title: Tuning Micellar Structures in Supercritical CO2Using 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-CO2(w/c) microemulsions are fluorinated analogues of sodium bis(2-ethylhexyl) sulfosuccinate (AOT): here it is proposed that mixtures of CO2-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 CO2-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 CO2. 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 CO2arising from elongated reversed micelles, as a function of surfactant fluorination and micellar aspect ratio.
authors.affiliation: School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
authors.forename: Jocelyn
authors.identifier: 0000-0002-7621-5513
authors.initials: J
authors.lastname: Peach
authors.affiliation: School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
authors.forename: Adam
authors.initials: A
authors.lastname: Czajka
authors.affiliation: School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
authors.forename: Gavin
authors.initials: G
authors.lastname: Hazell
authors.affiliation: School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
authors.forename: Christopher
authors.initials: C
authors.lastname: Hill
authors.affiliation: Faculty of Science and Mathematics, Department of Chemistry, University Pendidikan Sultan Idris , Tanjong Malim 35900, Perak, Malaysia.
authors.forename: Azmi
authors.initials: A
authors.lastname: Mohamed
authors.affiliation: School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Pegg
authors.affiliation: Rutherford Appleton Laboratory, ISIS Spallation Source, Chilton, Oxfordshire, OX11 0QT, United Kingdom.
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Rogers
authors.affiliation: School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
authors.forename: Julian
authors.identifier: 0000-0001-5706-8792
authors.initials: J
authors.lastname: Eastoe