Shape Modification of Water-in-CO₂ Microemulsion Droplets through Mixing of Hydrocarbon and Fluorocarbon Amphiphiles

Abstract

An oxygen-rich hydrocarbon (HC) amphiphile has been developed as an additive for supercritical CO2 (scCO2). The effects of this custom-designed amphiphile have been studied in water-in-CO2 (w/c) microemulsions stabilized by analogous fluorocarbon (FC) surfactants, nFG(EO)2, which are known to form spherical w/c microemulsion droplets. By applying contrast-variation small-angle neutron scattering (CV-SANS), evidence has been obtained for anisotropic structures in the mixed systems. The shape transition is attributed to the hydrocarbon additive, which modifies the curvature of the mixed surfactant films. This can be considered as a potential method to enhance physicochemical properties of scCO2 through elongation of w/c microemulsion droplets. More importantly, by studying self-assembly in these mixed systems, fundamental understanding can be developed on the packing of HC and FC amphiphiles at water/CO2 interfaces. This provides guidelines for the design of fluorine-free CO2 active surfactants, and therefore, practical industrial scale applications of scCO2 could be achieved.

Molecular-Scale Design of Hydrocarbon Surfactant Self-Assembly in Supercritical CO2

“…The self-assembly structure shows a sphere-to-rod transition after adding C 8 Benz. Through the analysis of microconformation of RMs, we found that the self-assembled RMs can be composed in three parts: two TC14 aggregate on the both ends and a C 8 Benz aggregates in the middle of RM which is difference from the suggestion of previous studies. − This dumbbell-like shape of rodlike RMs is regarded as origin mechanism of the sphere-to-rod transition of RMs. Further, the free energy profiles indicates that the high fusion ability of C 8 Benz aggregation is the crucial to generate this structures.…”

“…30 Why does the cosurfactant could induce the transformation from spherical RMs to rodlike RMs? Previously experimental study 28,29 suggests that the cosurfactants could insert into the surfactant film for modulating its packing curvature and thus facilitate the sphere-to-rod transformation. But, the fraction of cosurfactant is very small and limited to change the packing behavior.…”

“…Recently, the cosurfactant has been proved to transform the shape of hydrocarbon surfactant self-assembly. , TC14 surfactants is one of the most appreciated hydrocarbon surfactant that can form stable reverse micelles (RMs) in scCO 2 . And the research suggests that the TC14 surfactant shows shape transition with adding cosurfactants of C 8 Benz.…”

Tuning Micellar Structures in Supercritical CO2 Using Surfactant and Amphiphile Mixtures

“…21 The result of these changes is to destabilize spherical reversed micelles in favor of ellipsoidal, rod-like or lamellar structures, which have a greater degree of planarity. Following the packing parameter argument, it can also be proposed that, changing from a short singlechained additive, with a low chain volume (such as C2benz, Figure 1) to an analogue with greater chain volume (e.g., C8benz, Figure 1), P c should also decrease 21,23 toward 1 favoring ellipsoidal/rod-like/lamellar domains over highly curved reversed spherical micelles. Furthermore, it is interesting to see how a double chain additive, such as DIGSS (Figure 1), which is recognized as being CO 2 -philic 23 will affect preferred curvature compared to the single chain hydrotrope additives.…”

“…22 Importantly, the systems reported here have lower fluorination (surfactants + additives) in comparison to previously reported systems. 23 To emphasize the significance of these mixed systems for modification of CO 2 properties, intrinsic viscosities have been estimated and compared with all known similar CO 2 -philic systems from the literature. 6,20,22,23,28 This analysis provides a clear overview of the requirements for increasing scCO 2 viscosity through micellar elongation, and allows key micellar and surfactant parameters to be identified.…”