  We have used molecular simulation and methods of importance sampling to study
the thermodynamics and kinetics of ionic charge separation at a liquid
water-metal interface. We have considered this process using canonical examples
of two different classes of ions: a simple alkali-halide pair, Na$^+$I$^-$, or
classical ions, and the products of water autoionization, H$_3$O$^+$OH$^-$, or
water ions. We find that for both ion classes, the microscopic mechanism of
charge separation, including water's collective role in the process, is
conserved between the bulk liquid and the electrode interface. Despite this,
the thermodynamic and kinetic details of the process differ between these two
environments in a way that depends on ion type. In the case of the classical
ion pairs, a higher free energy barrier to charge separation and a smaller flux
over that barrier at the interface, results in a rate of dissociation that is
40x slower relative to the bulk. For water ions, a slightly higher free energy
barrier is offset by a higher flux over the barrier from longer lived hydrogen
bonding patters at the interface, resulting in a rate of association that is
similar both at and away from the interface. We find that these differences in
rates and stabilities of charge separation are due to the altered ability of
water to solvate and reorganize in the vicinity of the metal interface.
