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

Abstract

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).