(429a) Simulations Reveal Correlated Water Transport in Ion Exchange Membranes

Two models have been proposed for water transport in membranes: solution-diffusion, in which water molecules independently diffuse
down chemical potential gradients; and pore flow, in which water flows collectively in response to pressure gradients.
Previous simulations have explored membrane water transport by pushing water through a polyamide membrane with a graphene piston,
for which achieving steady state and performing long time averages are challenging.

In this work, we simulate water flow in polysulfone anion exchange membranes in periodic boundary conditions,
by applying a body force on the water corresponding to a pressure gradient. Steady state flow and long time averages are thereby easily obtained.
To investigate whether water transport is collective, we examine membranes with increasing water content.
For the same force per molecule, water flows faster in wetter membranes, qualitatively consistent with a pore flow model.
To quantify the pore dimensions, we introduce two novel length scales: one structural,
and one dynamic, based on correlations in the local water velocity; both are consistent with nanoscale collective water transport.

Pressure drops across real membranes may become large enough to deform the pore space.
To study this effect in simulations, we can measure the compressional modulus by "squeezing the sponge'' with external potentials
that act only on the membrane, not the water. With results of this kind, we can assess whether different membrane materials
will stand up to pressure-driven flow, or be compressed.