Nonaromatic hydrotropic cationic ammonium salts as a rheology modifier for an anionic/zwitterionic surfactant mixture.

In this article, we report additive-induced micellar growth and rheology modification for mixtures of anionic (sodium dodecyl sulfate, SDS) and zwitterionic (N-alkylated glycine derivative, Empigen BB or EBB) surfactants. Two nonaromatic hydrotropic salts (hexyltrimethylammonium bromide, C6TAB, and/or dibutylenebis(dimethylbutylammonium bromide), 4-4-4) are used as novel additives to induce micellar growth in these systems. Nuclear magnetic resonance (NMR), photon correlation spectroscopy (PCS), transmission electron microscopy (TEM), the Weissenberg effect, and rheology measurements were employed to assess mixed micelle formation, micellar growth, and rheology modifications. Finally, the manner in which the surfactants and hydrotropes self-assemble into aggregates has also been deduced from 2D NMR NOESY measurements. In this study, both hydrotropic ions have been found to contribute to similar structural modifications in the mixed micelles of the anionic and zwitterionic surfactants. However, the extent of the rheology modification in solution is found to be quite different when the gemini hydrotrope (4-4-4) versus the monomeric hydrotrope (C6TAB) is employed.