Enthalpic and Entropic Selectivity of Water and Small Ions in Polyamide Membranes

While polyamide reverse osmosis and nanofiltration membranes have been extensively utilized in water purification and desalination processes, the molecular details governing water and solute permeation in these membranes are not fully understood. In this study, we apply transition-state theory for transmembrane permeation to systematically break down the intrinsic permeabilities of water and small ions in loose and tight polyamide nanofiltration membranes into enthalpic and entropic components using an Eyring-type equation. We analyze trends in these components to elucidate molecular phenomena that induce water-salt, monovalent-divalent, and monovalent-monovalent selectivity at different pH values. Our results suggest that in pores that are either too small or contain an electrostatically repelling mouth, the thermal activation of ions in the form of ion dehydration is less likely, promoting entropically driven selectivity with steric exclusion of hydrated ions. Instead, larger uncharged pores enable ion dehydration, inducing enthalpic selectivity that is driven by differences in the ion hydration properties. We also demonstrate that electrostatic interactions between cations and intrapore carboxyl groups hinder salt permeability, increasing the enthalpic barrier of the transport. Last, permeation tests of monovalent cations in the loose and tight polyamide membranes expose opposite rejection trends that further support the phenomenon of ion dehydration in large subnanopores.

Automated summary

This study breaks down the permeabilities of water and ions in polyamide nanofiltration membranes using transition-state theory and Eyring equation, revealing molecular details governing selectivity and dehydration phenomena. The results suggest that ion dehydration is less likely in small pores with electrostatic repulsion, while larger uncharged pores promote ion dehydration and enthalpic selectivity. Permeation tests of monovalent cations show opposite rejection trends in loose and tight polyamide membranes, supporting the phenomenon of ion dehydration in large subnanopores.