Diffusion across a concentration step: Strongly nonmonotonic evolution into thermodynamic equilibrium

Abstract

Dynamical and statistical behaviour of the ionic particles in dissolved salts have long been known, but their hydration shells still raise unsettled questions. We engineered a “diffusion tunnel diode” that is structurally analogous to the well-known Esaki diode, but now concentration gradients serve as generalized voltages and the current means particle flow. In an equipartition sense, the hydrated ions enter a cavity as individual particles and later, upon increase of their concentration therein, they lose water molecules that henceforth are particles of their own. These temporarily attached water molecules thus are the tunnel current analogue. Unlike the original tunnel diode, our negative differential resistance has implications for the second law of thermodynamics, due to thermal effects of changes in the hydration shells.