Preferential sorption in ion-exchange pervaporation membranes: Sorption of water-ethanol mixture by sodium polyethylene sulphonate

Sorption of water and ethanol from aqueous ethanol solutions by sulphonated polyethylene membrane is studied both theoretically and experimentally, particular attention being paid to the effect of ionogenic-group content (capacity) on the sorption and sorption selectivity. A theoretical model is proposed based on models of salting-out and swelling of ion-exchangers. Examination of the experimental results obtained using the isopiestic method reveals a satisfactory agreement with the model, i.e. with a modified BET equation for sorption of water and a modified Setschenow equation for salting-out. Based on the results of this study, the following conclusions may be drawn: sorption selectivity toward water increases as capacity increases; the per equivalent uptake of water is determined primarily by water activity in the outside solution and the presence of ethanol plays only a minor role; the osmotic effect associated with deformation of the matrix has a negligible influence on selectivity. The model makes it possible to estimate sorption selectivity and may have applications in the calculation of pervaporation membrane performance. The sorption of water by the polymer was found to be far more pressure-dependent than that expected from thermodynamics, presumably due to the nonelastic behavior of the noncrosslinked polyethylene matrix.