Dissipation of a volatile compound or mixture from the skin surface after topical application involves both diffusion and evaporation. This report presents a detailed test of a previously described first-order kinetic approach to modeling this problem. Modified Franz diffusion cells fitted with a vapor trap were used to obtain absorption and evaporation data for benzyl alcohol (1% solution in ethanol) after application to human skin in vitro. Airflow over the skin surface (upsilon) was controlled in the experiment and accounted for in the model by allowing the evaporation rate constant(s) to vary as a function of upsilon. A linear dependence was found over the working range of the system, 10-100 mL/min. Three kinetic models were developed, all of which satisfactorily correlated cumulative absorption and evaporation results over the full range of upsilon (n = 120, s = 4-5%, r2 = 0.98-0.99). One of these was the model presented previously, in which all dissipation occurs from a single skin compartment. However, more details of the evaporation and absorption profiles could be accounted for by means of two-compartment models that explicitly consider the surface film present in the early stages post-application. The latter models seem to be better candidates for describing the time evolution of the volatile mixture evolving from the skin surface after topical application of, e.g., a complex fragrance or perfume.
Copyright 2003 Wiley-Liss, Inc. and the American Pharmacists Association