Infrared spectra of HCl adsorbed on high-area alkali-metal halides: dependence of the effective surface ionicities on the ionic radius ratio

The adsorption of HCl on the surfaces of a wide range of high-area alkali-metal halides gave broad HCl absorption bands caused by hydrogen bonding to the surface halide ions. The decrease in wavenumbers of the absorptions at near-monolayer coverage, Delta v(HCl), relative to the value for gas-phase HCl, used as a measure of the strength of the hydrogen bonding, showed systematic variations as a function of anion for a given cation, or vice versa. The values of Delta v(HCl) were shown to correlate with the radius ratio, r(+):r(-), of the anions. A scale of the effective surface ionicities of the alkali-metal halides, relative to CsCl assumed to have unit ionicity, was derived from the relative Delta v(HCl) values. The resulting scale gives a linear correlation with the charge separations in the gaseous diatomic alkali-metal halides as calculated earlier from the analysis of their dipole moments.