Heats of adsorption and site-energy distributions for Ar and N 2 on sublimated NaCl

Abstract

A sodium chloride particulate has been studied which possesses only residual surface heterogeneities. Freshly prepared particles are nearly spherical in shape and exist in a metastable glassy state. Subsequent to double heat treatment at 300°C under anhydrous conditions, particles are shown to have changed into the normal crystalline state. This change is accompanied by a 16% reduction in surface area, a reduction in heterogeneity, and minor modifications in particle morphology. Extensive adsorption measurements with Ar and N 2 at four temperatures between 72.0–90.1°K permitted determination of isosteric heats of adsorption and site-energy distributions. Experimental zero-coverage heats for Ar and N 2 were found to be 1.64 ± 0.08 and 3.04 ± 0.13 kcal/mole, respectively. Revised theoretical limiting heats were obtained from potential-energy curves utilizing more plausible estimates for the dispersion coefficients than those provided by the Kirkwood-Müller approximation. The values of 1.08 and 2.72 kcal/mole for Ar and N 2 lie below the experimental heats. The problem of predicting limiting heats is only partially solved.