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Critical micelle concentration (cmc) values were determined for the mixed zwitterionic/cationic gemini systems of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (ZW3–12)/N,N′-bis­(dimethyldodecyl)-α,ω-alkanediammonium dibromide (12-s-12) systems. The cmc values for the mixed systems were determined through conductivity measurements. The degree of nonideality of the interaction in the mixed micelle (βm), for each system, was determined according to Rubingh’s nonideal solution theory. In most cases, the systems exhibited negative deviations (−βm values) at high surfactant mole fractions of zwittergent (αZW3–12). Specifically, the ZW3–12/12–4–12 system displayed −βm values at αZW3–12 ≥ 0.5, while both the ZW3–12/12–5–12 and the ZW3–12/12–6–12 systems displayed −βm values over the entire mole fraction range. Except for the low mole fraction range in the 12–4–12 system, these mixed surfactant systems demonstrated almost identical behavior to the n-dodecyltrimethylammonium bromide/12–2–12 system studied by Bakshi et al. providing further evidence that ZW3–12 tends to behave as a cationic surfactant in mixed surfactant systems. The manner in which the cosurfactants aggregate in the micelles was determined via two-dimensional (2D) NOESY spectroscopy. In the case of both the ZW3–12/12–5–12 and the ZW3–12/12–6–12 systems, the 2D NOESY spectra exhibited strong cross peaks between the gemini and zwitterionic surfactants over the entire micellar composition range. In the case of the ZW3–12/12–4–12 system, little cross peak intensity was observed between the gemini and the zwitterionic surfactant at low micellar compositions of the zwittergent. The results suggest some micelle demixing is occurring between the gemini and the zwittergent certain micellar composition ranges, a phenomenon rarely associated with hydrocarbon surfactants.

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.