MICROCALORIMETRY OF THE ADSORPTION OF WATER VAPOR ON SODIUM CHLORIDE

Abstract

Using a microcalorimeter of the Calvet type, measurements have been made of the heats of adsorption of water vapor on microcrystals of sodium chloride, the rate of heat production being followed as a function of time. With crystals of surface areas between 1 and 15 square meters per gram it was found that three distinct processes occurred, in the following order: (1) simple adsorption of water on the surface, (2) decrease in the surface area, and (3) penetration of water into the lattice. During the second stage no adsorption could be detected. On crystals with larger surface areas there was considerable overlapping of the three phenomena, but the general mechanisms appear to be the same.