Ion−Dipole Interactions Are Asymptotically Unscreened by Water in Dipolar Nanopores, Yielding Patterned Ion Distributions

Abstract

The permeation, rejection, and transport of electrolytes in water-filled nanopores are critical to ion current gating and desalination processes in synthetic porous membranes and the functions of biological ion channels. While the effects of confinement, pore polarizability, and discrete channel charge sites have been much studied, the potentially dramatic impact of dipole-lined synthetic pores on electrolytes has seldom been addressed. Dipole layers naturally occur on the interior surfaces of certain nanopores, leading to intrinsic preference for cations or anions. This preference can be exploited when the membrane surface is functionalized differently from the pore interior or when there are alternating dipolar/nondipolar stretches inside a long pore. The dipole−ion interaction is asymptotically unscreened by water, leading to ionic, charge segregated, insulating behavior that can block ion transport, and potentially novel current−voltage (I−V) characteristics.