Method for dispersing metal oxide using water or aqueous solution as dispersion medium and water dispersion thereof

Abstract

To provide a good water dispersion of metal oxide in cosmetics.SOLUTION: A semiconductor or an alloy having silicon as a principal component is placed as an electrode in water, and plasma treatment is applied to a dispersion of metal oxide. In this treatment, silica can be coated on the metal oxide by a simple method. Thereby, a zeta potential of a surface of the metal oxide specifically decreases in a neutral pH range, and a uniform water dispersion without sedimentation or flotation of the metal oxide or a water dispersion which can be easily re-dispersed even with sedimentation, can be obtained by a dispersion effect due to electrostatic repulsive force.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for coating the surface of a metal oxide with silica by applying a voltage between air-water or underwater-underwater counter electrodes to a dispersion of the metal oxide in water or an aqueous solution as a dispersion medium to generate plasma, thereby dispersing the metal oxide well in a near-neutral dispersion medium having a pH of 6 to 8, and to an aqueous dispersion of the metal oxide obtained by this method. [Background technology]

[0002] Metal oxides are widely used in a variety of fields, including electronic materials, cosmetics, paints, and ceramics, due to their electrical and optical properties and color-tuning capabilities.

[0003] When this metal oxide is used as an aqueous preparation, it has been found that the dispersion stability of the metal oxide in water or an aqueous solution is very important for the metal oxide to exhibit the above-mentioned functions.

[0004] For example, in the field of cosmetics, metal oxides such as cerium oxide, fine particle titanium oxide, and zinc oxide are dispersed in formulations as UV protection agents, and the better the dispersion state of the metal oxide, the more transparent the product will be when applied to the skin, and the greater the UV protection ability.

[0005] One effective method for dispersing a metal oxide in water or an aqueous solution is a dispersion method that utilizes the electrostatic repulsion between particles by controlling the surface potential of the metal oxide particles.

[0006] Metal oxide particles dispersed in water or an aqueous solution are themselves positively or negatively charged, so ions of the opposite charge are adsorbed to the particle surface to maintain electrical neutrality. A layer of ions of the opposite charge is then formed around the particle, and this is called an electric double layer. The limit area where some of the ions surrounding the particle move as the particle diffuses or the liquid flows is called the "slip surface," and the electric potential of this area is called the zeta potential.

[0007] The magnitude of the zeta potential depends on the surface condition of the metal oxide, the pH of the dispersion medium, and the electrolyte concentration in the system. The pH at which the zeta potential becomes 0 is called the isoelectric point (IEP). In water or an aqueous solution with a pH lower than the IEP, the zeta potential of the metal oxide becomes positive, and in water or an aqueous solution with a pH higher than the IEP, the zeta potential becomes negative. The aggregation and dispersion behavior of metal oxides is related to the absolute value of the zeta potential. The larger the absolute value of the zeta potential, the greater the repulsion between charges of the same sign, making it easier to disperse. On the other hand, when the absolute value of the zeta potential is small, the repulsion is small, and the van der Waals forces between metal oxide particles become dominant, making them more likely to aggregate. In general, when the absolute value of the zeta potential is 25 mV or more, metal oxide particles tend to be stably dispersed in an aqueous solution. However, in the case of metal oxides with high density, the effect of sedimentation due to gravity cannot be ignored when the particle size is submicron or larger, as expressed by the Stokes' equation, and dispersion becomes difficult even if the absolute value of the zeta potential is large.

[0008] A simple method for dispersing metal oxides in water or an aqueous solution is to add a pH adjuster or a dispersant. For example, the metal oxides are dispersed when the pH of the system is adjusted to a region away from the IEP of the metal oxide by adding a pH adjuster. However, the IEP of many metal oxides is between about 5 and 9, as typified by titanium oxide with an IEP of about 5 to 6, and the absolute value of the zeta potential is small near the neutral range because it is close to the IEP, making it easy to aggregate. In particular, when using metal oxides in cosmetics, it is required to disperse them near neutral, considering that they will be applied directly to the skin. In addition, when using metal oxides as catalysts, the appropriate pH range and dispersant components are limited, taking into consideration the effect on the reaction system. Furthermore, when using a dispersant in an emulsion system, it is necessary to consider the effect on the emulsion stability of the system.

