Carbon dioxide clathrate hydrate formation at low temperature. Diffusion-limited kinetics growth as monitored by FTIR

Abstract

The formation and presence of clathrate hydrates could influence the composition and stability of planetary ices and comets; they are at the heart of the development of numerous complex planetary models, all of which include the necessary condition imposed by their stability curves, some of which include the cage occupancy or host-guest content and the hydration number, but fewer take into account the kinetics aspects. We measure the temperature-dependent-diffusion-controlled formation of the carbon dioxide clathrate hydrate in the 155-210~K range in order to establish the clathrate formation kinetics at low temperature. We exposed thin water ice films of a few microns in thickness deposited in a dedicated infrared transmitting closed cell to gaseous carbon dioxide maintained at a pressure of a few times the pressure at which carbon dioxide clathrate hydrate is thermodynamically stable. The time dependence of the clathrate formation was monitored with the recording of specific infrared vibrational modes of CO2 with a Fourier Transform InfraRed (FTIR) spectrometer. These experiments clearly show a two-step clathrate formation, particularly at low temperature, within a relatively simple geometric configuration. We satisfactorily applied a model combining surface clathration followed by a bulk diffusion-relaxation growth process to the experiments and derived the temperature-dependent-diffusion coefficient for the bulk spreading of clathrate. The derived apparent activation energy corresponding to this temperature-dependent-diffusion coefficient in the considered temperature range is E_a = 24.7 +/- 9.7 kJ/mol. The kinetics parameters favour a possible carbon dioxide clathrate hydrate nucleation mainly in planets or satellites.