m‑s‑m Cationic Gemini and Zwitterionic Surfactants–Spacer-Dependent Synergistic Interactions

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.