Computer Simulation of the Initial Stage of Water Vapor Nucleation on a Silver Iodide Crystal Surface: 1. Microstructure

Abstract

The nucleation of water vapors on the surface of a fragment of silver iodide crystal is simulated by the Monte Carlo method under the conditions similar to natural conditions in a humid atmosphere. A stable monolayer island of water molecules with clearly pronounced features of hexagonal symmetry and low orientational order is formed at the initial stage, when the vapor pressure is still lower than the saturating pressure. The island readily grows over the surface and, in the unsaturated vapor, does not grow in the direction perpendicular to the surface. The formed monolayer represents a substrate for further growth of a condensed phase and, eventually, is responsible for the mechanism of nucleation on the crystal surface. Water molecules are held by the substrate mainly owing to the directional electrostatic interaction between the negatively charged oxygen atoms and positively charged silver ions. The interaction with iodine ions lowers the binding of the island (nucleus) and the substrate. A point defect in the form of an extra ion on the surface does not change the planar shape of the nucleus and virtually does not distort its hexagonal structure. Indirect experimental data supporting the formation of a water monolayer at the stage preceding nucleation, as well as the data of observations indicating the important role of defects on a crystal surface, are reported.