REGULAR PAPERS
Aqueous Properties of Mixed Anionic Gemini Surfactant and Conventional Anionic Surfactant
2002 Volume 51 Issue 2 Pages 123-131
Details
2002 Volume 51 Issue 2 Pages 123-131
In order to determine the characteristics of a mixed system of an anionic gemini surfactant with N, N-dialkylamide and carboxylate groups and a conventional anionic surfactant, aqueous solutions of (CH2)2[NCO(C11H23)C2H4CO2Na]2 (212)/sodium dodecanoate (soap) mixture were studied. The following parameters were measured: critical micelle concentration (cmc), micelle composition, surface tension, foaming power, degree of micelle ionization, and pH values. For the sake of comparison, mixtures of SDSa/soap (SDSa=sodium N-dodecanoylsarcosinate, the corresponding monomer of 212) were also investigated. The cmc values of the SDSa/soap mixture monotonously increase with increasing molar fraction of soap at 30°C. In contrast, the cmc values of the 212/soap mixture are intermediate between those for the pure surfactants over a broad region at 30°C. The composition of 212/soap mixture in micelles is kept at a 2:1 molar ratio in the plateau region. With increase in temperature of 212/soap mixture, cmc values reach a maximum at a soap molar fraction of 0.6. This is obvious at 50°C. The surface tension of 212/soap mixture is minimal at a soap molar fraction of 0.6 at 30°C, at which the surface tension is the lowest among those of the pure surfactants. The foaming power of the 212/soap mixture is maximal at the soap molar fraction of 0.6, at which the foaming power is the largest among those of the pure surfactants. Greater degree of micelle ionization for 212/soap mixture than for SDSa/soap mixture was observed over a broad region. The reason for this is the larger proton uptake by the carboxylate group of 212 than that of SDSa, which releases Na+ into the bulk phase in the mixed micellization process. The evidence for this is the larger increase in pH in the mixed cmc solutions for 212/soap mixture than for SDSa/soap mixture. The unexpected synergistic interaction between 212 and soap molecules is due to stronger reduction of the mutual repulsion between the anionic head groups in the 212 molecule in the presence of a soap molecule.
