Studies of H₂O on ß-AgI Surfaces: An Effective Pair Potential Model

Effective pair potential functions are used to study the adsorption of a water molecule on surfaces of β-AgI. The water molecule is represented by a rigid point charge ST-2 model and the AgI substrate by an array of point atoms with effective charge ±0.6 e , Lennard-Jones cores, and ionic polarizabilities. Maximal binding energy surfaces and optimal H ₂ O configurations are generated for the water molecule adsorbed on the rigid and unrelaxed basal and prism AgI faces. Adsorption of the H ₂ O above a two layer ledge, an iodine vacancy, and an H ₂ O trapped in the vacancy are modeled for the iodine basal face and compared with results for the smooth substrates. These studies indicate the H ₂ O adsorption is favored at "interstitial" sites where no substrate atoms lie directly below either in the first or second layer. The prism face is found to attract the water molecule more strongly and provide larger energy barriers to surface diffusion. The model predicts maximal binding energies of 20 and 16 kcal/mole for the adsorbed H ₂ O on the preferred prism and basal face sites, respectively. The iodine vacancy produces an adsorption site with optimal binding energy 19 kcal/mole and the two layer ledge results in an extended region of strong binding sites of order 20 kcal/mole. The water molecule trapped in the I vacancy creates a surface charge "defect" around which additional water molecules could cluster with dipole moments in configurations favorable for H ₂ O-H ₂ O bonding.