Droplet Structure in a Water-in-CO₂ Microemulsion

Abstract

High-pressure small-angle neutron-scattering measurements show that a dichain hydrocarbon−fluorocarbon surfactant ((C7H15)(C7F15)CHSO4 -Na+) stabilizes spherical droplets, with mean water core radius around 25 Å, in a water-in-CO2 microemulsion (temperature = 25 °C, pressure = 500 bar). At lower pressures droplet clustering is observed. Our structural work on this system builds on initial phase stability studies by Johnston et al. (Langmuir 1994, 10, 3536).

Properties of surfactant films in water-in-CO2 microemulsions obtained by small-angle neutron scattering

“…The effect of pressure on SANS was also examined for a pure D2O core system and similar results were obtained as for the H2O/D2O mixed core systems. Furthermore, although formation of elongated w/c microemulsions has been reported in a few studies [14][15][16] , this is the first observation of anisotropic micellar structures triggered by changing CO2 pressure and bulk density. In Table 3, the fitting parameters obtained using the cylindrical form factor model have been compared for w/c systems stabilized by 4FS(EO)2 and 6FS(EO)2, together with other systems containing cylindrical micelles reported in the literature 11,15 .…”

“…A spherical structure was observed for the (micro)emulsified water cores, which provided the first direct evidence for the formation of w/c microemulsions 14 . Having shown the feasibility of using HP-SANS to characterize w/c microemulsions, this technique has been frequently applied in later studies with different surfactants 6,8,15 and structures 16,17,18 .…”

“…More importantly, due to the high contrast between different components containing hydrogen ( 1 H) and deuterium ( 2 H), and by controlling isotopic labeling the domains of interest, different nano-compartmentalized systems can be highlighted for detailed analysis 13 . Based on Johnston et al's phase studies 2 , Eastoe et al 14 .…”

Concentrated CO2-in-Water Emulsions with Nonionic Polymeric Surfactants

“…A variety of studies involving W/C emulsions (10)(11)(12) and W/C microemulsions (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) have been reported recently. Based on observations of the volume fractions of the upper and lower phases after shear, emulsions with a C/W morphology have been reported (25,26).…”

Design of Nonionic Surfactants for Supercritical Carbon Dioxide

“…Several laboratories have demonstrated progress in the micellization of aqueous and polar materials in many dense sc fluids including alkanes, chlorofluorocarbons, hydrofluorocarbons, and CO 2 (10)(11)(12)(13)(14)(15). However, success in designing generic surfactants for CO 2 has been hindered by the challenge of identifying lyophilic segments for CO 2 .…”