Formation of HCl center dot 6H(2)O 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 . 6H(2)O. Infrared spectroscopic measurements suggest that the hexahydrate is present as an amorphous solid. Upon the conversion of ice to HCl . 6H(2)O, 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 . 6H(2)O. 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 . 6H(2)O. The infrared spectra of such films were simulated by an appropriate linear combination of the spectra of ice and HCl . 6H(2)O. The sublimation kinetics of ice and HCl . 6H(2)O are analyzed, and the re suits used to infer that the enthalpy of HCl absorption by ice is -126 +/- 14 kJ mol(-1).
hcl center dot 6h(2)o ice ultrahigh vacuum hydrochloric acid fourier transform infrared spectroscopy