Microemulsion

Field of the Invention

The present invention relates to a microemulsion.

Background of the Invention

To obtain a microemulsion which is a liquid composition containing an oily component sollubilized in an aqueous phase or an aqueous component liquidized in an oil phase, surfactants, such as nonionic surfactants have been used in many cases. However, since polyethylene glycol-based nonionic surfactants such as polyethylene glycol alkyl ethers generally undergo a certain change in their affinity for water as the temperature rises, a microemulsion using such surfactants will have a very narrow temperature range of its stability from the critical solubilization temperature to the cloud point in the phase diagram.

Broadly, there are three methods known for improving the temperature stability of microemulsion even in the use of nonionic surfactants. The first known method includes using a nonionic surfactant in combination with an anionic surfactant In "Solution and Solubility" written by Kouzo Shinoda, p.177, published by Maruzen Co., Ltd., Tokyo, is disclosed a composition providing a microemulsion using an isocapryl alcohol monoglyceryl ether of a nonionic surfactant and a sodium palmitylsulfonate salt of an anionic surfactant Further, J. Phys. Chem., 92, 4702 (1988), JP-A-58-128311 and JP-A-58-131127 also disclose such methods as combining a nonionic surfactant with an anionic surfactant.

As stated above, it has been said that a combination of a specific nonionic surfactant and anionic surfactant is effective for maintaining the resultant microemulsion in its stable state even if the temperature changes. However, since this method requires the use of a specific nonionic surfactant and anionic surfactant at a specific formulation ratio, a problem will arise in that the degree of flexibility in formulation is inevitably limited when trying to apply such a microemulsion to cosmetic articles or like preparations.

A second method to improve the temperature stability of a microemulsion while using a nonionic surfactant includes controlling the composition of the aqueous phase or oil phase used. JP-A-6-293617 disdoses a composition providing a microemulsion, the composition using a nonionic surfactant as a surfactant and containing polar solvent as an aqueous component, but not containing water. This method is stated as being able to expand the temperature range in which a microemulsion can be stable. However, with such a composition without a water content, neither a fresh feeling of use nor a feeling of hydration upon use which is to no small extent desirable for cosmetic articles can be achieved.

JP-A-10-43573 disdoses a composition providing a microemulsion, containing as a nonionic surfactant a sucrose fatty acid ester, having a specific ester distribution, of a saturated or unsaturated fatty acid having a carbon number of 12 to 22 and, containing a monohydric alcohol having a carbon number of 4 to 20. This method is also stated as being able to expand the temperature range in which a microemulsion can be stable. In this method, however, if a monohydric alcohol having a smaller carbon number is used, its distinctive odor will result in a microemulsion-based cosmetic article having a degraded aroma, while if a monohydric alcohol having a larger carbon number is used, an oiliness of the alcohol will render it difficult to produce a feeling of use desired for cosmetic articles.

JP-A-63-126543 and JP-A-63-126544 describe a microemulsion containing a hydrophilic nonionic surfactant, an oil having its inorganic value and carbon number limited to a specific range on an organic conceptual diagram, and water. However, the microemulsions disclosed by those patent applications are hardly applicable to cosmetic articles, because the type of oil, the surfactant content and the mixing ratio of oil and water are limited to a specific range.

A third method to improve the temperature stability of a microemulsion while using a nonionic surfactant includes controlling the type and/or combination of nonionic surfactants used. JP-A-6-262060 discloses a microemulsion sucrose fatty acid ester, alkylglucoside or polyethylene glycol containing, as essential ingredients, a sucrose fatty acid ester, an alkylglucoside or polyethylene glycol, oily component, and water, wherein said sucrose fatty acid ester and said alkylglucoside or polyethylene glycol are used in such a combination that one of these ingredients exists as hydrophilic and the other as lipophilic. However, the temperature span in which the compositions disclosed there remain stable is at most on the order of 26 °C, and thus the compositions do not have a sufficient temperature span allowing their practical use as cosmetic materials. Besides, the sucrose fatty acid ester represents a surfactant relatively liable to hydrolysis among nonionic surfactants and thus is not necessarily preferred for cosmetic articles in view of stability in long term storage.

