Stoichiometric effect on the structural transformation and spatial variation of polyamide reverse osmosis membranes: A molecular dynamics study

Reverse osmosis based on polyamide membranes is currently the leading technology for desalination and water purification. However, the spatial variation of polyamide architecture, especially regarding stoichiometric criteria at the real reaction interface during membrane fabrication, is poorly understood. Here we conducted equilibrium molecular dynamic simulations, mimicking the practical construction of polyamide matrices using a variety of monomer component stoichiometries, to investigate resulting differences in both the morphological transformation and mass transport through the polymer network. Our simulations indicated that an equal stoichiometry of the reactive functional groups predominantly resulted in favorable polyamide matrix qualities such as a high crosslinking density, uniform pore size distribution, and adequate layer compactness. Despite the thickness-dependent water diffusion across the active layer, an analysis of polyamide membrane density revealed depth heterogeneity, particularly for membranes constructed under unequal concentrations. Consequently, uncontrollable defects are likely to be created, as evidenced by the partial penetration of Na+ across such membranes. In addition, the residual functional groups in the polyamide matrices (-COOH and -NH2) influenced the transport behavior of ions near the membrane surface. Overall, our simulations provide a visual platform for understanding the stoichiometry-induced relationship between membrane structure and performance, which may guide the future design of defect-free and scaling-resistant reverse osmosis membranes.