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.
The micellar and morphological properties of symmetric, cationic gemini surfactants have been well studied in the literature as a function of nature and type of the spacer group and the length and type of hydrophobic chain. In this paper, we have examined the effects of tail asymmetry on the properties of a series of cationic surfactants, the N-alkyl-1-N′-alkyl-2-N,N,N′,N′-tetramethyldiammonium dibromide. A novel synthetic method is used to prepare a series of these surfactants and the consequences of asymmetry on micellar properties are presented. This new method has been shown to be more efficient, with higher yields of the asymmetric surfactants than the yields of the accepted literature method. The critical micelle concentration values and the micelle sizes of the asymmetric gemini surfactants, 12-4-12, 12-4-10, 12-4-8, and 12-4-6 gemini surfactants, were obtained from conductivity and dynamic light scattering. With increasing chain asymmetry, the size of the micelle increased due to the formation of loose micelles. The addition of NaCl and Na2SO4 to the surfactant solutions increased the aggregate size, and this effect was more pronounced with increasing salt concentrations. These results are interpreted in terms of the effect these ions have on the “compactness” of the micelle structure.
The surface and self-assembly properties of a family of multiheaded, multitailed surfactants based on a tripentaerythritol backbone are described. Critical aggregation concentrations of these unusual surfactant systems have been determined from surface tension measurements; aggregate sizes in the presence and absence of a small amount of added electrolyte have been obtained via dynamic light scattering, and the morphologies of the aggregates were examined from electron microscopy measurements. In general, when compared to conventional ionic and two-headed surfactants (and other recently synthesized pentaerythritol derived surfactants from this group), these multiheaded surfactants exhibited some unusual trends in their aggregation behaviour and interesting aggregate structures in aqueous solution, as a function of alkyl chain length.
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