Carbon dioxide clathrate hydrate formation at low temperature

E Dartois,F Langlet

doi:10.1051/0004-6361/202140858

Abstract

Context.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.Aims.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.Methods.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 CO2with a Fourier Transform InfraRed spectrometer.Results.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 isEa= 24.7 ± 9.7 kJ mol−1. The kinetics parameters favour a possible carbon dioxide clathrate hydrate nucleation mainly in planets or satellites.

Highlights

Clathrate hydrates are inclusion compounds that trap molecules in a crystalline water ice network
The existence of clathrate hydrates in Solar System bodies has been explored in many models, principally on the basis of the stability curves for these inclusion compounds
The diffusion coefficient associated with the clathrate diffusion-controlled nucleation kinetics was deduced

Summary

Introduction

Clathrate hydrates are inclusion compounds that trap molecules in a crystalline water ice network. In a planetary or astrophysical context, clathrate hydrates are considered because they can retain volatile molecules at pressures higher than the pure compound that would otherwise sublimate under most astrophysical conditions, and/or influence the geophysics of large bodies (e.g. Fortes & Choukroun 2010; Choukroun et al 2010; Bollengier et al 2013). The existence of clathrate hydrates in Solar System bodies has been explored in many models, principally on the basis of the stability curves for these inclusion compounds. During the evolution of the solar nebula, the existence of clathrate hydrate, or rather its stability, depends on pressure and temperature. These thermodynamic properties represent a necessary but not sufficient condition. In particular small bodies such as comets, or low-temperature objects orbiting at large distances, such as transNeptunian objects, including Kuiper Belt objects, the kinetics for their formation becomes a primary driving factor to take into account in the modelling

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Conclusion