[0009] In cases where the methods of adjusting the pH of the dispersion medium or adding a dispersant are only applicable in limited opportunities, a method of coating the surface of the metal oxide with another compound is often used.

[0010] Patent Document 1 describes a manufacturing method for coating the surface of metal oxide fine particles with silica, but the silica coating requires multiple steps, time, and aging at high temperatures, and also requires the removal of additives such as silane coupling agents and organic solvents. The average particle size of the obtained silica-coated titanium oxide is 0.4 μm, which is difficult to say that the dispersion state has sufficient ultraviolet protection ability for cosmetics. Patent Document 2 describes a manufacturing method for cosmetics using an aqueous dispersion of such silica-coated metal oxide coated on the outside with a hydrophobic polymer, but it is clear that the protective colloid effect of the thickening polymer is essential to maintain the dispersion. Patent Documents 1 and 2 do not mention the zeta potential, and it is presumed that silica is used simply to improve the feel.

[0011] Patent Document 3 discloses a method for providing zinc oxide powder that hardly causes aggregation by coating it with water-repellent fine silica particles. However, this method is intended for dispersion in cosmetic compositions for which the composition is intended to be dispersed in oil-based preparations such as W / O or solvents, and does not suggest dispersion in water or an aqueous solution.

[0012] Patent Document 4 discloses a skin topical preparation that has good dispersion stability of metal oxides by coating with hydrophilic and lipophilic sucrose fatty acid esters. However, this also requires that a nonionic polymer compound or a naturally derived polymer compound and a wax be blended in the skin topical preparation composition to improve storage stability, and there is no suggestion of dispersing in water or an aqueous solution, so that the scope of application to cosmetics is limited.

[0013] Patent Document 5 describes a method for producing titanium oxide for aqueous dispersions having two silica coating layers on the titanium oxide surface. However, this also requires a multi-step process and time, and fine pH adjustment is required at each step. In particular, it is thought that the electrolyte concentration in the system is increased by dropping a large amount of acid or alkali to adjust the pH, but an increase in electrolyte concentration compresses the counterion cloud on the surface of the dispersed particles, thinning the electric double layer and reducing the absolute value of the zeta potential. Therefore, it is prone to aggregation, and it is thought that a well-dispersed aqueous dispersion of titanium oxide cannot be obtained unless a filtration or washing process is performed once.

[0014] As described above, it is not easy to maintain a good dispersion of metal oxides in neutral water or an aqueous solution using known techniques, particularly when the metal oxides are applied to cosmetics. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Patent No. 4836232 [Patent Document 2] Patent Publication No. 2009-161496 [Patent Document 3] Patent No. 5290544 [Patent Document 4] Patent No. 5212609 [Patent Document 5] Patent No. 3773220 Summary of the Invention [Problem to be solved by the invention]

[0016] In view of the above, the problem to be solved by the present invention is to provide a good aqueous dispersion of a metal oxide that can be applied to cosmetics in a simple manner without adding a dispersant or the like. [Means for solving the problem]

[0017] The present invention relates to a method for dispersing well the metal oxide by specifically lowering the zeta potential of the metal oxide in near-neutral water having a pH of 6 to 8, by placing a semiconductor or alloy mainly composed of silicon as an electrode in water or an aqueous solution and performing plasma treatment on the metal oxide aqueous dispersion to coat the surface of the metal oxide with silica having an IEP of 2 to 3, and to a good aqueous dispersion of the metal oxide obtained by this method. Note that a good aqueous dispersion in the present invention does not simply mean that the metal oxide is present in water, but refers to a state in which the metal oxide is stably and uniformly present in water without settling or floating, and a state in which the metal oxide is easily redispersed even if it has settled, and a dispersion in such a state is called a good aqueous dispersion. Effect of the Invention

[0018] The method of the present invention for coating metal oxide with silica can be carried out in one pot and one step, and a good aqueous dispersion of metal oxide can be obtained in a short time. Compared to conventional methods of treating with chemicals such as organic silanes and silicates, there is no need for a process of removing chemicals such as organic substances, and no need for heat aging from hydrated silica to a dense silica layer. In addition, since no electrolyte is added except for pH adjustment at the beginning of the reaction, a good aqueous dispersion of metal oxide can be obtained without removing the electrolyte after the reaction.

