Adsorption of Krypton on Alkali Halide Crystals Part 2.—On NaCI, KC1 and RbCl

Abstract

Adsorption of krypton on NaCI, KC1 and RbCl was measured at temperatures controlled to within ±0.005 K in the range 70-85 K. The equilibrium pressure was measured with a precision better than 0.5 % by carefully correcting for the thermal transpiration effect. Adsorption isotherms showed 2-dimensional condensation, and the critical temperatures of the 2-dimensional film were 80.0, 76.0 and 75.5 K for NaCI, KC1 and RbCl, respectively. Curves of the isosteric heat of adsorption against coverage had a unique form, and were analyzed by a model consisting of two kinds of domain of adsorption sites with a patchy structure. The lateral interaction energy between adsorbates was obtained from a?st against 0 curve, and the term of the third body effect was deduced by the method given in Part 1. Analyzing this term further, the surface electric field was estimated; the field strength on the adsorption site was of the order of 109 V/m. According to the results of Part 1, this means that the adsorption site is the C-site (above cation), and that the surface lattices of KC1 and RbCl are distorted as postulated by Benson and his coworkers. The entropy of adsorbates was analyzed on the basis of the localized adsorption model. The thermal (non-configurational) entropy of adsorbate obtained had reasonable values, supporting the assumed model. Finally, the origin of the surface heterogeneity is discussed.