Monte Carlo Studies of Water Monolayer Clusters on Substrates: Hexagonal AgI

A Monte Carlo procedure is used to study the stability and structure of small water clusters adsorbed on model basal and prism faces of hexagonal AgI. Effective pair potentials for the H ₂ O-AgI interaction [B. N. Hale and J. Kiefer, J. Chem. Phys. 73, 923 (1980)] and the revised central force potentials for the H ₂ O-H ₂ O interaction [F. H. Stillinger and A. Rahman, J. Chem. Phys. 68, 666 (1978)] are used to determine average internal cluster binding energy and average cluster-substrate binding energy for water clusters containing 6, 24, and 44 water molecules at temperatures 240, 265, and 298 K. The results (using an effective substrate point charge of 0.4 e ) give between 5 and 6 kcal/mol per molecule for each of the above average binding energies. The iodine exposed basal AgI face organizes the H ₂ O into five- and six-membered rings centered around the exposed I atoms. The prism face appears less effective in promoting ring structure and gives, in general, lower average internal cluster binding energies. The basal AgI face positions the water molecules (in H ₂ O-substrate high binding sites) about 2.8 A apart while the water-water interactions orient the H ₂ O dipole moments into configurations favorable for hydrogen bonding. The resulting six-membered ring structure is similar to that in ice I _h (crinkled) basal face layers, and appears to be qualitatively the same for all the temperatures studied.