Limited ion-ion selectivity of salt-rejecting membranes due to enthalpy-entropy compensation

Water and ion transport in salt-rejecting membranes, such as nanofiltration (NF) and reverse osmosis (RO), are exposed to different hindrance effects that result in selectivity between species. In this study, we explored sys-tematically the hindrance effects that govern water-salt and ion-ion selectivity in NF and RO membranes. More specifically, we measured the permeability of different species at varying temperatures and applied an Eyring-type equation to quantify the enthalpic and entropic barriers for their transport in three types of salt-rejecting membranes. We found that water-salt selectivity is entropically driven, where water molecules gain entropy in the membrane, while the salt ions face a substantial entropic barrier due to their larger size that reduces their possible configurations in the membrane. As the enthalpic barriers of the water and salt did not show a prom-inent difference (with minor effect on the water-salt selectivity), most ion-ion separations were restricted by an enthalpy-entropy compensation (EEC) effect; that is, an ion with a higher entropic barrier experienced a lower enthalpic barrier compared to the competing ion, resulting in minor variations in the free-energy barriers be-tween ions that limit the ion-ion selectivity. We conclude by proposing possible mechanisms that promote EEC during ion transport in salt-rejecting membranes.

Automated summary

This study systematically investigated the hindrance effects on water-salt and ion-ion selectivity in nanofiltration and reverse osmosis membranes. The research found that water-salt selectivity is driven by entropy, while ion-ion selectivity is limited by an enthalpy-entropy compensation effect. Possible mechanisms promoting this compensation effect during ion transport in salt-rejecting membranes were proposed.