Ion Pair Dissociation Dynamics in an Aqueous Premelting Layer

TL;DR

This study uses molecular dynamics simulations to explore how sodium chloride ions behave and dissociate in the unique, layered environment of an aqueous premelting layer at ice interfaces, revealing complex timescales and spatial effects.

Contribution

It provides new insights into ion dissociation dynamics and solvation structures specific to the premelting layer, a previously less understood environment.

Findings

Ion pair dissociation rate varies significantly across the layer.

Ions prefer the middle of the premelting layer due to interface distortion.

Solvation environment is intermediate between ice and liquid water.

Abstract

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.