Decoupling support-dependent transport profiles from molecular water motion
Abstract
Pressure and concentration profiles from non-equilibrium molecular dynamics (NEMD) simulations are often used to infer water transport mechanisms in dense and swollen polymer membranes. However, the mechanical restraints needed to stabilize simulated membranes can alter these profiles and their interpretation. Here, we test whether restraint-induced differences in pressure and water concentration profiles are accompanied by changes in molecular water dynamics. Crosslinked poly(ethylene glycol) diacrylate (PEGDA) membranes were simulated using three support strategies: a graphene support lattice, frozen membrane atoms near the permeate interface, and homogeneously distributed frozen membrane atoms. The restraints produced markedly different profiles, ranging from solution-diffusion-like to pore-flow-like, while overall water fluxes remained comparable. Molecular water dynamics, however, changed little. Interfacial exchange was strongly bidirectional and far exceeded the net permeation flux, membrane water diffusivities were similar, and complete crossings occurred in both directions. Local water associations were short-lived, and directional velocity correlations decayed rapidly with time and distance. Thus, substantially different pressure and concentration profiles can arise despite similar molecular water dynamics. Such profiles alone are therefore insufficient to identify transport mechanisms in mechanically restrained NEMD simulations. In PEGDA, water motion is dominated by stochastic diffusion with a small net directional bias.