The Effect of Temperature on Nucleation of Condensed Water Phase on the Surface of a beta-AgI Crystal. 2. Formation Work

摘要

In the condensation mechanism of heterogeneous ice formation, water crystallization occurs after a necessary amount of the liquid phase has accumulated on a substrate surface. In this way, the ice-forming activity of the surface is governed by its adsorption ability with respect to water vapor. The Monte Carlo canonical statistical ensemble method has been used to calculate the free energy, entropy, and work of nucleation of a disordered condensed water phase on the surface of crystalline silver iodide and to determine the surface tension. Comparative calculations have been performed at 260 and 320 K for the defect-free surface of a basal face of a crystal. The surface of a beta-AgI crystal is completely covered with a monomolecular film even in unsaturated water vapors. The surface tension at the growing nucleus-substrate interface substantially increases due to the formation of the underlying film, and the growth of the nucleus becomes possible only in a supersaturated vapor. As the vapor density increases, the thickness of the condensed water layer grows, and, at negative Celsius temperatures, conditions are created for its crystallization. The underlying film with pronounced hydrophobic properties hinders nucleation, thereby decreasing the ice-forming activity of the surface in the condensation process. Under these conditions, the observed abnormally high ice-forming activity of silver-iodide aerosol particles may be explained by the presence of numerous crystal defects on the particle surface, with these defects representing channels that provide overcoming the hindering action of the film.