CO2 Clathrate Dissociation Rates Below the Freezing Point of Water

Introduction: Gas hydrate clathrates are cage-like structures of water molecules surrounding a gas mole- cule such as carbon dioxide or methane. CO2 clathrate is structure I, indicating 5.75 H2O molecules : 1 CO2. Clathrates have properties similar to water ice, save their exceptional thermal expansivity and conductivity. Hydrates are stable at low temperature, modest pres- sure conditions making them likely to form in many environments throughout the solar system. Clathrates have been suggested sources of methane in the Martian atmosphere (1,2,3,4), and subsurface water and carbon reservoirs (5). However, experimental data is needed to better understand reaction mechanisms and con- strain the rates of the hydrate formation and dissocia- tion processes. This will lead to predictions of gas fluxes between subsurface aquifers, hydrate reservoirs, and planetary atmospheres within models of carbon reservoirs on Mars and other planets. Methods: A device has been constructed at the University of Oklahoma to measure the rate of clath- rate formation and dissociation by monitoring pressure and temperature conditions inside a reactor cooled within a commercial freezer. A small gas reservoir was also cooled within the freezer so that gas entering the reactor chamber is at thermal equilibrium with the ice in the reactor. The amount of gas present through- out the experiment was calculated using the Van der Waal's equation to account for intermolecular attrac- tion. Pressure and temperature measurements were taken every 15 seconds and recorded in Labview. A container of water, open only at the top was placed inside the reactor chamber and frozen before pressurization, leaving the top of the ice cylinder open and free to react with the gas in the headspace. In this arrangement the gas is reacting with a known surface area of ice, allowing surface area normalized rates to be measured. This geometry will also facilitate future work on diffusion rates during hydrate formation. Parallel experiments below atmospheric pressure