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 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 · 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.