DDD Conference

Dr. Razi Epsztein

Ion Dehydration as a Key Phenomenon to Achieve Selectivity in Nanofiltration Membranes

Technion – Israel Institute of Technology, Israel

 

Ion transport in salt-rejecting membranes, such as nanofiltration (NF) and reverse osmosis (RO), is exposed to different hindrance effects that result in selectivity between species. Among these effects, ion dehydration due to the pore confinement has been highlighted in numerous experimental and simulation works as a key phenomenon to achieve selectivity between similarly sized and charged ions. In a series of pressure-driven and diffusion-only experiments, we provide important indications of ion dehydration in NF membranes. We show that this phenomenon governs both partitioning into the membrane and intrapore diffusion, where in general, ions of weaker hydration (e.g., cesium) show a “stickier” behavior that promotes their partition into the membrane but hinders their translocation inside the pore compared to more strongly hydrated ions (e.g., lithium). We also connect ion dehydration with enthalpy-entropy compensation effects during the transport that limit the ion-ion selectivity observed; that is, strongly hydrated ions experience higher enthalpic barrier due the higher energy barrier associated with the removal of their bound water molecules but face a lower entropic barrier due to their smaller bare size that allows them to access more pores in the membrane when dehydration is feasible. Overall, our results highlight the role of ion dehydration in transmembrane permeation and invite future research to promote dehydration in a more selective manner.

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