Rate-Limiting Ion Transport Mechanisms in Nanoporous Polymer Membranes: Dehydration or Electrostatics?

Achieving global water security necessitates the treatment of unconventional and underutilized sources, for which pressure-driven desalination technologies are particularly well-suited. In these technologies, the membrane is the critical component and requires material breakthroughs to realize paradigm-shifting performance. Attempts to improve performance have been inconsequential over the last several decades - a result of our poor understanding of nanofluidic transport. Hence, this research aims to elucidate synthesis - structure - performance relationships in nanoporous membranes, focusing on ion-membrane electrostatics and their role in the properties and resulting performance of the membrane. We first demonstrate the importance of ion-membrane electrostatic interactions toward water-salt selectivity and the intrinsic charge of nanoporous membranes. Our findings indicate that increasing the active charge density within polyamide films can increase their water-salt selectivity fourfold without sacrificing water permeability. We then leverage machine learning to assess the role of molecular-level features in ion-selective transport. Notably, we find that electrostatic interactions are the predominant factor influencing the ion-ion selectivity of monovalent anions in nanoporous cellulose acetate, despite its relatively uncharged state. We contextualize these results to address a prominent question in membrane science: how important is ion (de)hydration in the selectivity of nanoporous polymer membranes? Our work highlights the need for new perspectives on ion dehydration and electrostatic effects - emphasizing the importance of deconvoluting the two mechanisms to improve the design of future membranes for desalination and resource recovery.

Cody Ritt, Massachusetts Institute of Technology