The metastable HCl · 6H<sub>2</sub>O phase – IR spectroscopy, phase transitions and kinetic/thermodynamic properties in the range 170–205 K

Abstract

In this laboratory study, 1 to 2 μm thick polycrystalline ice films have been grown under stirred flow reactor (SFR) conditions and subsequently doped with metered amounts of HCl under static conditions. A multidiagnostic approach including FTIR absorption spectroscopy in transmission, residual gas mass spectrometry (MS) and total pressure measurement was employed. Depending on the growth protocol controlling both temperature and partial pressure of HCl (P_HCl), either amorphous HCl/H_2O or crystalline HCl hexahydrate (HCl · 6H_2O) have been obtained. After controlled doping with HCl and evaporation of excess H_2O from the ice film, transmission FTIR of pure HCl · 6H_2O films and use of calibrated residual gas MS enabled the measurement of differential (peak) IR cross sections at several mid-IR frequencies (&sigma; = (6.5 ± 1.9) &times; 10^-19 cm² molec^-1 at 1635 cm^-1 as an example). Two types of kinetic experiments on pure HCl · 6H_2O have been performed under SFR conditions: (a) evaporation of HCl · 6H_2O under H_2O-poor conditions over a narrow <i>T</i> range, and (b) observation of the phase transition from crystalline HCl · 6H₂O to amorphous HCl/H_2O under H_2O-rich conditions at increasing <i>T</i>. The temperature dependence of the zero-order evaporation flux of HCl in pure HCl · 6H_2O monitored at 3426 cm^-1 led to log(J_ev) molec cm^-2s^-1= (36.34 ± 3.20) - (80 810 ± 5800)/2.303RT with <i>R</i>=8.312 JK⁻¹ mol^-1. 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 typical UT/LS conditions of HCl partial pressure (P(HCl)) and temperature. Water-rich HCl · 6H_2O undergoes a facile phase transition from crystalline to the amorphous/ supercooled/disordered state easily observable at <i>T</i>&ge; 195 K under both static and SFR conditions. This corresponds to low P(HCl) in the neighborhood of 10^-7 Torr that also prevails at the Upper Troposphere/Lower Stratosphere (UT/LS). The atmospheric importance of HCl · 6H_2O is questioned in view of its nucleation barrier and its dependence on temperature and P(HCl) compared to the amorphous HCl/H_2O phase

The mid‐IR Absorption Cross Sections of α‐ and β‐NAT (HNO3 · 3H2O) in the range 170 to 185 K and of metastable NAD (HNO3 · 2H2O) in the range 172 to 182 K

“…Similar to the case of HCl [ Iannarelli and Rossi , ], we assume that for all α ‐NAT samples, the measured composite spectrum is a superposition of the spectra of pure ice and pure α ‐NAT. We proceeded as follows to deconvolute the composite absorption spectrum: first, the spectrum of the pure ice film before HNO 3 deposition (red spectrum in Figure ) is used to calculate the initial number of H 2 O molecules in the condensed phase, using the cross section at 3233 cm −1 for pure ice, σ 3233 = (8.0 ± 0.8) × 10 −19 cm 2 [ Chiesa and Rossi , ], according to equation : where A Si is the area (one side) of the silicon film support in square centimeters and OD 3233 is the optical density in absorbance units.…”

“…In the case of NAD, akin to α ‐NAT, all samples are the sum of pure NAD and pure H 2 O ice. The same spectral subtraction procedure used for α ‐NAT has been applied: first, we calculate the initial number of H 2 O molecules in the pure ice film, N ice (H 2 O), using the absorption cross section at the characteristic ice peak at 3233 cm −1 , σ 3233 = (8.0 ± 0.8) × 10 −19 cm 2 [ Chiesa and Rossi , ] according to equation . Subsequently, we calculate the number of H 2 O molecules used to form the new NAD phase, namely, N NAD (H 2 O) = 2 N NAD (HNO 3 ), with N NAD (HNO 3 ) measured according to equation whose time‐dependent cumulative value is represented by the gray hatched area in Figure .…”

“…The experiments performed in this work have been conducted using a multidiagnostic stirred flow reactor (SFR) whose detailed characteristics, design, and operational functions have been reported before [ Chiesa and Rossi , ; Iannarelli and Rossi , ].…”

“…The gas admitted into the reactor therefore only interacts with the cold optical window, thus allowing the establishment of a 1:1 correspondence between changes in the gas partial pressure and thin film composition. Experimental proof of mass balance in the H 2 O/HCl system has been given previously for this cryostat/SFR combination [ Delval et al ., ; Chiesa and Rossi , ; Iannarelli and Rossi , ].…”

The Influence of HCl on the Evaporation Rates of H2O over Water Ice in the Range 188 to 210 K at small Average Concentrations

“…The time t ev in seconds to complete evaporation of an ice particle of radius r at a given relative humidity (rh) is given in equation 1 (Chiesa and Rossi, 2013;Iannarelli and Rossi, 2016a): (Iannarelli and Rossi, 2016a). Equation 1 is based on a simple layer-by-layer evaporation model of H 2 O(ice) from a spherical ice particle following a zero-order rate law for J ev or a first order rate law for its inverse, namely H 2 O adsorption or condensation.…”