Local ion environment in polyamide membranes revealed by molecular dynamics

TL;DR

This study uses molecular dynamics simulations to investigate the local ion environment within polyamide reverse osmosis membranes, revealing how ions interact with the polymer and how these interactions influence ion mobility and solvation structure.

Contribution

The paper provides detailed atomic-level insights into ion coordination and binding in hydrated polyamide membranes, highlighting differences from solution and introducing new measures for solvation analysis.

Findings

Ion-oxygen distances are similar in membrane and water.

Coordination number decreases due to solvation shell shifts.

Cations bind tightly to carboxylate and amide oxygens, affecting mobility.

Abstract

In reverse osmosis (RO) and nanofiltration (NF) membranes, the polymer structure and interactions with solvent and solutes dictate the permeability and selectivity. However, these interactions have not been fully characterized within hydrated polymer membranes. In this study, we elucidate the local atomic neighborhood around ions within a RO membrane using molecular dynamics (MD). We built a MD model of a RO membrane closely following experimental synthesis and performed long time scale simulations of ions moving within the polymer. We find that the ion-oxygen nearest neighbor distance within the membrane is essentially the same as in solution, indicating that ions coordinate similarly in the confined membrane as in water. However, we do find that the average coordination number decreases in the polymer, which we attribute primarily to shifting the outer portion of the solvation shell beyond the cutoff, rather than being entirely stripped away. We find that cations bind tightly to both the carboxylate and amide oxygen atoms within the membrane. Even in ionized membranes, binding to amide oxygen atoms appears to play a substantial role in hindering ion mobility. Finally, we find that commonly used measures of ionic solvation structure such as coordination numbers do not fully capture the solvation structure, and we explore other measures such as the chemical composition of the nearest neighbors and the radial distribution function.