Nanostructures in Water-in-CO₂ Microemulsions Stabilized by Double-Chain Fluorocarbon Solubilizers

Abstract

High-pressure small-angle neutron scattering (HP-SANS) studies were conducted to investigate nanostructures and interfacial properties of water-in-supercritical CO2 (W/CO2) microemulsions with double-fluorocarbon-tail anionic surfactants, having different fluorocarbon chain lengths and linking groups (glutarate or succinate). At constant pressure and temperature, the microemulsion aqueous cores were found to swell with an increase in water-to-surfactant ratio, W0, until their solubilizing capacities were reached. Surfactants with fluorocarbon chain lengths of n = 4, 6, and 8 formed spherical reversed micelles in supercritical CO2 even at W0 over the solubilizing powers as determined by phase behavior studies, suggesting formation of Winsor-IV W/CO2 microemulsions and then Winsor-II W/CO2 microemulsions. On the other hand, a short C2 chain fluorocarbon surfactant analogue displayed a transition from Winsor-IV microemulsions to lamellar liquid crystals at W0 =25. Critical packing parameters and aggregation numbers were calculated by using area per head group, shell thickness, the core/shell radii determined from SANS data analysis: these parameters were used to help understand differences in aggregation behavior and solubilizing power in CO2. Increasing the microemulsion water loading led the critical packing parameter to decrease to ~1.3 and the aggregation number to increase to > 90. Although these parameters were comparable between glutarate and succinate surfactants with the same fluorocarbon chain, decreasing the fluorocarbon chain length n reduced the critical packing parameter. At the same time, reducing chain length to 2 reduced negative interfacial curvature, favoring planar structures, as demonstrated by generation of lamellar liquid crystal phases.

Economical and Efficient Hybrid Surfactant with Low Fluorine Content for the Stabilisation of Water-in-CO2 Microemulsions

“…For decades, extensive studies were devoted to define the effect of the surfactant tail architecture [13,15,24,50,51]. However, only few works have alluded to the effects of hydrophilic-headgroup modifications on surfactant performance in CO2 [36,38,39].…”

“…Sulfoglutarate surfactants were prepared using the same alcohol precursors as used for sulfosuccinate surfactants. The fluorinated and hydrocarbon sulfoglutarate surfactants were synthesised using a Dean and Stark apparatus as shown in a previous work [39]. Meanwhile, an additional esterification step was included to obtain the hybrid sulfoglutarate surfactant.…”

Effect of Fluorocarbon and Hydrocarbon Chain Lengths in Hybrid Surfactants for Supercritical CO2

“…29 The solubilizing powers W0 max have been extracted from Figure 2 and are clearly presented in Figure 3. As mentioned above, the hydrocarbon FC6-HC6 analogue HC6-HC6 and FC4-HCn (n=2 and 4) always gave two-phases of W/CO2 mixtures, and were suggested to have no solubilizing power, namely On the other hand, the shortest HC-tail hybrid surfactant FC6-HC2 exhibited poor solubilizing power W0 max = 4.…”

Shape Transitions in Supercritical CO2 Microemulsions Induced by Hydrotropes

“…Unfortunately, these surfactants are highly fluorinated and therefore expensive and environmentally unfriendly 25,26 . To overcome this problem, low fluorine/hydrocarbon replacements have recently been developed [27][28][29] .…”