  Using molecular dynamics simulations and methods of importance sampling, we
study the thermodynamics and dynamics of sodium chloride in the aqueous
premelting layer formed spontaneously at the interface between ice and its
vapor. We uncover a hierarchy of timescales that characterize the relaxation
dynamics of this system, spanning the picoseconds of ionic motion to the
10s-100s of nanoseconds associated with fluctuations of the liquid-crystal
interface in their presence. We find that ions distort both local interfaces,
incurring restoring forces that result in the ions preferentially residing in
the middle of the layer. While ion pair dissociation is thermodynamically
favorable, these structural and dynamic effects cause its rate to vary by over
an order of magnitude through the layer, with a maximum rate significantly
depressed from the corresponding bulk value. The solvation environment of ions
in the premelting layer is distinct from that in a bulk liquid, being dominated
by slow reorganization of water molecules and a water structure intermediate
between ice and its melt.