JP-A-11-262653 discloses an oil-in-water type microemulsion containing polyglycerin fatty acid ester, a reaction product of a polyglycerin having an average degree of polymerization ranging from 5 to 15 and a fatty acid having 75 wt% or higher pure oleic acid content, and a polyhydric alcohol. However, this oil-in-water type microemulsion is not preferred for application to cosmetic articles, because it employs limitedly specified nonionic surfactants leading to low versatility and because in most cases the polyglyceryl fatty acid ester uses, as its fatty acid, oleic acid which is a typical unsaturated fatty acid involving a stability problem such as coloring or separation with time when stored for a longer period.

In Journal of Oleo Science, Vol.51, No.6, 379-386(2002), is described a method for producing a microemulsion using a sucrose fatty acid ester as the above-described oily component, water and nonionic surfactant However, for the microemulsion described there, no such compositions are available that are stable at around room temperatures where the microemulsion is used as cosmetic articles. Thus, the well-known methods of the prior art to produce a temperature-stable composition for a microemulsion using a nonionic surfactant have not been successfully satisfiable for applications to cosmetic articles in respect of ingredient composition, feeling of use preferable for cosmetic articles, or stability of the resultant cosmetics on skins, etc. Further, even if any microemulsions obtained by the prior art methods are applicable to cosmetic articles, such emulsions have not been able to exist stably over a sufficiently wide temperature range.

Summary of the invention

The present invention provides a microemulsion, containing the following ingredients (A) through (F): <ul id="ul0001" list-style="none" compact="compact"><li>(A) a hydrophilic nonionic surfactant having as a hydrophilic group a residue of a sugar, reducing sugar or polyglycerin having a hydrogen atom of at least one hydroxyl group thereof removed;</li><li>(B) a hydrophilic nonionic surfactant having a polyoxyethylene chain as a hydrophilic group;</li><li>(C) a water-soluble organic solvent selected from the group consisting of (C1) through (C3) listed below; <ul id="ul0002" list-style="none" compact="compact"><li>(C1) a compound having in a molecule thereof two or more oxypropylene groups (PO) and hydroxyl groups (OH), the ratio in number of these two groups (PO/OH) being smaller than 5;</li><li>(C2) a monohydric alcohol having a carbon number ranging from 2 to 6; and</li><li>(C3) a dihydric alcohol having a carbon number ranging from 2 to 6;</li></ul></li><li>(D) a lipophilic nonionic surfactant;</li><li>(E) an oily ingredient; and</li><li>(F) water.</li></ul> Further, the present invention provides a skin cosmetic and a skin deansing composition which contain the microemulsion according to the present invention.

Detailed Description of the Invention

The present invention provides a microemulsion having a stability over a wider temperature range.

The inventors have found out that a certain combination of three specific surfactants with specific water-soluble organic solvents can produce a microemulsion which is stable over a wider temperature range.

The microemulsion according to the present invention has an excellent stability over a wide temperature range.

The "microemulsion" may be defined in two ways, namely, in a broader sense and in a narrower sense. That is to say, there are one case ("microemulsion in the narrow sense") in which the microemulsion refers to a thermodynamically stable isotropic single liquid phase containing a ternary system having three ingredients of an oily component, an aqueous component and a surfactant, and the other case ("microemulsion in the broad sense") in which among thermodynamically unstable typical emulsion systems the microemulsion additionally includes those such emulsions presenting transparent or translucent appearances due to their smaller particle sizes (Satoshi Tomomasa, et al., Oil Chemistry, Vol. 37, No. 11 (1988), pp. 48-53). The "microemulsion" as used herein refers to a "microemulsion in the narrow sense," i.e., a thermodynamically stable isotropic single liquid phase.

The microemulsion refers to either one state of an O/W (oil-in-water) type microemulsion in which oil is solubilized by micelles, a W/O (water-in-oil) type microemulsion in which water is solubilized by reverse micelles, or a bicontinuous microemulsion in which the number of associations of surfactant molecules are rendered infinite so that both the aqueous phase and oil phase have a continuous structure. For properties, the microemulsion appears transparent or translucent and may exist as a solution in a monophasic state in which all the formulated ingredients and components are uniformly dissolved therein.

