An X-Ray Diffractometric Study of Hygroscopic Process of Alkali Halides

Abstract

Surface structure changes of alkali halides crystals on adsorbing water vapor molecules in multilayer have been studied by means of X-ray powder diffraction. Effects of particle size and some surface properties of the salts on the intensities of X-ray diffraction by the concerned samples were examined. Three salts, KI, KBr and KC1, were adopted as samples, and their evaporated films, fine crystals and powders ground to 200 mesh were prepared and used. The multilayer adsorption of water molecules on the salt results in a formation of the hydrated ions. At that time it is found that both the su-rface state and the X-ray diffraction intensity change and caking of the salt appears. The intensity of X-ray diffraction by the salt with low crystallinity, such as evaporated films of KI and KBr, increased above the vapor pressures of water molecules forming the hydrated ions. This fact might be attributed to a rearrangement of the surface ions. On the contrary when the salt had a high crystallinity, the depression of the diffraction intensity appeared in the vapor pressure range. The value of the vapor pressure at which adsorbed water molecules act to form the hydrated ions was peculiar to the material. Their values are 40, 51 and 55%rH for KI, KBr and KC1, respectively. From the above results it may be expected that X-ray diffractometry is an effective method for detecting formation of hydrated ions on a salt and for studying its caking-process.