Interaction in Binary Mixtures of Gemini Surfactant G12-6-12 and CTAB by NMR

The interaction between N'N0-bis(dimethyldodecyl)-1,6-hexanediammoniumdibromide (G12-6-12) and cetyltrimethylammonium bromide (CTAB) in D₂O aqueous medium has been investigated by NMR at 298 K. The measured critical micelle concentration (cmc) of G12-6-12 and CTAB are about 0.773 and 0.668 mmol/L, respectively. The cmc^* (cmc of mixture) values are less than CMC^* (cmc of ideally mixed solution) in the mixed system, and the interaction parameter β_M<0 at different molar fractions α of G12-6-12 in the mixed systems, but just when α≤0.3, cmc^* values are much smaller than CMC^*, and β_M satisfies the relation of |β_M|>|ln(cmc₁=cmc₂)|(cmc₁: cmc of pure G12-6-12 and cmc₂: cmc of pure CTAB). The results indicate that there exists synergism between G12-6-12 and CTAB, and they can form mixed micelles, which is further proven by 2D NOESY and self-diffusion co-efficient D experiments. There are intermolecular cross peaks between G12-6-12 and CTAB in 2D NOESY, and the radius of micelles in mixed solution is bigger than that in G12-6-12 pure solution in D experiments, indicating there are mixed micelles. However, when α>0.3, we find that cmc^*≈CMC^*, β_M≈0, obviously, the two surfactants are almost ideal mixing fitting the pseudo-phase separation model and regular solution theory.