Regardless of manufacturing processes, microemulsions may take the same state if they have the same formulation and same temperature. Therefore, the above-described three ingredients and the remaining ingredients may be added and mixed in any orders as appropriate and may be agitated using mechanical forces at any power to consequently yield a microemulsion having substantially the same state.

Thus, whether a composition containing the above-described three ingredients is a microemulsion or not may be determined based on its properties and manufacturing process. In respect of properties, the microemulsion takes a liquid state at room temperatures (about 25 °C) and whether a composition constitute a microemulsion may be determined by measuring its viscosity. For example, 10,000 mPa·s or lower viscosity (a viscometer manufactured by TOKIMEC INC., Tokyo, measurement conditions: rotor No. 1, 60 rpm) may be employed as a measure of the microemulsion state. For the manufacturing process, whether at least two processes involving varied mixing orders and mixing temperatures of ingredients yield the same state (in appearance, viscosity, feeling of use, etc.) at the same temperature may be depended on to determine the existence of a microemulsion state.

According to the present invention, the hydrophilic nonionic surfactant as the above-described ingredient (A) has as a hydrophilic group a residue of a sugar, reducing sugar or polyglycerin having a hydrogen atom of at least one hydroxyl group thereof removed and includes, for example, polyglyceryl fatty acid esters, polyglyceryl alkyl ethers, sucrose fatty acid esters, alkylpolyglucosides, or the like.

For the present microemulsion, preferable polyglyceryl fatty acid esters are those esters of a polyglycerin and a fatty acid having a carbon number ranging from 8 to 22, including, for example, polyglyceryl octanoate esters, polyglyceryl 2-ethylhexylate esters, polyglyceryl decanoate esters, polyglyceryl laurate esters, polyglyceryl myristate esters, polyglyceryl palmitate esters, polyglyceryl isostearate esters, polyglyceryl stearate esters, polyglyceryl oleate esters, polyglyceryl behenate esters, and so on. Among these esters, monoesters of a polyglycerin having a degree of polymerization ranging from 3 to 15 and a fatty acid having a carbon number ranging from 12 to 18 are more preferred.

Preferable polyglyceryl alkyl ethers are those ethers of a polyglycerin and an alkyl group having a carbon number ranging from 8 to 22, induding, polyglyceryl octyl ethers, polyglyceryl decyl ethers, polyglyceryl lauryl ethers, polyglyceryl myristyl ethers, polyglyceryl palmityl ethers, polyglyceryl isostearyl ethers, polyglyceryl stearyl ethers, polyglyceryl oleyl ethers, polyglyceryl behenyl ethers, and so on. Among these ethers, monoethers of a polyglycerin having a degree of polymerization ranging from 3 to 15 and an alkyl group having a carbon number ranging from 12 to 18 are more preferred.

Preferable sucrose fatty acid esters are those esters derived from a fatty acid having a carbon number ranging from 8 to 22 and sucrose, induding, for example, sucrose octanoate esters, sucrose 2-ethylhexanoate esters, sucrose decanoate esters, sucrose laurate esters, sucrose myristate esters, sucrose palmitate esters, sucrose isostearate esters, sucrose stearate esters, sucrose oleate esters, sucrose behenate esters, and so on. Among these, monoesters of a fatty acid having a carbon number ranging from 12 to 18 and sucrose are more preferred.

Preferable alkylpolyglucosides are those having an alkyl group with a carbon number ranging from 8 to 22 and a degree of glucoside unit condensation ranging from 1 to 7, induding, for example, octylpolyglucosides, 2-ethylhexylpolyglucosides, decylpolyglucosides, laurylpolyglucosides, myristylpolyglucosides, palmitylpolyglucosides, isostearylpolyglucosides, stearyl laurylpolyglucosides, oleylpolyglucosides, behenylpolyglucosides, and so on. More preferably, such alkylpolyglucosides as having an alkyl group with a carbon number ranging from 8 to 11 and having a degree of glucoside unit condensation ranging from 1 to 1.4 may be used and, further preferably, those having an alkyl group with a carbon number of 12 to 14 and degree of glucoside unit condensation ranging from 1.5 to 4.0 may be used.

For the hydrophilic nonionic surfactant as the above-described ingredient (A), it is more preferable to use polyglyceryl fatty acid esters, polyglyceryl alkyl ethers or alkylpolyglucosides, because the resultant composition may obtain a high stability in long term storage.

