(613a) Understanding Energy Barriers to Transport in Desalination Membranes

Major efforts in recent years have been directed towards understanding the molecular determinants of transport in reverse osmosis and nanofiltration membranes. Transition-state theory is an increasingly common approach to explore mechanisms of transmembrane permeation with molecular details. This theoretical framework relates molecular jumps to free energy barriers to membrane transport. However, most investigations treat all these free energy barriers in the membrane as equal, which does not isolate the associated molecular mechanisms. In this study, we expand the transition-state theory framework to include distributions of free energy barriers to better relate individual molecular mechanisms to the overall membrane permeability. We develop a novel expression relating individual barriers to experimentally observable free energy barriers. We show that experimentally observed energy barriers must be interpreted in terms of the underlying molecular barriers, and naive interpretations of these effective barriers can lead to incorrect assumptions about the transport mechanisms within polymeric membranes. Additionally, we use molecular simulations to observe ion and water transport within polyamide reverse osmosis membranes in order to identify possible mechanisms associated with individual energy barriers. Beyond size exclusion barriers, we also observe hopping-entrapment mechanisms for ions, where ions show little motion in local voids but larger jumps between them, and analyze the contribution of these barriers to overall transport.