Investigating the Molecular-Level Influence of Surfactants on Polyamide Reverse Osmosis Membrane Formation

Most thin-film composite reverse osmosis (RO) membranes are formed via interfacial polymerization (IP) between m-phenylenediamine (MPD) and trimesoyl chloride (TMC), yet controlling their structure and selectivity remains challenging. Surfactants can regulate this process, highlighting the need to understand the factors behind MPD transport and its interactions with surfactants. In this study, we provide a detailed molecular-level analysis by first conducting electronic structure calculations to investigate 17 surfactant/counterion combinations. Based on binding energies and partition coefficients, we select four surfactants for further interfacial MD simulations and eventually examine two surfactants in TMC-MPD cross-linking simulations, followed by structural characterization of the polymer membranes. Our results identify key molecular factors influencing membrane synthesis: electrostatic interactions (which affect surfactant binding energy with MPD and their orientation) and MPD partitioning into the organic phase (which impacts its stability at the interface). These factors dictate void connectivity through the membrane and may influence membrane performance, e.g., water permeance.