The ingredient (A) may contain one or more such hydrophilic nonionic surfactants with its content ranging preferably from 0.05 to 8 weight % and more preferably from 0.1 to 7 weight % of the microemulsion, because such a content allows the microemulsion to have a stability over a wider temperature range around the room temperatures and a good feeling of use as a cosmetic article.

According to the present invention, the hydrophilic nonionic surfactant as the ingredient (B) has a polyoxyethylene chain as a hydrophilic group and includes, for example, polyoxyethylene fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin monofatty acid esters, polyoxyethylene hydrogenated castor oils, polyoxyethylene hydrogenated castor oil monofatty acid esters, and so on

Preferable polyoxyethylene fatty acid esters are those esters derived from a fatty acid with a carbon number ranging from 8 to 22 and having a degree of polymerization of ethylene oxide (hereinafter, shall be abbreviated as "EO") ranging from 5 to 60, including, polyoxyethylene octanoate esters, polyoxyethylene 2-ethylhexanoate esters, polyoxyethylene decanoate esters, polyoxyethylene laurate esters, polyoxyethylene myristate esters, polyoxyethylene palmitate esters, polyoxyethylene isostearate esters, polyoxyethylene stearate esters,polyoxyethylene oleate esters, polyoxyethylene behenate esters, and so on. Among these, esters derived from a fatty acid with a carbon number ranging from 12 to 18 and having a degree of EO polymerization ranging from 5 to 60 are more preferred.

Preferable polyoxyethylene alkyl ethers are those ethers having an alkyl group with a carbon number ranging from 8 to 22 and having a degree of EO polymerization ranging from 5 to 60, including polyoxyethylene octyl ethers, polyoxyethylene decyl ethers, polyoxyethylene lauryl ethers, polyoxyethylene myristyl ethers, polyoxyethylene palmityl ethers, polyoxyethylene isostearyl ethers, polyoxyethylene stearyl ethers, polyoxyethylene oleyl ethers, polyoxyethylene behenyl ethers, and so on. Among these, ethers having an alkyl group with a carbon number ranging from 12 to 18 and a degree of EO polymerization ranging from 5 to 60 are more preferred.

Preferable polyoxyethylene sorbitan fatty acid esters are those esters derived from a fatty acid with a carbon number ranging from 8 to 22 and having degree of EO polymerization ranging from 5 to 60 and include, for example, polyoxyethylene sorbitan octanoate esters, polyoxyethylene sorbitan decanoate esters, polyoxyethylene sorbitan laurate esters, polyoxyethylene sorbitan myristate esters, polyoxyethylene sorbitan palmitate esters, polyoxyethylene sorbitan isostearate esters, polyoxyethylene sorbitan stearate esters, polyoxyethylene sorbitan oleate esters, polyoxyethylene behenate esters, and so on. Among these, esters derived from a fatty acid with a carbon number ranging from 12 to 18 and having a degree of EO polymerization ranging from 5 to 60 are more preferred.

Preferable polyoxyethylene glycerin monofatty acid esters are those esters derived from a fatty acid with a carbon number ranging from 8 to 22 and having degree of EO polymerization ranging from 5 to 60, including polyoxyethylene glycerin monooctanoate esters, polyoxyethylene glycerin monodecanoate esters, polyoxyethylene glycerin monolaurate esters, polyoxyethylene glycerin monomyristate esters, polyoxyethylene glycerin monoisostearate esters, polyoxyethylene glycerin monostearate esters, polyoxyethylene glycerin monooleate esters, polyoxyethylene glycerin monobehenate esters, and so on. Among these, esters derived from a fatty acid with a carbon number ranging from 12 to 18 and having a degree of EO polymerization ranging from 5 to 60 are more preferred.

