Polymeric membranes are widely used in separation processes. The distribution and transport of ionic species through these membranes controls the performance of these membranes and depends on factors like surface charge density, pore morphology, chemical structure and ionic species present in the fluid. Manning’s counter-ion condensation model is one of the simplest and most widely used activity coefficient models used to estimate the ion activity coefficients. However, it has been shown to work only for a limited range of ion concentrations. At low ion concentrations the model vastly over-predicts the ion activity coefficients. In the current work we show that this discrepancy is resolved if the variation in dielectric permittivity of the fluid medium with electric field strength is taken into account. A semi-analytical model is presented that accounts for the variation in the dielectric permittivity of water in the confined pore space of a charged polymer membrane. It is shown that such a model accurately predicts the ion activity in the membrane over a wide range of ion concentrations.

Effect of dielectric saturation on the ion activities in membranes

2019 Fall National Meeting

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