The kinetics of condensation and evaporation of H2O from pure ice in the range 173–223 K: a quartz crystal microbalance study

Abstract

The kinetics of evaporation and condensation on ice condensed from the vapor phase at 190 K has been studied in the range 173–223 K using a suitably calibrated quartz crystal microbalance that was put inside a vacuum chamber that could be used either as a molecular or a stirred flow reactor, depending on the measured pumping speed. The Arrhenius representation of the zero order evaporative flux J_ev (molecule cm⁻² s⁻¹) displays a discontinuity at 193 ± 2 K that could be observed owing to the degree of precision of the present measurements. For 223 > T > 193 K, J_ev = (1.6 ± 0.7) × 10²⁸exp((−10.2 ± 0.5)/RT) molecule cm⁻² s⁻¹and for 173 < T < 193 K, J_ev = (2.2 ± 0.9) × 10³⁰exp((−12.0 ± 0.5)/RT) molecule cm⁻² s⁻¹ where R = 1.987 × 10⁻³ kcal mol⁻¹ K⁻¹. The corresponding measured rate constant k_cond for H₂O condensation on ice satisfies the known vapor pressure P_eq (H₂O) together with J_ev thereby affording thermochemical closure of the kinetics which results in the heat of sublimation ΔH_subl⁰ = 11.7 ± 0.6 and 12.3 ± 0.5 kcal mol⁻¹ from the above high and low T-range, respectively. J_ev is significantly lower than the maximum theoretically allowed value throughout the T-range in agreement with an uptake coefficient for H₂O on ice that is significantly smaller than unity. In addition, the negative temperature dependence of k_cond or γ reveals a precursor-mediated adsorption/desorption process for the H₂O/ice system. The increased evaporative lifetime of pure ice may be important under atmospheric conditions.