Efficient silicon-containing di-chain anionic surfactants for stabilizing oil-water interfaces in microemulsions

New di-chain anionic surfactants containing silicon (Si) atoms in the hydrophobic chain-tips (trimethylsilyl (TMS) hedgehog surfactants) are able to reduce air-water (A-W) surface tension γ_cmc to as low as ≈22 mN m^-1 (A. Czajka, C. Hill, J. Peach, J. C. Pegg, I. Grillo, F. Guittard, S. E. Rogers, M. Sagisaka and J. Eastoe, Phys. Chem. Chem. Phys., 2017, 19, 23869). However, the extent to which these surfactants stabilize alkane oil-water (O-W) interfaces is unexplored. Here, it is shown that such TMS surfactants are able to stabilize water-in-oil microemulsions (W/O-µEs). The O-W interfacial tensions γ_o/w in these µEs are ultra-low, in the range of 10^-2 to 10^-4 mN m^-1, and µE-stability can be optimized by varying surfactant and solvent chemical structures. For example, with aliphatic n-alkanes and cycloalkanes, the surfactant AOT-SiC alone stabilizes W/O-µEs over a wide temperature window, but not with the aromatic solvent toluene. Likewise, AOT-SiB forms W/O-µEs, but preferably in aromatic solvents, such as toluene. Contrast-variation small-angle neutron scattering (SANS) measurements indicate that the water droplets in these W/O-µEs are stabilized by surfactant-monolayers. In all of these systems, the droplet morphologies and shapes are correlated with the proximity to (from) the µE-phase stability boundaries. The results show that Si-containing TMS surfactants are effective at O-W interfaces, promoting the ultra-low interfacial tensions necessary for stabilization of µEs. These TMS surfactants offer credible alternatives to environmentally damaging and health-hazardous fluorinated surfactants (FSURFs).