Preferable polyoxyethylene hydrogenated castor oils are those having a degree of EO polymerization ranging from 20 to 80 and more preferable from 30 to 60. Meanwhile, preferable polyoxyethylene hydrogenated castor oil monofatty acid esters are those esters derived from a fatty acid with a carbon number ranging from 8 to 22 and having a degree of EO polymerization ranging from 20 to 80 and indude, for example, polyoxyethylene hydrogenated castor oil monooctanoate esters, polyoxyethylene hydrogenated castor oil monodecanoate esters, polyoxyethylene hydrogenated castor oil monolaurate esters, polyoxyethylene hydrogenated castor oil monomyristate esters, polyoxyethylene hydrogenated castor oil monopalmitate esters, polyoxyethylene hydrogenated castor oil monoisostearate esters, polyoxyethylene hydrogenated castor oil monostearate esters, polyoxyethylene hydrogenated castor oil monooleate esters, polyoxyethylene hydrogenated castor oil monobehenate esters, and so on. Among these, esters derived from a fatty acid with a carbon number ranging from 12 to 18 and having a degree of EO polymerization ranging from 20 to 80 are more preferred.

For the hydrophilic nonionic surfactant as the above-described ingredient (B), it is more preferable to use polyoxyethylene fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene sorbitan fatty acid esters, or polyoxyethylene glycerin monofatty acid esters, because the resultant composition may obtain a good feeling of use as a cosmetic article.

The ingredient (B) may contain one or more such nonionic surfactants, and its content ranges preferably from 0.05 to 8 weight % and more preferably from 0.1 to 7 weight % of the microemulsion, because such a content allows the resultant microemulsion to have a stability over a wider temperature range around the room temperatures and a good feeling of use as a cosmetic article.

Further, for the ingredients (A) and (B), it is preferred to use hydrophilic nonionic surfactants having an HLB (hydrophile-lypophile balance) value above 8 and more preferably above 9. Here, the HLB value represents a measure of the molecular weight shared by the hydroxyl group portion of a surfactant in its total molecular weight and for polyoxyethylene-based nonionic surfactants it can be determined by the Griffin's formula shown below.

<maths id="math0001" num=""><math display="block"><mrow><mi mathvariant="normal">HLB value</mi><mo>=</mo><mi mathvariant="normal">E</mi><mo>/</mo><mn>5</mn></mrow></math><img file="EP1639989A1_D0001.tif" /></maths> where: E represents the quantity in weight % of the polyoxyethylene part contained in a surfactant molecule.

According to the present invention, the ratio (A)/(B) in weight of the ingredients (A) and (B) ranges preferably from 0.1 to 10 and more preferably from 0.2 to 5, because within this range the resultant microemulsion can have a stability over a wider temperature range around the room temperatures.

According to the present invention, the water-soluble organic solvent as the aforementioned ingredient (C) is selected from the group consisting of (C1) through (C3) listed below: <ul id="ul0003" list-style="none" compact="compact"><li>(C1) a compound having in a molecule thereof two or more oxypropylene groups (PO) and hydroxyl groups (OH), the ratio in number of these two groups (PO/OH) being smaller than 5;</li><li>(C2) a monohydric alcohol having a carbon number ranging from 2 to 6; and</li><li>(C3) a dihydric alcohol having a carbon number ranging from 2 to 6.</li></ul> The polypropylene glycol-based compound, namely the foregoing compound (C1) preferably has a PO/OH ratio smaller than or equal to 4. Further, it is preferred that the above compound (C) have a PO/OH ratio ranging from 1.5 to 4, because such a range allows the resultant microemulsion to have a stability over a wider temperature range around the room temperatures and a good feeling of use as a cosmetic article.

Preferable polypropylene glycol-based compounds include polypropylene glycols, polyoxypropylene trimethylpropane ethers, polyoxypropylene sorbitol ethers, polyoxypropylene monoglyceryl ethers, polyoxypropylene diglyceryl ethers, polyoxypropylene triglyceryl ethers, monoalkyl ethers having a polyoxypropylene polyglyceryl ether skeleton, condensates of a polypropylene glycol and a polyglucoside, condensates of a polypropylene glycol and sucrose, and so on.

Among these, polypropylene glycols, polyoxypropylene trimethylpropane ethers, polyoxypropylene sorbitol ethers, and compounds having a propylene oxide structure of a monoglycerin and/or a polyglycerin are preferred and, further specifically, polypropylene glycols, polyoxypropylene trimethylpropane ethers, polyoxypropylene sorbitol ethers, polyoxypropylene monoglyceryl ethers, polyoxypropylene diglyceryl ethers, polyoxypropylene triglyceryl ethers are more preferred.

