Aggregate transitions in mixtures of anionic sulfonate gemini surfactant with cationic ammonium single-chain surfactant.

Aggregation behaviors in mixtures of an anionic gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)-propane (C(12)C(3)C(12)(SO(3))(2)) and a cationic single-chain surfactant cetyltrimethylammonium bromide (CTAB) have been investigated in aqueous solutions at pH 9.5 by turbidity, rheology, isothermal titration microcalorimetry (ITC), cryogenic transmission electron microscopy, and dynamic light scattering. Reversible aggregate transitions from spherical micelles to wormlike micelles, vesicles, and back to wormlike micelles and spherical micelles are successfully realized through fine regulation over the mixing ratio of surfactants, i.e., the anionic/cationic charge ratio. The five aggregate regions display distinguished phase boundaries so that the aggregate regions can be well controlled. From thermodynamic aspect, the ITC curves clearly reflect all the aggregate transitions and the related interaction mechanism. The self-assembling ability of the C(12)C(3)C(12)(SO(3))(2)/CTAB mixtures are significantly improved compared with both individual surfactants. Micelle growth from spherical to long wormlike micelles takes place at a relative low total concentration, i.e., 2.0 mM. The wormlike micelle solution at 10 mM or higher shows high viscosity and shear thinning property. Moreover, the C(12)C(3)C(12)(SO(3))(2)/CTAB mixtures do not precipitate even at 1:1 charge ratio and relative high concentration. It suggests that applying gemini surfactant should be an effective approach to improve the solubility of anionic/cationic surfactant mixtures and in turn may promote applications of the surfactant mixtures.