Segregation of hydroxide ions to an ice surface.

Hydroxide ions that are initially buried within an ice film segregate to the ice film surface at elevated temperatures. This process was observed by conducting experiments with an ice film constructed with a bottom H(2)O layer and an upper D(2)O layer, with an excess of hydroxide ions trapped at the H(2)O/D(2)O interface as they were generated by Na hydrolysis. The transport of hydroxide ions from the interfacial layer to the surface was examined as a function of time using a low energy sputtering method. The progress of the H/D exchange reaction in surface water molecules was also monitored with the Cs(+) reactive ion scattering technique. At 90 K, only a small portion of buried hydroxide ions moved to the surface in the form of OD(-) species. This was due to hydroxide transport via proton hopping through a D(2)O layer, 3 BL thick, in the surface region. At 135 K, at which point water self-diffusion is active in the ice film, the majority of the buried hydroxide ions segregated to the surface after ∼1 h. Both OH(-) and OD(-) species were produced at the surface, at an OH(-)/OD(-) population ratio ≥1. Based on kinetic measurements for the transport of OH(-) and OD(-) species and the H/D exchange of surface water molecules, we concluded that the major transport channel for hydroxide ions in this regime is the migration of molecular hydroxide species. H/D exchange reactions also occur between surface hydroxide ions and water molecules. No evidence was observed for the occurrence of the hop-and-turn process at 135 K, although it is known as an important mechanism of proton transport in ice.