Meanwhile, it is preferred to use a monohydric alcohol having a carbon number of 2 or 3 and a dihydric alcohol having a carbon number ranging from 2 to 6 as the components (C2) and (C3), respectively, selectable for the ingredient (C).

Such monohydric and dihydric alcohols include ethanol, propanol, isopropanol, butanol, propylene glycol, isoprene glycol, 1,3-butylene glycol, 1,2-pentanediol, 1,2-hexanediol, hexylene glycol, etc. and, among these, ethanol, propylene glycol, isoprene glycol, 1,3-butylene glycol and hexylene glycol are more preferred.

The ingredient (C) may contain one or more such compounds as described above, and its content ranges preferably from 0.5 to 35 weight % and more preferably from 1.0 to 30 weight % of the microemulsion, because such a content allows the microemulsion to have a stability over a wider temperature range around the room temperatures and a good feeling of use as a cosmetic article.

According to the present invention, the lipophilic nonionic surfactant as the aforementioned ingredient (D) preferably has an HLB value not greater than 8. Here, the HLB value can be determined using the Griffin's formula described previously. Specifically, such surfactants include polyoxyethylene monofatty acid esters, polyoxyethylene difatty acid esters, polyoxyethylene monoalkyl ethers, polyoxyethylene dialkyl ethers, monoglycerin monofatty acid esters, monoglycerin difatty acid esters, diglycerin monofatty acid esters, monoglyceryl monoalkyl ethers, diglyceryl monoalkyl ethers, sorbitan fatty acid ester, and so on.

More specifically, for the polyoxyethylene monofatty acid esters and polyoxyethylene difatty acid esters, it is preferred to use monoesters or diesters derived from a fatty add having a carbon number ranging from 8 to 22 and a polyethylene glycol having a degree of EO polymerization ranging from 2 to 14, including, for example, polyoxyethylene octanoate esters, polyoxyethylene 2-ethylhexanoate esters, polyoxyethylene decanoate esters, polyoxyethylene laurate esters, polyoxyethylene myristate esters, polyoxyethylene palmitate esters, polyoxyethylene isostearate esters, polyoxyethylene stearate esters, polyoxyethylene oleate esters, polyoxyethylene behenate esters, and so on. Among these, esters derived from a fatty acid with a carbon number ranging from 12 to 18 and having a degree of EO polymerization ranging from 4 to 12 are more preferred.

For the polyoxyethylene monoalkyl ethers and polyoxyethylene dialkyl ethers, it is preferred to use monoethers or diethers having an alkyl group with a carbon number ranging from 8 to 22 and having a degree of EO polymerization ranging from 2 to 14, including polyoxyethylene octyl ethers, polyoxyethylene decyl ethers, polyoxyethylene lauryl ethers, polyoxyethylene myristyl ethers, polyoxyethylene palmityl ethers, polyoxyethylene isostearyl ethers, polyoxyethylene stearyl ethers, polyoxyethylene oleyl ethers, polyoxyethylene behenyl ethers, and so on. Among these, ethers having an alkyl group with a carbon number ranging from 12 to 18 and a degree of EO polymerization ranging from 4 to 12 are more preferred.

For the monoglycerin monofatty acid esters and monoglycerin difatty acid esters, it is preferred to use monoesters or diesters derived from glycerin and a fatty acid having a carbon number ranging from 8 to 22, including, monoglycerin octanoate ester, monoglycerin 2-ethylhexanoate ester, monoglycerin decanoate ester, monoglycerin laurate ester, monoglycerin myristate ester, monoglycerin palmitate ester, monoglycerin isostearate ester, monoglycerin stearate ester, monoglycerin oleate ester, monoglycerin behenate ester, and so on. Among these, monoglycerin 2-ethylhexanoate ester, monoglycerin laurate ester, monoglycerin myristate ester, monoglycerin palmitate ester, monoglycerin isostearate ester, monoglycerin stearate ester and monoglycerin oleate ester are more preferred.

For the diglycerin monofatty acid ester, it is preferred to use those ester derived from a diglycerin and a fatty acid having a carbon number ranging from 8 to 22, including, diglycerin octanoate ester, diglycerin 2-ethylhexanoate ester, diglycerin caprylate ester, diglycerin caprate ester, diglycerin laurate ester, diglycerin myristate ester, diglycerin palmitate ester, diglycerin isostearate ester, diglycerin stearate ester, diglycerin oleate ester, diglycerin behenate ester, and so on. Among these, diglycerin 2-ethylhexanoate ester, diglycerin laurate ester, diglycerin myristate ester, diglycerin palmitate ester, diglycerin isostearate ester and diglycerin stearate ester are more preferred.