[0019] Specific examples of the metal oxide used in the present invention include alumina, fine particle titanium oxide, fine particle zinc oxide, titanium oxide, zinc oxide, cerium oxide, silica, red iron oxide, yellow iron oxide, and black iron oxide.

[0020] The above-mentioned fine particle titanium oxide and fine particle zinc oxide refer to those having a primary particle diameter of 100 nm or less, while titanium oxide and zinc oxide refer to those having a primary particle diameter larger than that.

[0021] Of the metal oxides used in the present invention, fine particle titanium oxide, fine particle zinc oxide, titanium oxide, zinc oxide, and cerium oxide are preferred because they have high ultraviolet protection capabilities, and among these, fine particle titanium oxide is particularly preferred because it is versatile and inexpensive.

[0022] Furthermore, when titanium oxide or fine particle titanium oxide is used, it may be of the rutile type, anatase type, or brookite type, or a mixture selected from one or more of these types, but it is preferable to use the rutile type, which has a relatively low surface activity.

[0023] The shape and primary particle size of the metal oxide used in the present invention are not particularly limited, and examples of the shape include spherical, approximately spherical, spindle-shaped, needle-shaped, and cubic shapes. From the viewpoint of ease of dispersion, spherical, approximately spherical, and spindle-shaped shapes are preferred.

[0024] In the present invention, plasma treatment is performed on the liquid surface of a dispersion of a metal oxide in water or an aqueous solution as a dispersion medium.

[0025] The metal oxide concentration in the dispersion to be subjected to the plasma treatment is not particularly limited, but a low concentration will result in a lack of versatility when blended into cosmetics, while a high concentration will result in the interparticle distance of the metal oxide becoming closer and the van der Waals force becoming non-negligible, making aggregation more likely to occur. Therefore, the metal oxide concentration is preferably 0.1 to 15% by weight, more preferably 0.1 to 10% by weight, and even more preferably 0.1 to 5% by weight.

[0026] When the metal oxide concentration is high, extending the plasma treatment time increases the amount of silica coating, making it easier to disperse.

[0027] The dispersion medium used for the dispersion to be treated is not particularly limited as long as it is water or an aqueous solution in which a salt or the like is dissolved in water.

[0028] Specific examples of the components to be dissolved include sodium chloride, potassium chloride, magnesium chloride, copper chloride, silver chloride, gold chloride, ammonium chloride, sodium sulfate, sodium hydrogen sulfate, potassium sulfate, potassium hydrogen sulfate, magnesium sulfate, calcium sulfate, copper sulfate, silver sulfate, ammonium sulfate, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, magnesium carbonate, calcium carbonate, ammonium carbonate, sodium nitrate, potassium nitrate, magnesium nitrate, calcium nitrate, copper nitrate, silver nitrate, aluminum nitrate, ammonium nitrate, sodium hydroxide, potassium hydroxide, ammonium hydroxide, alum, ammonia, methanol, and ethanol, but are not particularly limited to these and may be mixtures of these. The amount of the additive to be added is also not particularly limited as long as the additive is dissolved.

[0029] In the present invention, the dispersed state of the metal oxide before the plasma treatment is not particularly limited, and the metal oxide is in a state of being immersed in the dispersion, but a part of the metal oxide may be precipitated.

[0030] The pH of the dispersion medium before the plasma treatment is not particularly limited, but in order to efficiently coat the metal oxide with silica, a neutral to alkaline pH of 6 to 14 is preferable, and a pH of 6 to 11 is more preferable because if the pH is too high, the electrolyte concentration will be high when neutralized after the treatment and aggregation will be likely to occur.

