Just Add Water

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Science  12 Jan 2001:
Vol. 291, Issue 5502, pp. 211-213
DOI: 10.1126/science.291.5502.211f

Supercritical carbon dioxide has been studied in part because it is an environmentally benign solvent and a candidate to substitute for organic solvents in many industrial processes. It also has important advantages in environmental remediation, because a supercritical phase has no liquid-vapor interface and thus no surface tension. An important disadvantage, however, is the low solubility of hydrophilic compounds and metals in supercritical carbon dioxide. Ligands can be added to increase the solubility of metals, but often they need to be added in large excess. This has hampered the use of supercritical carbon dioxide, for example, in metal extraction from contaminated waste.

Yates et al. use a different approach based on the surfactant-assisted formation of a microemulsion of water droplets in supercritical carbon dioxide. They show that these small droplets of water efficiently remove copper from a contaminated surface; copper is highly soluble in water, and the supercritical phase infiltrates small pores that would be inaccessible to bulk water. Very little surfactant is required, and the water and carbon dioxide phases can be separated readily by reducing the pressure. This approach may be particularly useful when extracting small amounts of metal dispersed in a large volume of solid waste. — JU

Chem. Comm.2001, 25 (2001).

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