Ultrathin Molecular-Layer-by-Layer Polyamide Membranes: Insights from Atomistic Molecular Simulations

In this study, we present an atomistic simulation study of several physicochemical properties of polyamide (PA) membranes formed from interfacial polymerization or from a molecular-layer-by-layer (mLbL) on a silicon wafer. These membranes are composed of meta-phenylenediamine (MPD) and benzene-1,3,5-tricarboxylic acid chloride (TMC) for potential reverse osmosis (RO) applications. The mLbL membrane generation procedure and the force field models were validated, by comparison with available experimental data, for hydrated density, membrane swelling, and pore size distributions of PA membranes formed by interfacial polymerization. Physicochemical properties such as density, free volume, thickness, the degree of cross-linking, atomic compositions, and average molecular orientation (which is relevant for the mLbL membranes) are compared for these different processes. The mLbL membranes are investigated systematically with respect to TMC monomer growth rate per substrate surface area, MPD/TMC ratio, and the number of mLbL deposition cycles. Atomistic simulations show that the mLbL deposition generates membranes with a constant film growth if both the TMC monomer growth rate and MPD/TMC monomer ratio are kept constant. The film growth rate increases with TMC monomer growth rate or MPD/TMC ratio. Furthermore, it was found on one hand that the mLbL membrane density and free volume varies significantly with respect to the TMC monomer growth rate, while on the other hand the degree of cross-linking and the atomic composition varies considerably with the MPD/TMC ratio. Additionally, it was found that both TMC and MPD orient at a tilted angle with respect to the substrate surface, where their angular distribution and average angle orientation depend on both the TMC growth rate and the number of deposition cycles. This study illustrates that molecular simulations can play a crucial role in the understanding of structural properties that can empower the design of the next generation RO membranes created from molecular-layer-by-layer (mLbL) on a silicon wafer.