Ion uptake and pairing in membranes: The pore model

Ion uptake plays a critical role in membrane separations used for water purification and electro-membrane processes. Although models of ion uptake usually consider free dissociated ions, ion-pairing was suggested to play an important role as well and but its understanding in the context of membrane transport is still insufficient. The paper systematically develops the pore model of ion-pairing in membranes, which considers the membrane as a micro-heterogeneous system, composed of interspersed water-rich domains (pores accommodating ions) and a low-dielectric matrix. It is shown that this picture significantly departs from the predictions of the primitive model of the membrane as a uniform dielectric, in which dielectric exclusion and pairing of ions are rigidly related thus pairing of mobile ions is always negligible. Within the pore model, the effect of the ion-pairing constant enhanced by the low-dielectric properties of the matrix may outweigh the opposite effect of dielectric exclusion thereby mobile ions may be present within the membrane mainly as ion-pairs rather than free ions. We discuss various implications of this result for salt and ion uptake, transport, and conductance in neutral and charged membranes and outline needs for further research towards predictive modeling. • Ion pairing and uptake analyzed using the model of interspersed water and matrix domains. • Ion-pair to free ion ratio in pores is enhanced compared with a dielectric continuum model. • Analysis indicates pairing may dominate salt uptake and eliminate Donnan exclusion. • Pairing may reduce effective fixed charge and suppress ion mobility, transport, and conductance.