Ion specific effects with CO2-philic surfactants

Due to its promise as a green alternative to standard petrochemical solvents, considerable effort has been devoted into making supercritical carbon dioxide (scCO2) a viable solvent. This has involved the design of additives able to solubilise water domains which can then themselves dissolve the small, polar/non-volatile molecules otherwise incompatible with pure CO2, and viscosifiers able to increase the viscosity of scCO2. Surfactants have been demonstrated to be able to achieve both of these objectives as long as they have optimal chemical structures which must be finely tuned. One way to do this is by controlling the nature of the surfactant counterion species and it is such counterion effects that are the focus of this review.