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 CO2 arising from elongated reversed micelles, as a function of surfactant fluorination and micellar aspect ratio.
Previous work (M. Sagisaka, et al. Langmuir 31 (2015) 7479-7487), showed the most effective fluorocarbon (FC) and hydrocarbon (HC) chain lengths in the hybrid surfactants FCm-HCn (sodium 1-oxo-1-[4-(perfluoroalkyl)phenyl]alkane-2-sulfonates, where m = FC length and n = HC length) were m and n = 6 and 4 for water solubilization, whereas m 6 and n 6, or m 6 and n 5, were optimal chain lengths for reversed micelle elongation in supercritical CO2. To clarify why this difference of only a few methylene chain units is so effective at tuning the solubilizing power and reversed micelle morphology, nanostructures of water-in-CO2 (W/CO2) microemulsions were investigated by high-pressure small-angle neutron scattering (SANS) measurements at different water-to-surfactant molar ratios (W0) and surfactant concentrations. By modelling SANS profiles with cylindrical and ellipsoidal form factors, the FC6-HCn/W/CO2 microemulsions were found to increase in size with increasing W0 and surfactant concentration. Ellipsoidal cross-sectional radii of the FC6-HC4/W/CO2 microemulsion droplets increased linearly with W0, and finally reached ∼39 Å and ∼78 Å at W0 = 85 (close to the upper limit of solubilizing power). These systems appear to be the largest W/CO2 microemulsion droplets ever reported. The aqueous domains of FC6-HC6 rod-like reversed micelles increased in size by 3.5 times on increasing surfactant concentration from 35 mM to 50 mM: at 35 mM, FC6-HC5 formed rod-like reversed micelles 5.3 times larger than FC6-HC6. Interestingly, these results suggest that hybrid HC-chains partition into the microemulsion aqueous cores with the sulfonate headgroups, or at the W/CO2 interfaces, and so play important roles for tuning the W/CO2 interfacial curvature. The super-efficient W/CO2-type solubilizer FC6-HC4, and the rod-like reversed micelle forming surfactant FC6-HC5, represent the most successful cases of low fluorine content additives. These surfactants facilitate VOC-free, effective and energy-saving CO2 solvent systems for applications such as extraction, dyeing, dry cleaning, metal-plating, enhanced oil recovery and organic/inorganic or nanomaterial synthesis.
A facile electrochemical exfoliation method was established to efficiently prepare conductive paper containing reduced graphene oxide (RGO) with the help of single chain anionic surfactant ionic liquids (SAILs). The surfactant ionic liquids are synthesized from conventional organic surfactant anions and a 1-butyl-3-methyl-imidazolium cation. For the first time the combination of SAILs and cellulose was used to directly exfoliate graphite. The ionic liquid 1-butyl-3-methyl-imidazolium dodecylbenzenesulfonate (BMIM-DBS) was shown to have notable affinity for graphene, demonstrating improved electrical properties of the conductive cellulose paper. The presence of BMIM-DBS in the system promotes five orders of magnitude enhancement of the paper electrical conductivity (2.71 × 10-5 S cm-1) compared to the native cellulose (1.97 × 10-10 S cm-1). A thorough investigation using electron microscopy and Raman spectroscopy highlights the presence of uniform graphene incorporated inside the matrices. Studies into aqueous aggregation behavior using small-angle neutron scattering (SANS) point to the ability of this compound to act as a bridge between graphene and cellulose, and is responsible for the enhanced exfoliation level and stabilization of the resulting dispersion. The simple and feasible process for producing conductive paper described here is attractive for the possibility of scaling-up this technique for mass production of conductive composites containing graphene or other layered materials.