The Surface Chemistry of Condensation Nuclei: II. The Preparation of Silver Iodide Free of Hygroscopic Impurities and Its Interaction with Water Vapor

Abstract

Silver iodide has been prepared in finely divided form by the reaction in vacuo between metallic silver and iodine and subsequent treatment with liquid ammonia. The compound thus prepared is considerably more stable toward sunlight than is AgI prepared by precipitation; the latter form contains a small amount of hygroscopic contaminant. Adsorption isotherms of water at 30C have been measured on AgI prepared by the direct reaction and on material prepared by precipitation. The isotherm on the latter material is essentially Type III; this is characteristic of cooperative adsorption on a hydrophobic surface. The isotherm on the salt prepared by the direct reaction is almost linear and does not fit into the Brunauer classification. The amount of water adsorbed per unit surface area at relative pressures exceeding 0.5 is considerably greater on the contaminated AgI. The results are discussed in terms of the surface properties of pure AgI as they affect the nucleation of ice from supercooled water vapor.