Phosphate Surfactants for Water-in-CO₂ Microemulsions

Interfacial phenomena at the compressed co2-water interface

“…It is also known from experimental observation that replacing the sulfosuccinate surfactant head group by phosphate, as the tail group is kept constant, has the effect of reducing the pressure required to stabilize a W/C microemulsion of about 10 MPa. This observation (Steytler, et al, 2001) clearly illustrates the relevance of a correct model description of the solvation of the head group, as well as the CO 2 -philic tail. A fully atomistic description of the surfactant is therefore essential for obtaining the correct physics of the problem.…”

“…Equally important is the reduction in the pressure required for stabilization of the aggregates. Subtle changes in the surfactant architecture have been shown to cause very significant reductions in the pressure required for stabilization of reverse microemulsions of water in CO 2 , Steytler, et al, 2001. Similar success has not been reported for hydrogenated surfactants.…”

“…While for the most part there seems to be a consensus regarding the use of detailed models for both solvents (water and CO 2 ), few research projects employ fully atomistic models to describe the chemistry of the surfactant (da Rocha, et al, 2002, Stone, et al, 2004. This is a surprising observation given the significant impact that subtle changes in surfactant architecture have on surfactant activity at the C|W interface , Steytler, et al, 2001. For example, substituting a single atom in the surfactant tail terminal group CF 3 to form CHF 2 in a double-tailed semi-fluorinated sulfosuccinate amphiphile requires an increase in pressure of approximately 9 MPa to stabilize a W/C microemulsion with a molar water/surfactant ratio of 10 .…”

“…The properties of the C|W interface are strikingly different from those of conventional oil-water (O|W) interfaces. Most marked, are (1) the much lower surfactant requirement at the onset of formation of reverse aggregates of water-in-CO 2 (W/C), than that in O|W systems (da Rocha, et al, 2000) and (2) the fact that subtle changes in surfactant chemistry have a profound effect on the pressure required to stabilize the reverse aggregates , Steytler, et al, 2001. Figure 1 is an idealized picture of W/C reverse microemulsions.…”

Water-in-CO2 microemulsions with a simple fluorosurfactant

“…However, this surfactant degraded over time to limit its applicability. Later, Johnston et al [4] [5,17,18] and phosphate fluorosurfactants [19,20] can also form W/CO 2 microemulsions. Recent work demonstrated that 0.03 M bis(2-ethylhexyl) sodium sulphosuccinate (AOT) in scCO 2 solution could solubilize 0.5-1 mol% water with the aid of ethanol (15-17 mol%) or pentanol (about 10 mol%) [21].…”