equipped with a thermometer, nitrogen inlet tube, and stirrer was used to prepare a fatty acid composition by adding 30 g of (Z)-9-hexadecenoic acid (palmitoleic acid) ("Palmitoleic acid", Thermo Scientific Chemicals) and 45 g of (E)-9-hexadecenoic acid (palmitoelaidic acid) ("trans-Palmitoleic acid", TargetMol) and stirring in a water bath under a nitrogen atmosphere for 10 minutes while heating to 40°C. This composition was used as the dispersant for Comparative Example 3.

[0045] Table 1 shows the content of each component (A) to (C) in the fatty acid composition for each dispersant in Examples 1 to 9 and Comparative Examples 1 to 3, the content ratio of component (A) to component (B) [(A):(B)] (mass ratio), and the total content of component (A) and component (B).

[0046] [Table 1]

[0047] [Examples 10-18, Comparative Examples 4-6] Dispersions of silver nanoparticles As described below, silver nanoparticles were prepared as metal nanoparticles by chemical reduction. Then, the surfaces of the silver nanoparticles were modified using the dispersants from Examples 1 to 9 and Comparative Examples 1 to 3. Dispersions of the silver nanoparticles surface-modified with the dispersants from Examples 1 to 9 and Comparative Examples 1 to 3 were prepared and designated as Examples 10 to 18 and Comparative Examples 4 to 6, respectively. Specifically, silver acetate was dissolved in ethanol to prepare a 0.1 M silver acetate ethanol solution. Next, 26.4 g of ascorbic acid was added to 1 L of this solution, and the reduction reaction was carried out at a constant stirring speed (300 rpm) at 40°C ± 2°C to generate silver nanoparticles. After this reduction reaction was completed, each dispersant of Examples 1-9 and Comparative Examples 1-3 was added in an amount equal to 2 times the mass of the silver nanoparticles (21.6 g), and the mixture was stirred at 500 rpm for 60 minutes at 60°C to modify the surface of the silver nanoparticles, thereby preparing the silver nanoparticle dispersions of Examples 10-18 and Comparative Examples 4-6.

[0048] [Example 19] Dispersion of gold nanoparticles 34.0 g of chlorauric acid (HAuCl4) was dissolved in 68.0 mL of water, and then 12.0 mL of water and 920.0 mL of ethanol were added to prepare 1,000 mL of a 0.1 M chlorauric acid solution with a 92 vol% aqueous ethanol solution. Next, 26.4 g of ascorbic acid was added to this 1,000 mL solution, and the reduction reaction was carried out at a constant stirring speed (300 rpm) at 40°C ± 2°C to generate gold nanoparticles. After this reduction reaction was completed, the dispersant of Example 1 was added in an amount twice the mass of the gold nanoparticles (39.4 g), and the mixture was stirred at 500 rpm for 60 minutes at 60°C to modify the surface of the gold nanoparticles, thereby preparing the gold nanoparticle dispersion of Example 19.

[0049] [Example 20] Platinum nanoparticle dispersion 40.9 g of hexachloroplatinum(IV) acid (H2PtCl6) was dissolved in 81.8 mL of water, and then 918.2 mL of ethanol was added to prepare 1,000 mL of a 0.1 M hexachloroplatinum(IV) acid solution with a 91.82 vol% aqueous ethanol solution. Next, 26.3 g of ascorbic acid was added to this 1,000 mL solution, and the reduction reaction was carried out at 40°C ± 2°C at a constant stirring speed (300 rpm) to generate platinum nanoparticles. After this reduction reaction was completed, the dispersant of Example 1 was added in an amount twice the mass of the platinum nanoparticles (38.9 g), and the mixture was stirred at 60°C at 500 rpm for 60 minutes to modify the surface of the platinum nanoparticles, thereby preparing the platinum nanoparticle dispersion of Example 20.

[0050] The dispersion stability and storage stability of metal nanoparticles in each dispersion of Examples 10-20 and Comparative Examples 4-6 were evaluated as follows. The evaluation results are shown in Table 2.

