KINETICS OF WATER-VAPOR SORPTION IN PORCINE STRATUM-CORNEUM

The kinetics of water vapor absorption-desorption was studied in the porcine stratum corneum (sc) at 25 degrees C. Dry sc samples were exposed to several relative humidities followed by exposure to dry air, and the change in weight was monitored over time by use of a thermal gravimetric analysis technique. Diffusion coefficients were estimated by simulation and data fit optimization to modifications of the non-steady-state solution of Fick's law. Very good fits of experimental data to theoretical equations were obtained. The stratum corneum-water absorption isotherm was characterized by a linear increase from the origin, extending to about 55% relative humidity (10% water gain), followed by an upward curve where water gain increased further by increasing relative humidity. Water absorption and water desorption in the porcine sc were asymmetrical processes, desorption being the slower one. Water diffusivity for the absorption process was related to sc water content in two phases. In the first (water gain < 9%) diffusion coefficients increased as water concentration increased. In the second (water gain > 9%) diffusion coefficients were independent of water concentration, equal to 1.17 X 10(-10) cm(2)/s. Water diffusivity in the desorption phase was shown to decrease linearly as sc water content decreased. Analysis of the absorption isotherm and of the derived diffusion coefficients suggested at least two forms of water associated with the sc. The first, termed ''firmly bound water'' and characterized by low diffusion coeficients, was most apparent with dry sc and with a sc water gain of up to 10%. The second, most apparent with wet sc of a water content higher than 10% was termed ''loosely bound water'', and was characterized by higher, water concentration-independent, diffusion coefficients.