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

180 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS for the stratum corneum treated by each humectant. The result is shown in Figure 9. The plasticizing ability of humectants depended on their water-holding capacities hence the higher the water-holding capacity, the more the plasticizing effect. This can be explained as follows. The elastic forces in the stratum corneum arise from keratin- chain interactions and the plasticization of the stratum corneum is caused by water which breaks hydrogen bonds in keratin (4,15). A plasticizer such as water may be adsorbed at polar sites in keratin and mask the centers of force for keratin-keratin intermolecular attraction by selectively solvating the stratum corneum at these points, widening the space between keratin chains. Therefore, the larger the number of water molecules present, the more the stratum corneum is plasticized. On the other hand, the water-holding capacity obtained in this experiment includes the water migrating to the stratum corneum in addition to that retained by the humectant. As the quantity of migrating water seems to be proportional to the value of water-holding capacity, the application of an aqueous humectant solution with a high water-holding capacity may induce an increase in water content within the stratum corneum. Consequently, the higher the water-holding capacity, the more the plasticizing effect. 1,0- DPG••itol PEG 200 ß Na-PCA • actate I I I I I I I I I 0 10 20 30 40 50 60 70 80 90 Water Holding Capacity (mgH20/100mg humectant) Figure 9. Relation between water-holding capacity of humectants and their stratum corneum plasticizing effects. It can be concluded from these results that the skin-softening effects of low molecular weight humectants such as glycerin or sodium lactate, which themselves have no effect on the stratum corneum, may be determined by their water-holding capacities.