[0031] In the present invention, it is sufficient to apply plasma to an aqueous dispersion of a metal oxide. In addition to liquid surface plasma (see Figure 1), which is generated between the air electrode and the surface of the dispersion by placing electrodes in the air and the dispersion, liquid plasma can also be generated between electrodes in the dispersion.

[0032] It is preferable to use liquid surface plasma because it reduces electrode wear and enables discharge without the need for additives such as salts to increase the electrical conductivity of the aqueous dispersion.

[0033] When liquid surface plasma is used, one or more electrodes may be placed in the air (above the surface of the dispersion) and in the dispersion (with at least a portion of the electrode in contact with the aqueous dispersion).

[0034] In the plasma used in the present invention, at least one electrode placed in the dispersion is a semiconductor or alloy mainly composed of silicon. Examples of such semiconductors include silicon semiconductors, semiconductors composited with other elements such as silicon-germanium and silicon carbide, and alloys such as ferrosilicon. The silicon semiconductor in the present invention includes intrinsic silicon semiconductors as well as P-type and N-type semiconductors in which silicon is doped with other elements. The silicon content in the electrodes is preferably 50% or more, more preferably 90% or more, and even more preferably 99% or more.

[0035] In each plasma generation method, when a silicon semiconductor is used as an electrode placed in a dispersion, the exposed crystal face may be (100), (110), or (111), but is not particularly limited and may be any of them. In addition, there are high-resistance and low-resistance silicon semiconductors, but low-resistance silicon semiconductors are preferred in consideration of electrical conductivity.

[0036] The shape of the electrode is not particularly limited, and examples include a flat, rod-like, needle-like, etc. shape. A flat silicon wafer is preferred because the larger the surface area, the better the reaction efficiency.

[0037] Specific examples of the material of the electrode placed in the air include copper, tungsten, graphite, titanium, stainless steel, molybdenum, aluminum, iron, platinum, silver, gold, etc., but there is no particular limitation, and electrodes plated with these metals may also be used. Considering the red heat of the electrode, platinum is preferred.

[0038] The shape of the electrode placed in the air is not particularly limited, and possible shapes include a plate, rod, needle, cylinder, sphere, coil, a comb shape having multiple needle or rod shapes, a pin-holder shape, etc., of which, rod, needle, pin-holder and comb shapes are preferred because an uneven electric field is generated, which lowers the breakdown voltage and allows plasma to be generated at a lower voltage, and the size and thickness of the electrode are also not particularly limited.

[0039] The number of electrodes used for plasma generation is not particularly limited and may be selected appropriately depending on the plasma generation conditions. When a large applied voltage or current is used, it is preferable to use multiple electrodes because this reduces the burden on each electrode.

[0040] In the present invention, the power source used to generate plasma can be of various types, such as a DC power source, a pulse power source, a nano-pulse power source, a low-frequency AC power source, or a high-frequency AC power source.

[0041] When a large applied voltage or current is used, the current or voltage may be increased by connecting multiple power sources in parallel and in series.

[0042] In addition, in order to reduce the red heat of the electrodes, a rectifier circuit, a current branching unit that branches the current, a burst control unit that can adjust the ratio of ON and OFF output, etc. may be used.

[0043] In particular, when using a large applied voltage and current in liquid surface plasma, it is possible to reduce the glow of the electrodes without reducing the processing efficiency, so it is preferable to use a rectifier circuit so that the electrode placed in the air is positive and the electrode placed in the dispersion is negative, or to use a current branching unit to generate multiple plasmas from multiple electrodes.

[0044] When a pulsed power supply is used, the pulse width and frequency can be selected arbitrarily, and can be appropriately selected depending on the amount and efficiency of the treatment, and the shape and number of the electrodes.

[0045] In the present invention, a plurality of power sources may be provided, and plasma may be generated from each of the power sources for processing.

