Studies on Gemini–Conventional Surfactant Mixtures

This article provides a detailed description of micelles formed upon mixing of cationic gemini alkanediyl-α,ω-bis(dimethylcetylammonium bromides) (henceforth referred as 16-s-16 where the spacer s = 4, 5, 6) with a cationic conventional surfactant cetyltriphenylphosphonium bromide (CTPB). For comparison, the conventional surfactant cetyltrimethylammonium bromide (CTAB) was also mixed with CTPB. The critical micelle concentration (cmc) values were obtained by tensiometric measurements. The results analyzed as per the Clint, Rubingh and Rosen models suggest strong synergism in the mixed systems. Ideal cmc values (cmc*) are always greater than the experimental values. The micellar mole fraction (X1m), interaction parameter (βm), obtained from Rubingh’s model, and interfacial mole fraction (X1σ), interaction parameter (βσ), obtained from Rosen’s model, show that the contribution of CTPB is less but the interaction is strong in mixed micelles as compared to a mixed monolayer. The CTPB molecule contains three phenyl rings and its adjustment at the planar surface (at the air/water interface) is easier than in the micellar surface. The surface excess (Γmax) and minimum area per head group (Amin) values also support this conclusion, as these values are closer to that of pure CTPB. Thermodynamic parameters were also calculated. All these results suggest that mixed micelles are formed with strong synergism between the two components. The 1H NMR studies show that the gemini with spacer s = 4 is more de-shielded than the other geminis, suggesting that these micelles are crowded with more N-methyl protons.