What is the influence of ion aggregation and counterion condensation on salt transport in ion exchange membranes?

Concerted motions of ions arising due to ion aggregation and counterion condensation are often neglected when interpreting salt transport in ion exchange membranes. In this study, we apply the Onsager transport framework to atomistic molecular dynamics simulations to study the impact of such physics on salt transport properties in a model cation exchange membrane as a function of membrane water volume fraction, sulfonation fraction, and cation identity. Cations and anions, distinct cations, and distinct anions tend to move in the same direction due to the formation and resulting motion of ionic aggregates. Such motions, in general, increase the salt diffusion coefficient and decrease the conductivity relative to “ideal” values which envision the ions to move independently of each other. In contrast, condensation-induced slowdown inhibits the motion of ionic aggregates since the condensed counterions serve as a connection between aggregates and the immobile polymer matrix. Such effects decrease deviations of salt transport properties from the ideal assumption. We demonstrate that experimentally extracted cation/anion self-diffusion coefficients can exhibit significant errors if correlated motions arising from ion aggregation are neglected.