The metastable HCl center dot 6H(2)O phase - IR spectroscopy, phase transitions and kinetic/thermodynamic properties in the range 170-205 K

In this laboratory study a multidiagnostic experimental approach including Fourier transform infrared (FTIR) absorption of 1 to 2 mu m thick polycrystalline ice films, residual gas mass spectrometry (MS) and total pressure measurement were employed. Both amorphous HCl-H2O and crystalline HCl hexahydrate (HCl center dot 6H(2)O) have been investigated. After controlled doping with HCl and evaporation of excess H2O from the ice film, transmission FTIR of pure HCl center dot 6H(2)O films and use of calibrated mass spectrometry enabled the measurement of differential (peak) IR cross sections at several mid-IR frequencies, for example sigma = (6.5 +/- 1.9) x 10(-19) cm(2) molec(-1) at 1635 cm(-1). Two types of kinetic experiments on pure HCl center dot 6H(2)O have been performed under SFR conditions: (a) evaporation of pure HCl center dot 6H(2)O over a narrow T range after evaporation of excess H2O, and (b) observation of the phase transition from crystalline HCl center dot 6H(2)O to amorphous HCl-H2O under H2O-rich conditions at increasing T. The temperature dependence of the zero-order evaporation flux of HCl in pure HCl center dot 6H(2)O led to logJ(ev) molec cm(-2) s(-1) = (36.34 +/- 3.20) - (80 810 +/- 5800)/2.303 RT with R = 8.314 JK(-1) mol(-1), which turned out to be rate-limiting for evaporation. HCl center dot 6H(2)O has a significant intrinsic kinetic barrier to HCl evaporation of 15.1 kJ mol(-1) in excess of the HCl sublimation enthalpy of 65.8 kJ mol(-1) at 200 K but is kinetically unstable (metastable) at T >= 173 K. The atmospheric importance of HCl center dot 6H(2)O is questioned in view of its large nucleation barrier and its dependence on T and P(HCl) compared to the amorphous HCl-H2O phase at upper tropospheric-lower stratospheric (UT/LS) conditions.