Carbon dioxide clathrate

Carbon dioxide hydrate is a snow-like crystalline substance composed of water ice and carbon dioxide. It normally is a Type I gas clathrate. However, there has been some experimental evidence for the development of a metastable Type II phase at A temperature near the ice melting point. The clathrate can exist below 283K (10 °C) at a range of pressures of carbon dioxide. It is quite likely to be important on Mars due to the presence of carbon dioxide and ice at low temperatures.

The first evidence for the existence of CO₂hydrates dates back to the year 1882, when Zygmunt Florenty Wróblewski reported clathrate formation while studying carbonic acid. He noted that gas hydrate was a white material resembling snow, and could be formed by raising the pressure above a certain limit in his H₂O - CO₂ system. He was the first to estimate the CO₂ hydrate composition, finding it to be approximately CO₂•8H₂O. He also mentions that "...the hydrate is only formed either on the walls of the tube, where the water layer is extremely thin or on the free water surface... (from French)" This already indicates the importance of the surface available for reaction (i.e. the larger the surface the better). Later on, in 1894, M. P. Villard deduced the hydrate composition as CO₂•6H₂O. Three years later, he published the hydrate dissociation curve in the range 267 K to 283 K. Tamman & Krige measured the hydrate decomposition curve from 253 K down to 230 K in 1925 and Frost & Deaton (1946) determined the dissociation pressure between 273 and 283 K. Takenouchi & Kennedy (1965) measured the decomposition curve from 45 bars up to 2 kbar (4.5 to 200 MPa). The CO₂ hydrate was classified as a Type I clathrate for the first time by von Stackelberg & Muller (1954).

On Earth, CO₂ hydrate is mostly of academic interest. Tim Collett of the United States Geological Survey (USGS) proposed pumping carbon dioxide into subsurface methane clathrates, thereby releasing the methane and storing the carbon dioxide (Michael Marshall, 2009). As of 2009, ConocoPhillips is working on a trial on the Alaska North Slope with the US Department of Energy to release methane in this way, (ConocoPhilips, January 2010, New Scientist, no. 2714, p. 33). At first glance, it seems that the thermodynamic conditions there favor the existence of hydrates, yet given that the pressure is created by sea water rather than by CO₂, the hydrate will decompose.

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