Preferable monoglyceryl monoalkyl ethers are those ethers of glycerin and an alkyl group having a carbon number ranging from 8 to 22, including, monoglyceryl 2-ethylhexyl ether, monoglyceryl octyl ether, monoglyceryl decyl ether, monoglyceryl lauryl ether, monoglyceryl myristyl ether, monoglyceryl palmityl ether, monoglyceryl stearyl ether, monoglyceryl isostearyl ether, monoglyceryl oleyl ether, monoglyceryl behenyl ether, and so on. Among these, monoglyceryl 2-ethylhexyl ether, monoglyceryl lauryl ether, monoglyceryl myristyl ether, monoglyceryl palmityl ether, monoglyceryl stearyl ether and monoglyceryl isostearyl ether are more preferred.

Preferable diglyceryl monoalkyl ethers are those ethers derived from diglycerin and an alkyl group having a carbon number ranging from 8 to 22, including diglyceryl 2-ethylhexyl ether, diglyceryl octyl ether, diglyceryl decyl ether, diglyceryl lauryl ether, diglyceryl myristyl ether, diglyceryl palmityl ether, diglyceryl stearyl ether, diglyceryl isostearyl ether, diglyceryl oleyl ether, diglyceryl behenyl ether, and so on. Among these, diglyceryl 2-ethylhexyl ether, diglyceryl lauryl ether, diglyceryl myristyl ether, diglyceryl palmityl ether, diglyceryl stearyl ether and diglyceryl isostearyl ether are more preferred.

Preferable sorbitan fatty acid ester are those ester derived from a fatty acid having a carbon number ranging from 8 to 22, including, sorbitan octanoate ester, sorbitan 2-ethylhexanoate ester, sorbitan caprylate ester, sorbitan caprate ester, sorbitan laurate ester, sorbitan myristate ester, sorbitan palmitate ester, sorbitan isostearate ester, sorbitan stearate ester, sorbitan oleate ester, sorbitan behenate ester, and so on. Among these, sorbitan 2-ethylhexanoate ester, sorbitan laurate ester, sorbitan myristate ester, sorbitan palmitate ester, sorbitan isostearate ester and sorbitan stearate ester are more preferred.

For the ingredient (D), it is preferred to use polyoxyethylene difatty acid esters, polyoxyethylene dialkyl ethers, monoglycerin monofatty acid esters, monoglycerin difatty acid esters, diglycerin monofatty acid esters, monoglyceryl monoalkyl ethers, diglyceryl monoalkyl ethers or sorbitan fatty acid esters. Among these, monoglycerin monofatty acid esters, monoglycerin difatty acid esters, monoglyceryl monoalkyl ethers, diglyceryl monoalkyl ethers and sorbitan fatty acid esters are more preferred.

The ingredient (D) may contain one or more such compounds as described above, and its content ranges preferably from 0.05 to 8 weight % and more preferably from 0.1 to 7 weight % of the microemulsion, because such a content allows the resultant microemulsion to have a stability over a wider temperature range around the room temperatures and a good feeling of use as a cosmetic article.

According to the present invention, it is preferred that the total content (A) + (B) + (D) of the foregoing nonionic surfactant ingredients (A), (B) and (D) range from 0.2 to 10 weight % and more preferably from 0.4 to 9 weight % of the entire composition, because such a content allows the resultant microemulsion to have a stability over a wider temperature range around the room temperatures and a good feeling of use as a cosmetic article.

