Water adsorption and ice nucleation on silver iodide surfaces

摘要

The potentials and the isosteric heats of water adsorption on the basal planes of β‐AgI were computed, using 1–4‐6–12‐type potentials. The heat ranges from 12.5 to 13 kcal mole-1 at low coverage. Using a simple model, it is shown that the lateral interaction among water molecules adsorbed at identical positions (semilayer) lowers the adsorption potential and makes the molecular orientation more random. The interaction between two different semilayers at their minima allows a further and larger reduction in the potential, by as much as 15%, indicating the formation of isolated islands of adsorbed molecules as the coverage increases. Considering the special feature of the potential on the Ag+ exposed surface which is likely to appear at the beginning of photodecomposition of AgI, an explanation is presented for the mechanism of the activated ice nucleation experimentally observed. The results are applied to explain the tendency for random orientation and easier rotation among adsorbed molecules as the adsorption proceeds. It is shown that this easier rotation acquired by the molecules at larger coverage allows the system to practically avoid the entropy effect in ice nucleation.