Effect of Organic Cation Adsorption on Ion-Transport Selectivity in a Cation-Permselective Nanopore Membrane

A key knowledge gap in the emerging field of nanofluidics concerns how the ionic composition and ion-transport properties of a nanoconfined solution differ from those of a contacting bulk solution. We and others have been using potentiometric concentration cells, where a nanopore or nanotube membrane separates salt solutions of differing concentrations to explore this issue. The membranes studied contained a fixed pore/tube wall anionic charge, which ideally would prohibit anions and salt from entering the pore/tube-confined solution. We have been investigating experimental conditions that allow for this ideally permselective cation state to be achieved. Results of potentiometric investigations of a polymeric nanopore membrane (10 ± 2 nm-diameter pores) with anionic charge due to carbonate are presented here. While studies of this type have been reported using alkaline metal and alkaline earth cations, there have been no analogous studies using organic cations. This paper uses a homologous series of tetraalkylammonium ions to address this knowledge gap. The key result is that, in contrast to the inorganic cations, the ideal cation-permselective state could not be obtained under any experimental conditions for the organic cations. We propose that this is because these hydrophobic cations adsorb onto the polymeric pore walls. This makes ideality impossible because each adsorbed alkylammonium must bring a charge-balancing anion, Cl^-, with it into the nanopore solution. The alkylammonium adsorption that occurred was confirmed and quantified by using surface contact angle measurements.