Effect of size-asymmetric electrolyte on single-file osmosis

Single-file osmosis through a 0.9 nm diameter semipermeable carbon membrane using various size-asymmetric electrolyte solutions is investigated using molecular dynamics simulations. In an uncharged pore, the osmotic flux with a KCl solution is found to be higher than that of in a NaCl solution, i.e., J _KCl>J _NaCl, for same concentration gradient of solute. Mean force and hydration analysis indicate that distinct ion-water and ion-pore molecular interactions cause a differential cation and anion affinity to the pore, leading to the observed size-asymmetric electrolyte dependence on osmotic flux. The water orientation in the pore during osmosis is also related to the ion affinity and exhibits a size-asymmetric electrolyte dependence.