Role of Ion Dehydration in Ion-Ion Selectivity of Dense Membranes.

Fabricating polymeric membranes with ion-specific selectivity has been targeted in recent years to address the growing challenges of water and resource scarcity. Inspired by discoveries of the selectivity mechanisms in biological channels, ion dehydration has been increasingly recognized as a key phenomenon governing the transport and selectivity in dense polymeric membranes and other synthetic nanochannels. However, understanding the molecular details of this phenomenon and leveraging and controlling it to increase the selectivity between ions in state-of-the-art membranes remain elusive. In this Perspective, we discuss the foundations of ion dehydration and explore opportunities to study and leverage this phenomenon for improving ion-ion selectivity in membranes. We first introduce the fundamentals and measurements of ion's hydration properties in solution, distinguishing between static and dynamic hydration properties. Next, we discuss simulation and experimental techniques to study ion dehydration under confinement, highlighting critical knowledge gaps that impede our understanding of this phenomenon. We then discuss effects of ion dehydration on the energy landscape of ion transport and analyze attempts in the literature to improve ion selectivity by promoting dehydration of specific ions. We conclude by proposing research directions to enhance our understanding of ion dehydration and fabricate sustainable and robust membranes with ion-specific selectivity.