Cellulose acetate membranes exhibit exceptional monovalent to divalent cation selectivities

Salt transport properties of cellulose acetate membranes are reported for a series of chloride salts with monovalent and divalent cations (LiCl, NaCl, MgCl2, CaCl2). Measurements include salt permeability and sorption, with diffusivity values calculated from the permeability and sorption results. We report an exceptionally high LiCl/MgCl2 selectivity of 750:1. Salts with similar valence (LiCl and NaCl; MgCl2 and CaCl2) have similar transport properties. The high monovalent/divalent selectivity arises from differences in both sorption and diffusion, with a LiCl/MgCl2 solubility selectivity of about 11 and a diffusivity selectivity of about 70. Atomistic molecular dynamics simulations show that ions tend to reside in isolated clusters of water. Increasing ion charge strengthens ion–water interactions relative to ion–polymer interactions, explaining the reduced sorption of divalent ions. Diffusion of ions through the membrane occurs via hop-like motion between water clusters. Lithium diffuses faster than magnesium due to weaker ion–water coordination for lithium, which allows for greater mobility within water clusters and more frequent hopping. Overall, our atomistic simulations suggest that the high LiCl/MgCl2 selectivity is linked to cellulose acetate’s high water/salt selectivity and is a consequence of low water content and relatively uniform water distribution. [Display omitted] •Cellulose acetate membranes display high monovalent to divalent cation selectivities.•Selectivity arises due to differences in both salt diffusion and sorption.•Molecular dynamics simulations probe the experimental trends’ origins.