[0046] In liquid surface plasma, the contact area between the silicon electrode placed in the dispersion and the dispersion to be treated is not particularly limited, but may be adjusted appropriately depending on the conditions for generating plasma, such as voltage and current, in order to perform stable plasma treatment.

[0047] In the present invention, plasma can be generated in a closed system filled with gas or under reduced pressure.

[0048] Specific examples of the gas include hydrogen, oxygen, nitrogen, carbon dioxide, helium, neon, argon, and the like. These gases may be mixed, and the type of gas is not particularly limited.

[0049] Moreover, as a method for introducing gas into an open system, there is a method for blowing gas around the area where plasma is generated, which is particularly effective when using an electrode that is rod-shaped, needle-shaped, etc., and that can easily limit the location where plasma is generated. The gas may be introduced either in the dispersion (bubbling) or above the surface of the dispersion.

[0050] In the present invention, the efficiency of the plasma treatment and dispersion treatment can be further increased by combining the liquid surface plasma treatment with a mechanical disintegration force or a mechanical stirring force.

[0051] Specific examples of the homogenizer include a magnetic stirrer, a mixer, an ultrasonic bath, a homogenizer, an ultrasonic homogenizer, a high-speed homomixer, a high-pressure homogenizer, a colloidal mill, a stamp mill, a rod mill, a ball mill, a bead mill, a jaw crusher, a kneader, and a planetary mill, but are not limited to these, and two or more of them may be used in combination.

[0052] In addition, it is preferable to use an ultrasonic bath, an ultrasonic homogenizer, a high-speed homogenizer, a bead mill, or a planetary mill in combination, since this allows for efficient application of stirring and disintegrating forces simultaneously.

[0053] In the present invention, these mechanical stirring and disintegrating forces and the positions of the electrodes to be installed can be arbitrarily selected and combined, and the combinations are not particularly limited.

[0054] In ultrasonic treatment, the frequency of ultrasonic waves is 15 to 150 kHz, which is the level normally used for cleaning, and 30 to 50 kHz is preferable in consideration of the stirring power and installation costs. The output depends on the amount to be treated and the dispersion characteristics of metal oxides, but it is sufficient to use an ultrasonic wave of 2500 W or less that is normally available commercially, and 1200 W or less is preferable in consideration of costs.

[0055] In the present invention, the treatment time may be appropriately adjusted depending on conditions such as the treatment volume and the concentration of the dispersion.

[0056] In the present invention, the obtained dispersion can be used for various purposes, such as being used as the dispersion itself, or being subjected to a surface treatment and then dried. [Brief description of the drawings]

[0057] [Figure 1] FIG. 1 is a schematic diagram of an apparatus for using liquid surface plasma in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] Next, the method for dispersing a metal oxide in water or an aqueous solution of the present invention, and the silica-coated metal oxide aqueous dispersion will be described in detail with reference to examples, but the present invention is not limited to these. EXAMPLES

[0059] Metal oxides were dispersed in water using liquid surface plasma (Figure 1). <Preparation of aqueous dispersion of titanium dioxide particles> In a 1 L separable beaker, 5 g of titanium dioxide microparticles MT-500B (manufactured by Teika Corporation, shape: roughly spherical) with a primary particle diameter of 35 nm were added, and 100 μL of ammonia water (28 vol%) was added to adjust the pH at which the liquid surface plasma treatment was to begin. Ion-exchanged water was then added to a total of 1000 g to prepare a dispersion of titanium dioxide microparticles before plasma treatment (concentration: 0.5 wt %, treatment amount: 1000 g, pH 10.0).

