474 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 3 000 I 000 • 300 E -- •4 lOCI -- o E $0 .•_ o CaCI 2 IO --© NH4NO: • ß I t I I 40 60 80 1 O0 % rh Figure 2. Elastic modulus vs relative humidity for solvent extracted stratum comeurn (C) in the vapour phase (D) in saturated solutions of the salts shown. .... Stratum ___ corneum Powder funnel InsIron clamps Clamp Saturated sol solution --Rubber bung Figure 3. Experimental arrangement for obtaining modulus values after equilibration in saturated salt solution.

EFFECT OF SALT SOLUTIONS ON STRATUM CORNEUM 475 previously described (1). Included in Fig. 1 for comparison is the curve (A) previously obtained (1), of the elastic modulus of stratum corneum as a function of relative humidity in the vapour phase. The elastic modulus of the corneum in a saturated sucrose solution which has an equivalent relative humidity of 85,2/0 (13) falls on this curve. Fig. 2 shows similar data for stratum corneum which had previously been exhaustively extracted with chloroform/methanol (2: 1, v: v) and then with water (2). This technique has been shown to remove most lipid and water-soluble hygroscopic material from the corneum leaving behind mainly proteinaceous components. The relatively high values recorded for the modulus in solutions of K:COa and CaCI: (Fig. 2) were observed to be due to the inability of these viscous solutions to wet the solvent-extracted corneum. DISCUSSION The main fact to emerge from the data in Figs. 1 and 2 is that in most cases modulus values in salt solutions are significantly less than those ob- tained when corneum strips are equilibrated in the vapour phase. Since at a particular relative humidity the water activity is the same in the vapour as in the salt solution the lower modulus values observed must reflect the effect of the salts. At this initial stage of the investigation, salts were selected with a view to creating as wide a range of water activities as possible and as a result the number of different ions investigated is relatively small. Consequently, it would be incorrect to rule out a specific ion effect (i.e. dependence of modulus on ion charge, size etc.). However, it was possible to determine the significance of the total number of ions present in the various solutions no correlation could be found between this and the lowering of the modulus [EA-EB at rh values of the salts studied (Fig. 1)]. This fact, combined with the regular variation of modulus with rh (Figs. 1 and 2) in the solutions considered so far, suggests that the water activity is still the overriding factor in determining the corneum modulus. It has been suggested (2) that the role of the naturally-occurring hygro- scopic materials in the corneum is to disrupt the protein matrix owing to their location between protein chain segments (similar arguments can be invoked for polysaccharide-polysaccharide or polysaccharide-protein interactions if these prove to be important). This leads to increased protein hydration (as compared with the situation in which the hygroscopics are absent) and a consequent reduction in modulus (2). It is known that high