Cation–Ligand Interactions Dictate Salt Partitioning and Diffusivity in Ligand-Functionalized Polymer Membranes
journal contributionposted on 2022-03-04, 20:33 authored by Everett S. Zofchak, Zidan Zhang, Nico Marioni, Tyler J. Duncan, Harnoor S. Sachar, Alyssa Chamseddine, Benny D. Freeman, Venkat Ganesan
Membranes are an attractive alternative to current thermal separations due to their scalability and energy efficiency in desalinating water. Unfortunately, many of the conventional membrane materials available today are unable to differentiate between ionic solutes, especially alkali cations, compromising their use in ion–ion separations. Inspired by the ion-specific interactions exhibited by biological ion channels, recent research efforts have focused on synthesizing and characterizing new polymeric materials that incorporate ligands into polymer networks to bias solubility and/or diffusivity of one cationic species over another. Despite these efforts, little is known about the influence of incorporating ligands into polymer membranes on solubility and diffusivity of the complexing species. In this study, we first build a qualitative model of salt partitioning, diffusivity, and permeability in generic cation-complexing ligand-functionalized polymer membranes. Next, to validate our model and hypotheses, we perform atomistic molecular dynamics simulations of a 12-crown-4-functionalized membrane in the presence of alkali halide salts at low concentration. Generally, cation complexation enhances cation solubility but decreases diffusivity. Interestingly, the reduction in diffusivity is predicted to be larger than the enhancement in solubility for materials which operate by the mechanisms proposed in our physical picture, ultimately resulting in a reduction in the permeability of the selectively complexing ion.
specific interactions exhibitedespecially alkali cationsalkali halide saltsone cationic speciesbiological ion channelsrecent research effortsselectively complexing ionfunctionalized polymer membranespolymer membranescomplexing speciesfunctionalized membranepolymer networksultimately resultingsalt partitioningphysical picturemechanisms proposedlow concentrationionic solutesincorporating ligandsincorporate ligandsgeneric cationfirst buildenergy efficiencydesalinating waterattractive alternative