SORPTION AND TRANSPORT OF ETHANOL AND WATER IN POLY(ETHYLENE-CO-VINYL ACETATE) MEMBRANES

The ability to control the fluxes and the relative composition of multiple species permeating across a membrane would be invaluable to the design of controlled release systems. The counter-diffusion of ethanol and water across poly (ethylene-co-vinyl acetate) [P(E-co-VAc)] membranes was investigated as a case study. Ethanol and water fluxes across these membranes were measured from the changes of the refractive indices in the donor and receiver solutions. Ethanol and water solubilities in the membranes were determined by a sorption and desorption method. Ethanol absorption followed a Flory-Huggins isotherm, whereas water absorption exhibited a maximum due to opposing factors between the plasticization of the membrane by ethanol and the driving force of water. Ethanol and water diffusivities in the membranes were calculated from the flux and solubility data. In a 37% vinyl acetate P(E-co-VAc) membrane, ethanol permeability was found to increase exponentially with ethanol activity in the membrane; whereas water permeability decreased with water activity in the membrane. Transport models were developed to estimate ethanol and water fluxes across a 37% vinyl acetate P(E-co-VAc) membrane, given any set of compositions in the donor and receiver solutions. The counterdiffusing fluxes of ethanol and water across this membrane were found to be approximately linearly dependent. Additionally, the model suggested that a 37% P(E-co-VAc) membrane slightly favors water over ethanol, in separating ethanol and water by pervaporation.