[0051] (1) Evaluation of the dispersion stability of metal nanoparticles in each dispersion of Examples 10-20 and Comparative Examples 4-6 The dispersion stability of metal nanoparticles in each dispersion of Examples 10-20 and Comparative Examples 4-6 was evaluated by measuring the zeta potential and the variability of the average particle size. (i) Sample The dispersions of each example and comparative example prepared above were used as samples. (ii) Measurement of zeta potential Using the "Zeta Potential, Particle Size, and Molecular Weight Measurement System ELSZ-2000" (Otsuka Electronics Co., Ltd.), each sample was diluted with ethanol to a sample concentration of 30% by mass using a standard dilution cell, packed into the cell, and the zeta potential was measured at 25°C. (iii) Evaluation of the dispersibility of metal nanoparticles based on zeta potential measurement results Based on the zeta potential measurements, the dispersion stability of metal nanoparticles was evaluated according to the following criteria. <Evaluation Criteria> ◎(Excellent dispersion stability observed): The absolute value of the zeta potential is 25mV or higher. ○ (Slightly good dispersion stability is observed): The absolute value of the zeta potential is between 10mV and 25mV. × (Poor dispersion stability): The absolute value of the zeta potential is less than 10mV. (iv) Evaluation of changes in average particle size due to storage Each prepared sample was diluted with ethanol to 0.05 mg / mL, and the average particle size was measured at 25°C using the "Zeta Potential, Particle Size, and Molecular Weight Measurement System ELSZ-2000" (Otsuka Electronics Co., Ltd.) by dynamic light scattering. Then, each sample was stored at 25°C for 30 days, and the average particle size after storage was measured in the same manner. The average particle diameter before the start of storage (immediately after preparation) was used as the reference value, and the rate of change in the average particle diameter was calculated from the measured average particle diameter after the above storage period using the following formula (I). Variance rate (%) = [(Reference value - Average particle size after storage period) / Reference value] × 100 (I) Based on the calculated variability, the dispersion stability of the metal nanoparticles was evaluated according to the following evaluation criteria. <Evaluation Criteria> ◎(Excellent variance stability observed): Volatility is 5% or less. ○ (Slightly good variance stability is observed): Volatility is greater than 5% and less than or equal to 10%. × (Poor variance stability): Volatility exceeds 10%

[0052] (2) Evaluation of storage stability of metal nanoparticles in each dispersion of Examples 10-20 and Comparative Examples 4-6 The storage stability of the metal nanoparticles in each dispersion of Examples 10-20 and Comparative Examples 4-6 was evaluated by measuring the iodine value of the dispersant used for surface modification of the metal nanoparticles in the preparation of each dispersion according to the standard oil and fat analysis method JOCS3.3.3. Specifically, the iodine value of the dispersants (each fatty acid composition) used for surface modification of metal nanoparticles in Examples 1-9 and Comparative Example 3, as well as the dispersants (each fatty acid) in Comparative Examples 1 and 2, was measured. Then, each dispersant (each fatty acid composition and each fatty acid) was stored at 25°C, and the iodine value was measured again after a 30-day storage period. The iodine value at the start of storage (immediately after preparation) was used as the reference value, and the rate of change in iodine value was calculated from the measured iodine values ​​after the aforementioned storage period using the following formula (II). Rate of change (%) = [(Reference value - Iodine value after storage period) / Reference value] × 100 (II) Storage stability was evaluated based on the rate of change calculated above, according to the following evaluation criteria. <Evaluation Criteria> ◎(Excellent storage stability observed): Fluctuation rate is 1% or less. ○ (Slightly good storage stability observed): Variability is greater than 1% and less than or equal to 3%. × (Poor storage stability): Fluctuation rate exceeds 3%

[0053] [Table 2]

