EVALUATION OF SKIN SOFTENING 175 1.0 0.5 ß ß o o o o ooo o • -w•.. •, o: o.5,•1 ß: 4.0,•1 I I I I I I I I I I I I I I t •10 I I I I I 0 0 30 60 120 Time (rnin) Figure 3. Time dependence of dynamic loss modulus of the stratum corneum after application of distilled water at 25øC, 50% RH. OILS Figure 4 shows the effect of oils on the softness of the stratum corneum. In addition to mineral oil and jojoba liquid wax, polar oils such as glyceryl diisostearate and trimethyl propane triisostearate were examined. 1.0 mineral oil • 0.5 ß ß ß . i.. - . ß -.. ß ß . . . o . jojoba liquid wax ' ' ' 0 •) 15 3•0 45 0 0 15 0 5 60 Time (min) Time (min) (a) (b) Figure 4. Time dependence of dynamic elastic modulus of the stratum corneum after application of oil at 25øC, 50% RH. (a) mineral oil, (b) jojoba liquid wax.

176 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS From these results it was confirmed that both polar and non-polar oils which are widely used in cosmetic products did not soften the stratum corneum at all as Blank had reported (1). SURFACTANTS Figure 5 shows the effects of aqueous surface active agent solutions (100 mM/1, i.e., above the c.m.c.) on the mechanical properties of the stratum comeurn. The stratum 1.50 1.00 0.50 oo SLS Polyoxyethylene (15) Laurylether 0 I I I I I I I I I I I I I I I I I I I I I 0 30 60 90 120 Time (min) Figure 5. Time dependence of dynamic elastic modulus of the stratum corneum after application of aqueous surfactant solution (100 mM/1) at 25øC, 50% RH. corneum was plasticized by aqueous surfactant solutions more than by water immedi- ately after application. The reason is that the aqueous surfactant solutions have lower surface tensions than distilled water, spreading more readily and penetrating the stratum corneum (9,10). However, the recovery of the elastic modulus started sooner for surfactant-treated stratum corneum since water evaporated more rapidly. Finally, the elastic modulus became markedly higher than the pretreatment levels. Thus, the effects of application of aqueous surfactant solutions on the stratum corneum were characterized by increased elasticity after water evaporation. The extent of elasticity increase depended on the characteristics of the surfactant. Sodium lauryl sulfate (SLS) was more effective than polyoxyethylene (15) lauryl ether. The mechanism of stratum corneum hardening by application of surfactant solution is not clear, but this result may be related to the taut feeling that occurs after washing the face with soap.