Atomistic Simulations of Polymer Crosslinking at Solution Interface for Reverse Osmosis Polyamide Membrane

Reverse osmosis polyamide membranes are of significant importance due to their widespread applications in water purification and desalination processes. The critical properties of the membrane, such as thickness, chemical composition, crosslinking structure, and surface roughness, are sensitive to the conditions of the interfacial polymerization process, posing a remaining challenge for their precise prediction.

In this work, extensive molecular dynamics simulations and experiments were performed to study the polymerization and crosslinking of trimesoyl chloride (TMC) and metaphenylene diamine (MPD) at the interface of water and organic solvent. The simulations employ all-atom models and a distance-based event-driven criterion, successfully presenting the experimental interfacial polymerization process from the initial free distribution of monomers to the complete formation of a crosslinked thin layer of polyamide.

Two distinct organic solvents, namely hexane and cyclohexane, are employed in the simulations to investigate their influence on the diffusion of TMC and the crosslinking process at the solution interface. Various structural and topological properties of the resulting crosslinked membranes are calculated and analyzed, including the degree of polymer crosslinking, density distribution, order parameter, radial distribution function, and pore size distribution. Experimental studies were also conducted to synthesize polyamide membranes by mixing TMC and MPD monomers, each dissolved in their respective solvents of water and hexane. The structural characteristics of the synthesized membranes are investigated through total scattering pair distribution function analysis by synchrotron X-ray diffraction. The simulation results are compared with the experimental observations, facilitating a comprehensive evaluation of the dynamics and topology of the crosslinking process and the formed membranes.