Skin performs very important functions as a barrier against the external environment, protecting from biological, chemical and physical attacks of microorganisms, chemical substances, ultraviolet rays, etc. and preventing loss of biologically essential components such as moisture. It is a stratum corneum having a thickness of about 20 μm, the outermost layer of the epidermis, that functions as the barrier. The stratum corneum is made up of flat corneous cells laid like bricks cemented with intercellular lipids. It is known that ceramides act as a key component of the intercellular lipids for making up the lipid barrier, playing an important role in keeping skin soft and fresh (see Downing D. T., et al., J. Lipid Res., vol. 24, p. 759 (1983) and Dowing D. T., et al., J. Invest. Dermat., vol. 84, p. 410 (1985)). It has been revealed that the skin of those suffering from rough skin, dry skin or atopic dermatitis has a noticeably reduced ceramide content in the intercellular lipids as compared with healthy skin. Attempts to improve skin conditions by supplementing rough skin with the intercellular lipids containing ceramides have been made. For example, external preparations containing ceramides or ceramide-containing intercellular lipids for application to the skin have been proposed.
However, ceramides are highly crystalline high-melting compounds and, because of their peculiar amphiphatic structure, have extremely low solubility in most of oil-soluble or water-soluble bases (solvents) for cosmetics. For this it has been difficult to formulate ceramides into stable preparations. That is, preparations having a high ceramide content easily undergo precipitation, or some lubricants which are used to dissolve a larger amount of a ceramide are unfavorable for safety.
It has recently been demanded to supply ceramides in the form of a clear solution for use in cosmetics or pharmaceuticals. To meet the demand JP-A-9-315929 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes a clear lipid composition comprising a sphingolipid, a lysophospholipid, and a polyhydric alcohol, the ratio of the sphingolipid to the lysophospholipid being 2/1 or less. This composition is a water-free mixture, however. JP-W-9-505065 (the term “JP-W” as used herein means a “world patent application (PCT) transfer originating from abroad”) discloses a lipid composition in which a phytosphingosine-containing ceramide is stably suspended in a C6-C100 ester base and solubilized in a C8-C22 monofatty acid ester. It is desired for this composition to be free from water. In other words, the compositions of JP-A-9-315929 and JP-W-9-505065 have difficulty in keeping clear when mixed with water as is frequently used in formulation into final products, or in providing a clear preparation upon dilution with water to an arbitrary concentration.
Japanese Patent 3008212 teaches transparent or translucent cosmetics comprising (A) an amphiphatic lipid, (B) a nonionic surface active agent, (C) an ionic surface active agent, and (D) an aqueous medium, the ratio of (A) to [(B)+(C)] being 0.2 to 10. However, the amphiphatic lipids that are actually used are pseudoceramides. In addition, there is a fear of skin irritation due to the ionic surface active agent which is not preferred for cosmetics.