Equilibrium thermodynamics of HC1 adsorption on H{sub 2}O ice

We have measured the adsorption of HCl onto neat H{sub 2}O ice in the laboratory under conditions which span those of the stratosphere. We utilize a nonlinear light scattering technique to obtain adsorption spectra which reflect both the thermodynamic phase and the fraction of total ice surface area covered by the adsorbed HCl. We also measure the HCl vapor pressure over the ice sample by mass spectrometry. The amount of H{sub 2}O ice substrate covered by HCl as a function of the total amount of HCl adsorbed is determined by the kinetics of multilayer adsorption for this system. Analysis of the data using classic BET multilayer adsorption theory yields adsorption enthalpies for this system that agree with those in the literature. We present a preliminary surface phase diagram for this system for temperatures from 140 K to 210 K and HCl hydrate coverage of 0.1 to 10 monolayers.