Supercritical Carbon Dioxide as a Solvent for Life

[stevebowers]

This book should help
The Limits of Organic Life in Planetary Systems

especially this chapter
https://www.nap.edu/read/11919/chapter/8#76

Supercriticality in an environment does not, in itself, prohibit life. Some terran enzymes are known to be active in supercritical fluids.(30)-(32) Subsequent reviews can be found in Aaltonen and Rantakyla,(33) Kamat et al.,(34) and Aaltonen.(35) Although most of that work concerns supercritical carbon dioxide as the solvent, fluorinated hydrocarbons (HCF3) and simple alkanes (e.g., ethane, propane) have also been reported,(36) providing a formal demonstration that terran-derived proteins can function in these media. Any enzyme adapted to the supercritical media would undoubtedly be different from those used in the studies cited.

Organic chemists have been attracted for a variety of reasons to supercritical media as an environment for performing reactions. These reasons include, especially for CO2 and H2O, the environmental friendliness of the medium. The fact that supercritical fluids can be removed without a residue is an advantage. Other advantages include the solubility of gases within supercritical mixtures, the high diffusion rates, and the variable and adjustable density, solvent power, and dielectric constant of the medium. Ordinary gases, such as O2 and H2, are miscible with supercritical fluids, such as supercritical CO2. That property removes a limitation that is well known in standard liquids, including water, in which reactions are limited by the poor solubility of gases in the standard liquid. Table 6.1 shows some of the solvents studied.

Supercritical water is a key species in mineralogy, as well as in the commercial synthesis of crystalline inorganic species, including alpha-quartz and gemstones.(37) It has rarely been used in organic chemistry, although there are examples of its use for the oxidation of methane, the degradation of biopolymers, and the dehydration of alkenes.(38) Its high critical temperature (647 K) implies that many organic species are unstable in supercritical water. Nevertheless, some reactions are known.(39)