[0060] The plasma generation and processing conditions were set as follows: <Plasma generation conditions> ·power supply Pulse power supply (pulse width: 1.6 microseconds, frequency: 15 kHz) Applied voltage: approx. 4 kV Current: about 0.3A Using a rectifier circuit, the electrode placed in the air was made positive, and the electrode placed in the aqueous dispersion was made negative. ·electrode Electrode placed in aqueous dispersion: Silicon wafer (diameter = 100 mm, thickness = 0.5 mm, P-type semiconductor, silicon content 99% or more) Contact area of ​​the electrode placed in the aqueous dispersion: (50) 2 π×2=5,000[mm 2 ] Electrode placed in air: platinum, rod-shaped (diameter = 1 mm, length = 30 mm) Distance between liquid surface and electrode: 5mm <Processing conditions> Atmosphere: The liquid surface was plasma treated for up to 2 hours while introducing argon gas. Liquid temperature: 10±5℃ Mechanical stirring and disintegration force: The plasma irradiation part is an ultrasonic cleaner (frequency: 40 kHz, output: 80 W)

[0061] The obtained dispersion was an extremely good aqueous dispersion with little sediment, even though it was a 0.5% by weight aqueous dispersion of fine titanium dioxide particles. When X-ray absorption fine structure (XAFS) spectroscopy was performed to identify the chemical structure of the material coated on the titanium dioxide surface, a prominent peak was observed at 1848 eV, and the shape of the peak indicated the presence of amorphous silica on the surface. Furthermore, SEM-EDX analysis detected silicon throughout the entire titanium dioxide surface, and X-ray fluorescence analysis using the glass bead method revealed that the amount of silicon present was 3.82 mg per 1 g of titanium dioxide.

[0062] Ion-exchanged water was added to 5 g of fine titanium dioxide particles MT-500B so that the total weight was 1000 g, and then ultrasonic treatment was performed for 2 hours. The result was not subjected to the plasma treatment, and was used as Comparative Example 1. From the SEM-EDX analysis, no silicon was detected on the surface.

[0063] To 5 g of commercially available silica-coated titanium oxide fine particle TS-04 (manufactured by Teika Corporation, shape: roughly spherical), ion-exchanged water was added to make a total of 1000 g, and then ultrasonic treatment was performed for 2 hours. The result was not subjected to the above plasma treatment, and was used as Comparative Example 2. SEM observation of the dried product showed that larger aggregates than the titanium oxide of Example 1 had been formed. (Evaluation of Dispersion State in Example 1 and Comparative Examples 1 and 2)

[0064] In order to evaluate the dispersion state of the aqueous dispersions of Example 1 and Comparative Examples 1 and 2 near neutral pH, the zeta potential was measured at pH 7 (±1) using a zeta potential measurement system (ELS-Z1, manufactured by Otsuka Electronics Co., Ltd.). In addition, the average dispersed particle size (median size, d50) at pH 6, pH 7, and pH 8 was measured by dynamic light scattering (DLS) using a multi-analyte nanoparticle size measurement system (nanoSAQRA, manufactured by Otsuka Electronics Co., Ltd.). The results are summarized in Table 1. In Example 1, the surface was covered with silica with a low IEP, so the zeta potential near neutral pH was lowered by 20 mV or more than in Comparative Example 1, and the average dispersed particle size was 200 nm or less over the entire neutral range of pH 6 to 8 due to the electrostatic repulsion, and the dispersed state was maintained well. On the other hand, in Comparative Example 1, sufficient electrostatic repulsion was not obtained near neutral pH, and the particles aggregated and settled at pH 7 or less, which is close to the IEP of titanium oxide. On the other hand, in Comparative Example 2, the zeta potential was low because the particles were coated with silica, but the particle size was large and they quickly settled. As agglomerates were observed in the SEM observation, it is believed that agglomerations are likely to occur during the process in the conventional silica coating method, and if that happens, it will be difficult to achieve good dispersion stabilization due to the effect of gravity. [Table 1] EXAMPLES

[0065] Example 2 was prepared by changing the concentration of fine titanium oxide particles in Example 1 to 0.1% by weight. EXAMPLES

[0066] Example 3 was prepared by changing the concentration of fine titanium oxide particles in Example 1 to 0.25% by weight. EXAMPLES

[0067] Example 4 was prepared by changing the concentration of fine titanium oxide particles in Example 1 to 5.0% by weight. EXAMPLES

