Effective Ion Exclusion Requires Hydration Shell Stripping

Sulfonated polystyrene membranes exhibit a nanostructure characterized by tortuous, interconnected hydrophilic pores enclosed by a hydrophobic polymer domain. The characteristic pore size plays a crucial role in determining the salt partition coefficients. To directly relate pore size to salt partitioning, we construct a simplified pore space confined between flat sulfonated polystyrene walls. By adjusting the separation between the polymer walls, we vary the pore size. Pores larger than subnanometer exclude ions because of the entropic barrier associated with the intrinsic counterion concentration. However, their concentration within the pore is far from uniform. The concentration at the pore center is low, resulting in a low entropy barrier and salt partition coefficients that are too large for practical ion exclusion applications like desalination and energy storage. Narrow pores, with dimensions smaller than hydrated ions, are required for effective ion exclusion. We find that ions begin to lose their hydration shell about half a nanometer away from the pore wall. Electrostatic interactions near the pore wall stabilize ions even as their hydration shells shrink, suggesting that neutral pores could be more effective for desalination.