2016 AIChE Annual Meeting
Molecular Dynamics Modeling of Water and NaCl Transport in Sulfonated Poly(arylene ether sulfone) Desalination Membranes
Highly charged polymer membranes have been examined as possible replacements for polyamide membranes in desalination applications. While these membranes may reduce pre and post-processing costs, optimization of separation performance is challenging, due in part to an insufficient understanding of the link between atomic scale chemistry and permeate transport. In this work, a series of 140,000 atom molecular dynamics models are used to study NaCl and water transport through sulfonated poly(arylene ether sulfone) desalination membranes. A state-dependent analysis framework is developed, using custom analysis tools to identify dominant transport mechanisms within the membrane. Salt bridging and Na-sulfonate cluster formation play a key role in mediating Na interactions with the membrane, and increase the binding time of Na to fixed charges on the polymer backbone. Additionally, we find that the bulk diffusion of water, Na, and Cl is subdiffusive within the membrane for time intervals shorter than 1.5 ns. Position-dependent mean squared displacement calculations suggest this behavior could result from intermittent binding interactions of ions with sulfonate groups and slow interpore transport. This result highlights a disconnect between short-timescale molecular interactions and the ultimate bulk diffusion of membrane permeates. Further numerical modeling and more extensive characterization of the effects of membrane morphology are necessary to connect atomistic membrane interactions to the bulk transport of permeates.