Benchmarking Forcefields for Molecular Dynamics Simulations of Polyamide-Based Reverse-Osmosis Membranes

Abstract

Molecular dynamics simulations offer unique insights about solvent and solute transport through the active layer of reverse-osmosis membranes at sub-nanometer to nanometer length scales. Over the last two decades, several cross-linked polyamide membranes, formed by trimesoyl chloride and m-phenylenediamine, have been simulated with different forcefields and water models. However, a clear rationale for choosing a forcefield-water combination is missing. In this work, the accuracy of eleven forcefield-water combinations is evaluated by a direct comparison of properties against experimentally synthesized polyamide membranes with similar chemical compositions. Overall, six forcefields and three water models are compared. The polyamide membranes are simulated in equilibrium in both dry and hydrated states, as well as in a non-equilibrium reverse osmosis. The best-performing forcefields predicted the experimental pure water permeability of 3D printed polyamide membranes within a 95% confidence interval. Finally, these models were used to elucidate water transport mechanisms for a range of desalination conditions. In high pressure pure-water permeance, dense interconnected free volume regions (pores) were observed in the membrane throughout the simulation that connected feed- and permeate-side. The tortuosity of the shortest path through the free volume regions of the membrane decreased with increase in pressure suggesting pressure-driven restructuring. The desalination studies demonstrated increased salt partitioning with feed-side pressure, however, at very high pressures partitioning decreased due to membrane compaction.

Keywords: Reverse osmosis, Desalination, molecular dynamics, water transport, pore analysis