244 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Reviews of previous work on the permeability of skin to organic materials have been presented by Scheuplein and Blank (8) and Tregear (9). Scheuplein and Blank (8) .::.. report some data on the sorption and diffusion behavior of a series ofn-alkanes and al- cohols in stratum corneum. King and Cassie (10) studied the vapor sorption and diffu- sion of methanol in horn keratin and wool and of ethanol in wool. Acetone, ether, hexane, and other common solvents were found to damage the skin, but the most effective solvents were found to have both polar and nonpolar character and the most potent of all appear to be mixed solvents like chloroform-methanol (2:1) and ether-ethanol (10:1) (1, 4, I 1, 12). In our work, we have assessed the effect of treating guinea pig corneum in the chloro- form-methanol mixture in two ways: (1) comparison of the water vapor sorption and diffusion characteristics in a treated and untreated sample and (2) examination of the treated corneum in the scanning electron microscope (SEM). I Figure 1. Gravirnetric technique for vapor sorption and desorption

THE STRATUM CORNEUM 245 EXPERIMENTAL The gravimetric technique used to study benzene and toluene vapor sorption or de- sorption is shown in Fig. 1. The method employs a recording microbalance and stirred organic solutions to generate the required relative vapor pressure. The comeurn sample was hung on a very thin glass rod, which was suspended from the microbalance terminal (A) into a 500 ml flask containing the organic vapor system (B). The distance between the sample and the organic solution was maintained at about 1 in. To eliminate draughts, the glass rod was shielded by a glass tube, the upper end of which fitted tightly into the base of the microbalance, while the lower end was connected to a ground glass joint. The flask was placed on a small magnetic stirrer (C), which gently rotated a small magnetic bar placed in the solution. The magnetic stirrer was placed on a regular lab jack (D), which could be conveniently raised or lowered to allow quick change of flasks containing solutions of varying vapor pressures. The microbalance was placed on a specially constructed base which was secured to a stand (G). The whole set- up was placed on antivibration base (H) in a constant temperature and humidity room (23øC, 55 per cent RH). In Fig. 1, E and F depict the balance control of a Beckman*- microbalance L-600 and a chart recorder, respectively. This method was also used to examine the sotpriori, desorption, and diffusion of water vapor in keratins using satu- rated salt solutions. The salt solutions used to generate the required relative humidities at 23øC are as follows: Lithium chloride Potassium acetate Sodium iodide Sodium dichromate Copper chloride Ammonium chloride Potassium chromate Ammonium dihydrogen phosphate 11.5 per cent 21.5 per cent 38.0 per cent 54.0 per cent 68.0 per cent 78.0 per cent 87.0 per cent 93.0 per cent A flask containing drierite provided zero humidity. The partial pressure P of the or- ganic solvents was controlled by using solutions of benzene or toluene in hexadecane, the latter being essentially nonvolatile. According to Raoult's law, assuming ideality p = po X where p0 is the vapor pressure of pure benzene or toluene and X is the mole fraction of benzene or toluene in the hexadecane. The sorption experiments were carried out in the relative vapor pressure range from about 0.3 to 0.94. Lower relative vapor pressure conditions were employed in a number of cases. The stratum comeurn samples were initially dried over Drierite to obtain a base weight. The experimental approach was as follows: after drying out the comeurn, it was exposed to the organic vapor atmosphere of a given relative vapor *Beckman Instruments, Irvine, CA.