Abstract
A series of alkanediyl-α,ω-bis(dimethylalkylammonium bromide) surfactants, referred to as dimeric m-s-m surfactants (where m and s are the carbon numbers of the side alkyl chains and of the alkanediyl spacer), have been investigated by time-resolved fluorescence quenching (determination of micelle aggregation number of the 12-s-12 series) and transmission electron microscopy at cryogenic temperature (determination of the nature and shape of the aggregates present in solutions of 12-s-12 and 16-s-16). These m-s-m surfactants can be considered as the dimers of the quarternary ammonium surfactants having two unequal alkyl chains with carbon numbers m and s/2. The 12-s-12 surfactants were found to form spherical micelles at concentration close to the critical micelle concentration. The homologues with s=6, 8, and 10 showed only a small to moderate micellar growth upon increasing concentration, suggesting that the micelles remained spheroidal in the concentration range investigated. However, the homologues with s=3, 4, and 5 showed a significant micellar growth, which increased as s decreased. The solutions of 12-2-12 showed shear-thickening at the concentration of 1.4% and became viscoelastic above 2%. CryoTEM showed the presence of giant entangled threadlike micelles in the aqueous solutions of this surfactant. Elongated micelles were also seen in solutions of 12-3-12. Only spherical or spheroidal micelles were present in solutions of 12-4-12, 12-8-12, and 12-12-12, at the concentration investigated, in agreement with the fluorescence quenching results. Vesicles, often double-lamellar were observed in solutions of 12-16-12 and 12-20-12. Vesicles were also present in the solutions of the corresponding «monomeric» surfactants. In the case of the 16-s-16 series, vesicles, bilayer membrane fragments and threadlike micelles were observed in 16-3-16 solutions. Only threadlike micelles were seen in 16-4-16 solutions, whereas 16-6-16 solutions showed slightly elongated micelles and 16-8-16 solutions spherical micelles. The above results clearly indicate that the dimeric surfactants investigated tend to form in aqueous solutions aggregates of lower curvature than the corresponding «monomeric» surfactants
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