A variety of analytical models have been used in the literature to describe molecule transport through the polyamide active layers of reverse osmosis (RO) membranes. These analytical models include formulations positing a solution-diffusion mechanism, a pore-flow mechanism, and/or hindered versions of these two mechanisms (e.g., friction, steric hindrance, etc.). All of these models have been shown to adequately describe experimental permeation data for RO membranes. However, given the lack of experimental procedures to assess transport mechanisms at the nano- or molecular scale, it has been difficult to arrive at an incontrovertible conclusion on which analytical model best describes actual transport mechanisms. Towards elucidating dominant molecule transport mechanisms, molecular dynamics (MD) simulations enable the study of molecule transport through polyamide films at the nano- and molecular scale. At present, MD simulations of polyamide films can be performed much more reliably than a decade ago since important quantitative information on polyamide properties (e.g., density, charge, crosslinking degree) has become available, therefore enabling validation that simulated polyamide ‘domains’ approximate those fabricated in the lab. In this work, we combine the evaluation of experimental RO permeation data via analytical transport models with MD simulations of representative polyamide domains to obtain insights on the mechanisms of molecule transport through RO membranes and identify steps toward advancing analytical models.

Mechanisms of molecule transport through polyamide reverse osmosis membranes: Combined lessons from analytical and atomistic modeling

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