Solvent interaction with polydimethylsiloxane membranes and its effects on benzocaine solubility and diffusion.

The effect of polar solvents and polar cosolvent mixtures on the transport properties of benzocaine in polydimethylsiloxane (PDMS) was studied. Methanol, ethanol, n-propanol, and n-butanol, as well as aqueous cosolvent mixtures of each n-alkanol, were used as vehicles for benzocaine. A constant activity gradient was maintained in all diffusion studies, with the membrane exposed to saturated donor suspensions of drug, and sink conditions maintained in the receiver. In spite of the constant activity gradient, steady-state benzocaine flux was substantially enhanced with increasing n-alkanol volume fraction and reached a maximum for the pure n-alkanol in each case. At any given composition, the degree of benzocaine flux enhancement generally increased with n-alkanol carbon number. In terms of the appropriate Fick's first law expression for this system, these observations were attributed to simultaneous changes in benzocaine concentration within the PDMS membrane, the diffusion coefficient of benzocaine in PDMS, fillerless membrane volume fraction, tortuosity, and the membrane thickness. These parameters were in turn correlated with the cosolvent composition in contact with the membrane. Both membrane solubility and diffusion coefficient were found to increase substantially, but decreases in tortuosity and increases in fillerless membrane volume fraction and membrane thickness were minor.