Effect of Surfactant Headgroup on Low-Fluorine-Content CO2-Philic Hybrid Surfactants

The article addresses an interesting issue in the development of hybrid surfactants for waterin-CO2 (w/c) microemulsion stabilisation: the role of surfactant headgroup on the surfactant performance. The synthetic procedure, aqueous properties, and phase behaviour of a new hybrid sulfoglutarate surfactant are described. The compound resembles sulfosuccinate surfactants, commonly used to stabilize w/c phases, but with an extra methylene group incorporated into the hydrophilic headgroup. For comparison purposes, the related hydrocarbon (AOT14 and AOT14GLU) and fluorocarbon (di-CF2 and di-CF2GLU) surfactants are used to form w/c microemulsions. In general, the aqueous properties and w/c phase stability of both sulfoglutarates and sulfosuccinates are found to be similar, which shows the secondary role of the hydrophilic headgroup. Interestingly, the newly synthesised hybrid CF2/AOT14GLU (sodium (4H,4H,5H,5H,5H-pentafluoropentyl-2,2-dimethyl-1-propyl)-2sulfoglutarate) proved to be more efficient than the normal sulfosuccinate, hybrid CF2/AOT14 (Ptrans = 383 bar, γcmc = 26.8 mN m ) in terms of the aqueous behaviour and w/c phase stability. Switching to the sulfoglutarate compound, hybrid CF2/AOT14GLU (Ptrans = 232 bar, γcmc = 20.6 mN m) more effectively decreases the air-water surface tension by about ~ 5 mN m as compared to the sulfosuccinate. High-pressure phase behaviour studies show significant improvements in stabilising w/c microemulsions at much lower cloud pressures. The results indicate distinct effects of the headgroup structure on the phase behaviour and physicochemical properties, particularly for this hybrid surfactant.