[0068] Example 5 was prepared by changing the concentration of fine titanium oxide particles in Example 1 to 10.0% by weight. EXAMPLES

[0069] The silicon wafer in Example 1 was replaced with an N-type semiconductor to form Example 6. EXAMPLES

[0070] Example 7 was prepared by changing the reaction temperature in Example 1 to 80±5°C. EXAMPLES

[0071] Example 8 was prepared by using sodium hydroxide instead of the aqueous ammonia in Example 1 and adjusting the pH at the initial stage of the reaction to 12. (Evaluation of Dispersion State in Examples 2 to 8)

[0072] XAFS spectrum and SEM-EDX analysis revealed that the entire surface of titanium oxide was covered with amorphous silica in all of Examples 2 to 8. The dispersion state of the aqueous dispersions of Examples 2 to 8 near neutrality was evaluated by the same method as in Example 1 and Comparative Examples 1 and 2. The results are summarized in Table 2. From Examples 1 to 4, when the titanium oxide concentration was in the range of 0.1 to 5% by weight, the average dispersed particle size was 300 nm or less over pH 6 to 8 after 2 hours of plasma treatment, and the particles were dispersed well. From Example 5, when the plasma treatment time was 4 hours, the average dispersed particle size was 300 nm or less over pH 6 to 8 even when the titanium oxide concentration was 10% by weight, and the particles were dispersed well. In Example 8, the electrolyte concentration was increased by pH adjustment during neutralization, so the absolute value of the zeta potential at neutrality was low and the particles were not dispersed. [Table 2]

[0073] The fine titanium dioxide particle aqueous dispersion after plasma treatment had a zeta potential in the neutral region of approximately -25 mV or less, and its average dispersed particle diameter was approximately 300 nm or less, making it a good aqueous dispersion. For example, if the average primary particle diameter is 100 nm or less and the average dispersed particle diameter is 300 nm or less, it has excellent ultraviolet absorption and scattering capabilities and transparency, making it useful for cosmetic applications. [Industrial Applicability]

[0074] According to the present invention, the zeta potential of the metal oxide surface is specifically decreased in the near-neutral pH range, and a good aqueous dispersion is obtained due to the dispersion effect caused by electrostatic repulsion. The present invention is capable of coating silica on the metal oxide in a simpler manner than the conventional technology, and shows excellent dispersion stability without using a dispersant or the like. This good aqueous dispersion of metal oxide is expected to be applied to, for example, aqueous cosmetics. [Explanation of symbols]

[0075] 1. Storage tank for water dispersion 2 Metal oxide water dispersion 3 Power supply 4. Electrode placed in the air above the water dispersion 5. Electrodes placed in aqueous dispersion 6. Insulating tube 7. Plasma 8. Argon Gas 9 Constant temperature water 10. Ultrasonic cleaner

Claims

1. A method for producing a silica-coated metal oxide, characterized by placing at least one electrode made of a silicon-based semiconductor or alloy in water or an aqueous solution containing a metal oxide, and generating plasma by applying a voltage between the electrode and a pair of electrodes, thereby coating the surface of the metal oxide with silica.

2. The manufacturing method according to Claim 1, characterized in that one or more electrodes are placed in the air above the water or aqueous solution as the pair of electrodes.

3. The manufacturing method according to claim 1 or 2, characterized in that the metal oxide is fine-particle titanium oxide with a primary particle diameter of 100 nm or less.

4. An aqueous dispersion of fine titanium oxide particles, the surface of which is coated with silica at least partially, (a) Substantially free of organic solvents, surfactants and polymeric dispersants, (b) The zeta potential at pH 6 to 8 is -25 mV or less, (c) The fine titanium dioxide particles measured by dynamic light scattering at pH 6 to 8 The average dispersion particle size (median diameter, D50) is between 150 and 300 nm. A characteristic feature is the aqueous dispersion of fine titanium dioxide particles.

5. A cosmetic and composition containing the fine particle titanium dioxide aqueous dispersion described in Claim 4.

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