Tuning Micellar Structures in Supercritical CO₂ Using Surfactant and Amphiphile Mixtures

Abstract

For equivalent micellar volume fraction (ϕ), systems containing anisotropic micelles are generally more viscous than those comprising spherical micelles. Many surfactants used in water-in-CO (w/c) microemulsions are fluorinated analogues of sodium bis(2-ethylhexyl) sulfosuccinate (AOT): here it is proposed that mixtures of CO-philic surfactants with hydrotropes and cosurfactants may generate elongated micelles in w/c systems at high-pressures (e.g., 100-400 bar). A range of novel w/c microemulsions, stabilized by new custom-synthesized CO-phillic, partially fluorinated surfactants, were formulated with hydrotropes and cosurfactant. The effects of water content (w = [water]/[surfactant]), surfactant structure, and hydrotrope tail length were all investigated. Dispersed water domains were probed using high pressure small-angle neutron scattering (HP-SANS), which provided evidence for elongated reversed micelles in supercritical CO. These new micelles have significantly lower fluorination levels than previously reported (6-29 wt % cf. 14-52 wt %), and furthermore, they support higher water dispersion levels than other related systems (w = 15 cf. w = 5). The intrinsic viscosities of these w/c microemulsions were estimated based on micelle aspect ratio; from this value a relative viscosity value can be estimated through combination with the micellar volume fraction (ϕ). Combining these new results with those for all other reported systems, it has been possible to "map" predicted viscosity increases in CO arising from elongated reversed micelles, as a function of surfactant fluorination and micellar aspect ratio.

Molecular Architecture Effects on Bulk Nanostructure in Bis(Orthoborate) Ionic Liquids

“…[73] Changing the solvent alters the solubility, because of variable polarity, cohesive energy density (aka the Gordon parameter), [74] and directional solvation interactions. [75][76][77][78] In a pure IL, changing the alkyl chain length changes both of these parameters simultaneously, [79,80] meaning that there are likely to be local maxima of aggregate size as a function of alkyl chain length, as the two terms intersect. In other words, if the alkyl chain of an IL is excessively long, the driving force for self-assembly could conceivably reduce, because the difference between polar and apolar parts is diminished as the solvent itself also becomes on average less polar.…”

Anisotropic reversed micelles with fluorocarbon-hydrocarbon hybrid surfactants in supercritical CO2

“…[η] and viscosity of reversed micelle/CO2 solutions were approximated as described below [21,49]. The [η] value is very sensitive to particle shape, for hard spheres [η] = 2.5, whereas for one-dimensional, anisotropic particles [η] is greater than this and can be approximated using equation 3 [η] = 2.5 + 0.4075 (Xmicelle -1) 1.508 (3)…”

Thickening CO2 with Direct Thickeners, CO2-in-Oil Emulsions, or Nanoparticle Dispersions: Literature Review and Experimental Validation

“…At a concentration of ∼6 wt % Ni-diHCF 4 , at 25 °C and 350 bar, with 10 mol of water per surfactant mole ( W = 10), a ∼50% increase in viscosity occurred and up to a 100% increase occurred using NaF 7 H 4 at a concentration of 4.4 wt % and W = 12.5 mol of water per surfactant molecule . Eastoe and co-workers summarized the literature regarding water-in-CO 2 microemulsions, with an informative look at any surfactant structures aimed at attaining both CO 2 solubility and viscosity enhancement. Although there have been developments in designing surfactants that form ellipsoidal micelles in CO 2 , ,, the extraordinarily long micelles required to generate the necessary large increases in viscosity at dilute concentration have not been realized.…”