Cation Permselectivity Through Synthetic Nanopore Membranes: The Role of Surface Charge

Thomas T. Volta, Stevie N. Walters, Charles R. Martin

University of Florida

For decades, multiporous polymer membranes have been used as model systems for studying selective ion transport. Permselectivity is an important transport phenomenon observed in these membranes. The selectivity typically occurs due to charged groups on the membrane face and pore walls. In this work, commercially available track-etched membranes are gold plated using an electroless deposition technique. By varying gold plating time, gold membranes can be reliably produced with nanopores as small as 2 nm. When exposed to a chloride salt, these membranes could display ideal cation permselectivity due to adsorbed chloride anions. Permselectivity of gold nanotube membranes with varying diameters were explored potentiometrically using a varying concentration cell. We could then determine the effect of diameter and cation on the permselectivity of these membranes. A new double-layer overlap model is proposed here to explain the extent of cation permselectivity. Ion-specific effects on the permselectivity of these membranes occur and are drastic under confinement. These effects cause a pathway for anion and salt transport which explains lower than ideal transference numbers for some cations. Using well-known gold thiol chemistry, it is possible to chemisorb any functional group of interest. By adding a carboxylate group to the pore wall, cation permselectivity significantly changed when compared to chloride. This effect can be explained due to ion-pair formation between certain cations and the carboxylate group. The difference in permselectivity between the functional groups will be discussed.