PARABEN PERMEATION THROUGH MODEL MEMBRANES 441 lO 0 I • I i I , 0.0 .2 .4 .6 .8 1.0 ETHANOL MOLE FRoeT ION Figure 10. Imbibition of solvent as a function of ethanol mole fraction from ethanol-water mixtures. 10-fold over the non-interactive systems from neat ethanol. The diffusivity was also increased, to a lesser degree, from the ethanol-water systems. Imbibed solvent changes the physical-chemical properties of the membrane, allowing a higher membrane solu- bility and reduced segmental interactions (plasticization). Adsorption of methylparaben to filler was also influenced by the imbibition of solvent (Figure 12). Adsorption de- clined abruptly with the introduction of ethanol and then fell off gradually in a linear fashion. A postulated mechanism is direct competition between the imbibed solvent and the less polar paraben for adsorption sites on the filler surface. CONCLUSIONS The flux of parabens through polydimethylsiloxane membranes has been studied uti- lizing a number of solvent systems. With this membrane, water, several polyols, and mixtures of the two behave as non-interactive solvents. This is shown by the fact that flux values from saturated solution, membrane solubilities, and diffusion coefficients were not significantly different despite wide variation in solubility in these vehicles. However, ethanol and ethanol-water mixtures behaved quite differently diffusion coef- ficient and, especially, membrane affinity for paraben increased as the ethanol content was increased. Imbibition of solvent altered the barrier properties of the membrane. Binding to the silica filler was directly related to paraben polarity and was reduced in the presence of vehicles containing ethanol, presumably due to competition. PDMS membranes differ from stratum corneum in composition and structure. Vehicle

442 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 4O A SO 2O 10 0.0 .2 .4 .• .B •.0 ETHANOL MOLE FRAETION Figure 11. Membrane solubility versus ethanol content for methylparaben from saturated ethanol-water mixtures.