Fast and slow proton transfer in ice: the role of the quasi-liquid layer and hydrogen-bond network.

The proton mobility in ice is studied through molecular dynamics simulations carried out with a newly developed ab initio-based reactive force field, aMS-EVB3/ice. The analysis of both structural and dynamical properties of protonated ice as a function of temperature indicates that the mobility of excess protons at the surface is largely suppressed, with protons becoming essentially immobile at temperatures below 200 K. In contrast, fast proton transfer/transport can exist in bulk ice Ih at low temperature through connected regions of the proton-disordered hydrogen-bond network. Based on the simulation results, it is shown that the mechanisms associated with proton transfer/transport in both bulk and interfacial regions of ice are largely dependent on the local hydrogen-bond structure surrounding the charge defect. A molecular-level picture of the mechanisms responsible for proton transfer/transport in ice is then developed and used to interpret the available experimental data.