LIQUID CRYSTAL MAKE-UP REMOVER 25 Dissolution of dirt Oily dirt '...•••. ß .'..'-•,'. .::.': .:.',' wipe-off .::.'....'.....'. !:':ii.S, k!n. ::" •"Wl:O :." :: ::?.' :.' .":" Evaporation of water Oil Water • • • Dissolution • • • of dirt Wipe-off \ e•! [ I I I ß :" .:: '..'. ,' :.",' ' .::.".' :...:Phase ...:.:.:•: ..'?:.'.:,' 2.':'•.:'." '. :.! Skin':'.. '•:':.:'::".' :"'. " " ...... '" "'O•W' :" : "::."::::'::'"inversibn':".':." Reinversion Rinse-off + wazer .[ .: :.'... :.-.: :' 2 :::.:'.' :.':.•.'. Figure 2. Schematic illustrations showing cleansing processes of two types of cleansing cream. ever, were speculated as a cause of skin irritation and skin trouble by the physical trauma of wiping off. The physiological influence in terms of change in the area size of cor- neocytes on human upper arms was studied after treatment with two different kinds of cleansing methods: wiping off with tissue and rinsing off with water. Figure 3 shows the comparison in area size of corneocytes isolated from the surface of stratum corneum after three weeks of treatment. The size of corneocytes isolated from the wiped-off area became smaller than those of the rinsed-off and control areas. Within the two products of O/W cleansing cream (rinsing-off type) and liquid cystal gel, the difference was not significant. It appears that only the physical means of the removal process of cleansing products contributes to the size of corneocytes. We suspect that the wiping, a physical stress, stimulated the skin turnover, thus bringing immature corneocytes to the surface of the stratum corneum. Furthermore, the texture of the skin surface with rinsing-off treatment obviously looked better compared to that observed after wiping-off treatment (Figure 4). CONCEPT OF A NEW MAKE-UP REMOVER AND REQUIREMENTS OF FORMATION OF LIQUID CRYSTAL MAKE-UP REMOVER Several features and characteristics are required for an ideal make-up remover that can remove oily materials effectively without damaging the skin. Here four characteristics and formulating technology required in such a product are shown: 1) Easy to apply to the skin. -- Structural system retaining a large amount of oil. 2) Dissolves oily dirt quickly. -- Oily system or a system of low interfacial tension to oil. 3) Easy to rub. -- Thixotropic property. 4) Easy to rinse off. -- Hydrophilic or self-emulsification property.

26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1000 -- "" 900 -- o 800 - o •- 700 -- o .N_ 600 - A B A B I ControlJ I Washing-Off ] Wiping-Off J Figure 3. Area sizes of corneocytes isolated from the surface of the human stratum comeurn where the different cleansing methods were undertaken for three weeks. A: a cleansing gel. B: a cleansing cream. Control: non-treated skin. In order to satisfy the antagonistic conditions of 2) and 4), possessing oily and hydro- philic properties simultaneously, a bicontinuous liquid crystalline system composed of oil, water, and surfactant was chosen. A liquid crystal made of higher associates of amphiphilic molecules and a lameliar liquid crystal can retain oil and water in the lipophilic and hydrophilic moieties, respectively. It is known that fine emulsions can be formed when liquid crystals are formed as an intermediate stage of the emulsification process (8-11). Using these properties, the oil phase dissolving the dirt can be easily dispersed in water as fine droplets. In such a case, it is important to choose a surfactant that yields a liquid crystalline structure easily and does not remain on the skin. In general, surfactants with branched alkyl chains effectively form higher associates such as liquid crystals, compared to surfactants with linear alkyl chains (12). The amount of removers remaining on the skin after the rinsing process was measured spectrophotometically by using Parsol MCX as a marker. The samples were prepared by mixing various kinds of surfactants and oils. Figure 5a shows the results as a function of the hydrophilic-lipophilic balance (HLB) number of the surfactant. It was suggested that a polyoxyethylene alkyl ether, which possessed branched Y-shaped alkyl chains, was suitable for this purpose. The hydrophilic-lipophilic balance (HLB) of nonionic surfactants is due to oxyethylene (EO) chain length. The lipophilic surfactants of shorter EO chains are favorable for the