A Molecular Dynamics Model of Interfacial Polymerization: Growth and Properties of Polyamide Membranes

We have developed a novel molecular dynamics technique for simulating interfacial polymerization (IP), especially as it relates to the growth of thin films for reverse osmosis, nanofiltration, and ion-exchange applications. Through the careful modification and extension of the OPLS-AA force-field, 'reactivity' is implemented via special non-bonded potentials such that the kinetics and thermodynamics of 'reactions' can be tuned by adjusting the potential parameters. Combining this reaction scheme with external fields allows IP to be simulated in a physically-meaningful and efficient manner, in contrast to the ad-hoc, heuristic approaches used previously. We apply the technique to studying the growth and performance of the well-known polyamide membranes that are ubiquitous in reverse osmosis applications, finding that dense films can be grown with topologies and hydration levels that agree well with experiment. Importantly, the properties of these films arise organically from the IP model itself, rather than from a forced conformity to target values. Synthesis conditions can be systematically varied to produce films with different properties, yielding insight into the relationships between experimental parameters, structure, and performance. The IP model is readily extensible to any polycondensation chemistry, allowing for the connection of monomer-level properties to film performance and thus the bottom-up design of membranes for technological applications.