178 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.5 1.0 0.5 ! 0 30 60 90 120 Time (min) Figure 7. Time dependence of dynamic elastic modulus of the stratum corneum after application of aqueous formic acid solution (90%) at 25øC, 50% RH. of a plasticizer for the stratum corneum than water immediately after application. However, the elastic modulus started to increase in about 5 minutes, and as the formic acid evaporated, finally reached two times the non-treated level. This suggests that formic acid has remarkable astringent effects on the stratum corneum. HUMECTANTS Table I shows the hygroscopicities and the water-holding capacities of humectants. Generally, the water content of hygroscopic substances at a certain relative humidity is higher when measured by a desorption path rather than by a sorption path. Our results corresponded to these results, and it was shown that water-holding capacity is higher than hygroscopicity, though the former parameter does not correspond strictly to the water content in equilibrium on a desorption path. Since most cosmetic products contain fairly large amounts of water whose activity is very high and release water rather than absorbing it, water-holding capacity seems to be a more useful parameter than hygroscopicity in estimating the skin hydrating capacity of humectants. Judging from these results, sodium lactate and sodium pyrrolidone carboxylate are better hu- mectants than glycerin which is widely used in the cosmetic industry.

EVALUATION OF SKIN SOFTENING 179 Table I Hygroscopicities and Water-Holding Capacities of Humectants at 25øC, 50 RH Hygroscopicity Water-Holding Capcity Humectant (H20 mg/100 mg) (H20 mg/100 rag) DPG • 12 8 Sorbitol 1 21 PEG 2002 20 22 Glycerin 25 40 Na-PCA 3 44 60 Na-lactate 56 84 Dipropylene glycol. Polyethylene glycol (M.W. 200). Sodium pyrrolidone carboxylate. Figure 8a shows the change of dynamic elastic modulus of the stratum corneum with time after the application of 10% aqueous glycerin solution. The stratum corneum was plasticized to the same extent as by distilled water. However, the processes of dynamic elastic modulus recovery were very different from each other. The plasticization of the stratum corneum treated by glycerin solution lasted for a longer time than that obtained with distilled water. Other humectant solutions (10%) examined in this experiment also softened the stratum corneum like glycerin solution. However, the extent of stratum corneum plasticization at 120 min after the application varied with the type of humectant. On the other hand, neat glycerin did not affect the mechanical properties of the stratum corneum to any extent, as shown in Figure 8b. The elasticity was gradually decreased, and the stratum corneum was only slightly plasticized even after 120 min. 1.0 0.5 Glycerin 10% Aqueous Solution 1.0 Glycerin o • .... a'o .... do .... •o .... •io o 6 .... io .... •o .... •o .... do Time (min) Time (min) (a) (b) Figure 8. Time dependence of dynamic elastic modulus of the stratum corneum after application of humectant at 25øC, 50% RH. (a) 10% aqueous glycerin solution, (b) glycerin. The average values of the ratio, E't/E't= 0, from 90 min to 120 min after the appli- cation of aqueous humectant solutions, were plotted against water-holding capacities