Role of the alkyl chain number and head groups location on surfactants self-assembly in aqueous solutions

Role of the alkyl chain number and head groups location on surfactants self-assembly in aqueous solutions was investigated. A series of quaternary ammonium bromide surfactant oligomers with dodecyl chains connected at the level of head groups by a short ethylene spacer (from dimer to tetramer) and their catanionic surfactant mixtures with monomeric anionic surfactant, sodium dodecyl sulfate, were used as model systems. Adsorption and aggregation of cationic surfactant oligomers in aqueous solutions strongly depend on the competition between the number of hydrophobic tails and bimodal charge distribution of ionic head groups at both the air/solution and micelle/solution interfaces. In catanionic surfactant mixtures, the size and shape of mixed aggregates can be modulated by the electrostatic interactions at the aggregate/solution interface as well as by alkyl chain packing determined from the one side by the length of the spacer and from the other side by ion pairing. Experimental equilibrium area per molecule at the air/solution solution interface, before and after mixing, and the regular solution theory were used to explain surfactant molecular interactions in mixed monolayer and mixed aggregates. The increase in the number of dodecyl chains decreases the molar fraction of SDS in the mixed monolayer. In contrast, negatively charged mixed vesicles indicated that aggregates were enriched with SDS.