Additivity of ionic currents in mixed electrolyte solutions and confined geometries

Abstract

Electrolyte mixtures in aqueous solutions are central to biophysical chemistry and chemical engineering. In many cases, ions are confined to nanoscale volumes with surface electrical charges. Here, we analyze experimentally the additivity of ionic currents across negatively charged conical nanopores in mixed electrolyte solutions. These pores show an asymmetric axial distribution of carboxylic acid groups on the pore surface, which gives rectified current (I)-voltage (V) curves when a single-pore membrane is bathed by two symmetrical aqueous solutions at neutral pH values. We consider KCl, LiCl, NaCl, CsCl, CaCl2, and K2SO4 pure electrolytes as well as the mixed electrolyte systems LiCl + KCl, NaCl + KCl, CsCl + KCl, CaCl2 + KCl, and K2SO4 + KCl for concentrations ranging from 2 to 500 mM and applied voltages between −2 V and 2 V. The difference between the sum of the pure electrolyte currents and the mixed electrolyte current is studied in terms of the electrolyte concentration, the sign of the applied voltage in the current–voltage curves, and the mixed electrolyte type.