  Dynamical and statistical behavior 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.
