Formation of HCl·6H2O from ice and HCl under ultrahigh vacuum

The interactions of hydrogen chloride (HCl) with ultrathin (10–100 monolayers thick) films of ice between 120 and 180 K were studied using temperature programmed desorption (TPD) and Fourier transform infrared reflection absorption spectroscopy (FTIRAS). HCl is initially absorbed by ice at 120 K to form the stoichiometrically fixed hexahydrate phase, HCl·6H2O. Infrared spectroscopic measurements suggest that the hexahydrate is present as an amorphous solid. Upon the conversion of ice to HCl·6H2O, HCl is adsorbed to the thin film surface, possibly with the H–Cl bond intact. The amorphous hexahydrate can be crystallized by annealing at 165 K for several minutes. Infrared spectra of the resulting films closely resemble those reported elsewhere of authentic, macroscopic samples of crystalline HCl·6H2O. If the H2O to HCl ratio is less than 1:6, two phases are present: pure or nearly pure ice, and pure or nearly pure HCl·6H2O. The infrared spectra of such films were simulated by an appropriate linear combination of the spectra of ice and HCl·6H2O. The sublimation kinetics of ice and HCl·6H2O are analyzed, and the results used to infer that the enthalpy of HCl absorption by ice is −126±14 kJ mol−1.