ALKANEDIYL-ALPHA,OMEGA-BIS(DIMETHYLALKYLAMMONIUM BROMIDE) SURFACTANTS(DIMERIC SURFACTANTS) .5. AGGREGATION AND MICROSTRUCTURE IN AQUEOUS-SOLUTIONS

A series of alkanediyl-alpha,omega-bis(dimethylalkylammonium bromide) surfactants, referred to as dimeric m-s-m surfactants (where m and s a re the carbon numbers of the side alkyl chains and of the alkanediyl s pacer), have been investigated by time-resolved fluorescence quenching (determination of micelle aggregation number of the 12-s-12 series) a nd transmission electron microscopy at cryogenic temperature (determin ation of the nature and shape of the aggregates present in solutions o f 12-s-12 and 16-s-16). These m-s-m surfactants can be considered as t he dimers of the quarternary ammonium surfactants having two unequal a lkyl chains with carbon numbers m and s/2. The 12-s-12 surfactants wer e found to form spherical micelles at concentration close to the criti cal micelle concentration. The homologues with s = 6, 8, and 10 showed only a small to moderate micellar growth upon increasing concentratio n, suggesting that the micelles remained spheroidal in the concentrati on range investigated. However, the homologues with s = 3, 4, and 5 sh owed a significant micellar growth, which increased as s decreased. Th e solutions of 12-2-12 showed shear-thickening at the concentration of 1.4% and became viscoelastic above 2%. Cryo-TEM showed the presence o f 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 1 2-4-12, 12-8-12, and 12-12-12, at the concentration investigated, in a greement with the fluorescence quenching results. Vesicles, often doub le-lamellar were observed in solutions of 12-16-12 and 12-20-12. Vesic les were also present in the solutions of the corresponding ''monomeri c'' surfactants. In the case of the 16-s-16 series, vesicles, bilayer membrane fragments and threadlike micelles were observed in 16-3-16 so lutions. Only threadlike micelles were seen in 16-4-16 solutions, wher eas 16-6-16 solutions showed slightly elongated micelles and 16-8-16 s olutions spherical micelles. The above results clearly indicate that t he dimeric surfactants investigated tend to form in aqueous solutions aggregates of lower curvature than the corresponding ''monomeric'' sur factants.