[0054] As shown in Table 1, the dispersions of metal nanoparticles surface-modified with the dispersants of Examples 1 to 9 (Examples 10 to 20), which contain (Z)-9-hexadecenoic acid (palmitoleic acid) and (E)-9-hexadecenoic acid (palmitoelaidic acid) in a content ratio (mass ratio) of 63.0:37.0 to 99.9:0.1, with a total content of 52.0% by mass or more, were evaluated as having good dispersion stability of metal nanoparticles and good storage stability, as shown in Table 2. In particular, the dispersions of metal nanoparticles surface-modified with the dispersant of Example 1 (Examples 10, 19, and 20), in which the content ratio of (Z)-9-hexadecenoic acid (palmitoleic acid) to (E)-9-hexadecenoic acid (palmitoelaidic acid) was 99.68:0.32 by mass ratio and the total content of these was 95% by mass, exhibited excellent dispersion stability as evaluated by the zeta potential and the rate of variation of the average particle size, and were also evaluated as having excellent storage stability. Furthermore, in the silver nanoparticle dispersion (Example 11) surface-modified with the dispersant of Example 2, where the content ratio of (Z)-9-hexadecenoic acid (palmitoleic acid) to (E)-9-hexadecenoic acid (palmitoelaidic acid) was 99.78:0.22 by mass ratio and the total content of these was 90% by mass, both the zeta potential and the dispersion stability, as evaluated by the rate of variation of the average particle size, were excellent. In addition, in the silver nanoparticle dispersion (Example 14) surface-modified with the dispersant of Example 5, where the content ratio of (Z)-9-hexadecenoic acid (palmitoleic acid) to (E)-9-hexadecenoic acid (palmitoelaidic acid) was 95.26:4.74 by mass ratio and the total content of these was 95% by mass, the dispersion (Example 14) surface-modified with the dispersant of Example 5 was evaluated as having excellent dispersion stability, as evaluated by the zeta potential, and excellent storage properties.

[0055] On the other hand, in the dispersion of silver nanoparticles surface-modified with (Z)-9-hexadecenoic acid (palmitoleic acid) (dispersant in Comparative Example 1) (Comparative Example 4), the dispersion stability evaluated by zeta potential was excellent, but the dispersion stability and storage stability evaluated by the rate of variation of average particle size were evaluated poorly. This suggests that in the dispersant of Comparative Example 1, which does not contain component (B), degradation due to auto-oxidation of (Z)-9-hexadecenoic acid (palmitoleic acid) occurred, leading to a decrease in dispersibility during storage and aggregation of silver nanoparticles. In the dispersion of silver nanoparticles surface-modified with (E)-9-hexadecenoic acid (palmitoelaidic acid) (dispersant in Comparative Example 2) (Comparative Example 5), it was found that the dispersion stability evaluated by zeta potential and the rate of variation of average particle size was poor. Furthermore, in a dispersion of silver nanoparticles surface-modified with a fatty acid composition (dispersant of Comparative Example 3) having a mass ratio of (Z)-9-hexadecenoic acid (palmitoleic acid) to (E)-9-hexadecenoic acid (palmitoelaidic acid) of 40:60 (Comparative Example 6), it was found that the dispersion stability, as evaluated by the zeta potential and the rate of variation of the average particle size, was inferior. [Industrial applicability]

[0056] As detailed above, the present invention provides a dispersant for metal nanoparticles that can impart excellent dispersion stability to metal nanoparticles and also exhibits excellent storage stability. Furthermore, the present invention provides a dispersion of metal nanoparticles whose surface is modified by the above-mentioned dispersant, resulting in excellent dispersion stability and storage stability. In particular, the present invention effectively suppresses aggregation between metal nanoparticles, as evaluated by zeta potential measurement, and provides a dispersion of metal nanoparticles with excellent dispersion stability. Moreover, the metal nanoparticle dispersant provided by the present invention can impart excellent dispersion stability and storage stability to metal nanoparticles without requiring synthetic dispersants or antioxidants used in conventional technologies, thus offering high safety and reducing environmental impact. This invention is not only expected to have applications in high-performance catalysts and electronic materials in the field of nanotechnology, but it can also be developed for cosmetics, pharmaceuticals, and other applications.

Claims

1. A dispersant for metal nanoparticles, comprising (A) a fatty acid having 16 carbon atoms and a cis double bond at the 9th position, and (B) a fatty acid having 16 carbon atoms and a trans double bond at the 9th position, wherein the content ratio of component (A) to component (B) [(A):(B)] is 99.99:0.01 to 60:40 by mass ratio.

2. A dispersion of metal nanoparticles, wherein the metal nanoparticles are metal nanoparticles whose surface has been modified with the dispersant described in claim 1.

3. The dispersion according to claim 2, wherein the metal nanoparticles are metal nanoparticles surface-modified with the dispersant according to claim 1 in an amount of 0.5 to 5 parts by mass of fatty acid per 1 part by mass of metal nanoparticles.

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