ADSORPTION OF WATER ON ALKALI-HALIDE SURFACES STUDIED BY SCANNING POLARIZATION FORCE MICROSCOPY

The adsorption of water on several alkali halide surfaces was studied using scanning polarization force microscopy. Water adsorption leads t o an overall increase of surface potential and ionic mobility. At a cr itical humidity that is characteristic of each salt (NaCl, KCI, KBr, a nd KI), important changes in the rate of increase of the surface poten tial and ionic mobility occurred. Topographical changes occurred as we ll, in the form of step motion. These were observed to occur at a fast rate above the critical point, while little step motion occurred at l ower humidity. Dissolution of the crystal (deliquescence) is observed at high humidity. Below the critical humidity, contact potential image s indicate that preferential solvation of cations occurs at steps prod ucing a large enhancement of the step contrast in the images.