Proton transfer and H/D isotopic exchange of water molecules mediated by hydroxide ions on ice film surfaces

The effect of hydroxide ions on proton transfer and H/D isotopic exchange of water molecules was examined at the surface of amorphous ice films at temperatures of 92-140 K. Excess hydroxide ions were provided onto a D2O-ice film by the hydrolysis of Na atoms, and H2O was adsorbed onto the surface for a submonolayer coverage. The H/D isotopic exchange between H2O and D2O molecules on the ice film surface was monitored as a function of reaction time and temperature by using the techniques of reactive ion scattering and low-energy sputtering. The result was compared with that obtained on a hydroxide-free ice film. At a temperature of 92 K, proton transfer occurred from water mostly to adjacent hydroxide ions. The proton transfer distance and the H/D exchange reaction rate increased with increase in temperature above 105 K. The H/D exchange reaction propagated to several water molecules on the surface at 100-120 K. Kinetic measurement in this temperature range deduced the Arrhenius activation energy for the reaction, E-a=9.6 +/- 2.0 kJ mol(-1). The study shows that hydroxide ions promote the H/D exchange reaction on the ice surface compared with that on a hydroxide-free ice surface, but the promotion effect is moderate and the H/D exchange occurs on a substantial energy barrier. It is suggested that the stabilization of hydroxide ions at the ice surface produces an energy barrier for the proton transfer.