The metastable HCl · 6H2O phase – IR spectroscopy, phase transitions and kinetic/thermodynamic properties in the range 170–205 K
Abstract. In this laboratory study a multidiagnostic experimental approach including Fourier transform infrared (FTIR) absorption of 1 to 2 μm thick polycrystalline ice films, residual gas mass spectrometry (MS) and total pressure measurement were employed. Both amorphous HCl–H2O and crystalline HCl hexahydrate (HCl · 6H2O) have been investigated. After controlled doping with HCl and evaporation of excess H2O from the ice film, transmission FTIR of pure HCl · 6H2O films and use of calibrated mass spectrometry enabled the measurement of differential (peak) IR cross sections at several mid-IR frequencies, for example σ = (6.5 ± 1.9) × 10−19 cm2 molec−1 at 1635 cm−1. Two types of kinetic experiments on pure HCl · 6H2O have been performed under SFR conditions: (a) evaporation of pure HCl · 6H2O over a narrow T range after evaporation of excess H2O, and (b) observation of the phase transition from crystalline HCl · 6H2O 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 · 6H2O led to logJev 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 · 6H2O 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 · 6H2O 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.