According to the present invention, preferable oily components as the aforementioned ingredient (E) may be those materials ordinarily used for cosmetic articles, including, for example, hydrocarbon oils such as liquid paraffin, liquid isoparaffin, squalane, etc.; ester oils such as cholesteryl isostearate, isopropyl palmitate, isopropyl myristate, neopentylglycol dicaprate, isopropyl isostearate, octadecyl myristate, cetyl 2-ethylhexanoate, isononyl isononanoate, isotridecyl isononanoate, glyceryl tri-2-ethylhexanoate, glyceryl tri(caprylate/caprate), etc.; ether oils such as alkyl-1,3-dimethylethyl ether, nonylphenyl ether, etc.; silicone oils such as methyl polysiloxane, decamethylcyclopentasiloxane, octamethylcyclotetrasiloxane, etc.; higher fatty acids having a carbon number ranging from 8 to 22, such as lauric acid, myristic acid, palmitic acid, stearic acid, etc.; higher alcohols having a carbon number ranging from 8 to 22, such as lauryl alcohol, myristyl alcohol, palmityl alcohol, stearyl alcohol, cetyl alcohol, etc.; animal and vegetable oils such as fish oil, soybean oil, olive oil, etc.; ceramides, phospholipids, glycolipids, etc.; or terpene oils.

Among these, liquid paraffin, liquid isoparaffin, neopentylglycol dicaprate, isopropyl isostearate, cetyl 2-ethylhesanoate, isononyl isononanoate, glyceryl tri(caprylate/caprate), alkyl-1,3-dimethylbutyl ether, methyl polysiloxane having a molecular weight ranging from 100 to 500, decamethylcyclopentasiloxane, octamethylcyclotetrasiloxane, higher fatty acids having a carbon number ranging from 12 to 22, higher alcohols having a carbon number ranging from 12 to 22, soybean oil, olive oil, ceramides, glycolipids and terpene oil are more preferred and, further, hydrocarbon oils are also preferred.

The ingredient (E) may contain one or more such compounds as described above, and its content ranges preferably from 1 to 50 weight % and more preferably from 2 to 45 weight % of the microemulsion, because such a content allows the microemulsion to have a stability over a wider temperature range around the room temperatures and a good feeling of use as a cosmetic article.

Further, it is preferred that the present microemulsion contains the above-described ingredient (F), namely water, in a quantity ranging from 40 to 95 weight % and more preferably from 45 to 90 weight %, because such a content allows the microemulsion to have a stability over a wider temperature range around the room temperatures and a good feeling of use as a cosmetic article.

The microemulsion of the present invention may be mixed as appropriate with such additional ingredients that are typically used for cosmetic articles, including any surfactants or water-soluble solvents other than those specified hereinbefore, thickeners, bactericides, humectants, wetting agents, colorants, preservatives, feel improving agents, perfumes, antiinflammatory agent, skin-lightening cosmetics, antihidrotics, ultraviolet absorbers, etc.

The microemulsion of the present invention may be produced by any typical methods known in the art and may be applied in the form of skin cleansing composition such as cleanser, face wash, body wash, etc.; skin cosmetic such as skin toning lotion, essence, skin-lightening cosmetic, antiwrinkling agent, anti-UV skin care cosmetic, etc. Further, the present microemulsion may be applied in the form of such cosmetic as combined with woven textiles, nonwoven fabrics or like sheet materials.

Examples

Preferred examples 1 through 16 and comparative examples 1 through 6

Microemulsions having the formulations shown in Tables 1 through 3 were prepared to evaluate the temperature stabilities. Tables also show the results of evaluation.

Process

For preparation of the microemulsions, the process basically proceeded in the following way. All ingredients of each intended composition was loaded into a mixing vessel as a batch. To dissolve those normally solid components or those gelled components produced by mixing at room temperatures, the batch was heated at 70 to 75 °C under agitation. After the content was fully dissolved, the batch was cooled down to room temperature to obtain a microemulsion. It is to be noted here that neither the order and manner of mixing the ingredients nor the speed of their agitation is not particularly limited according to the present invention. Also, the heating temperature is not limited to those temperatures mentioned above.

Evaluation method

For each microemulsion prepared as above, five 20 mℓ samples were separately put into a capped dear glass vessel and held at 5 °C, 10 °C, 25 °C, 40 °C and 45 °C, respectively, for 12 hours. Then, the microemulsion samples were externally observed visually and rated as good "o" when the samples were uniform without turbidity and their viscosity was sufficiently low, while rating as bad "×" when the samples underwent separation. <tables id="tabl0001" num="0001"><img file="EP1639989A1_D0002.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP1639989A1_D0003.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP1639989A1_D0004.tif" /></tables>