Phase Behavior in Mixtures of Cationic Dimeric and Anionic Monomeric Surfactants

Abstract

The formation of mixed aggregates has been investigated in the mixture of oppositely charged surfactants vastly differing in molecular geometry and size. The systems considered is mixture of the cationic gemini surfactant, ethanediyl-1,2-bis(dodecyldimethylammonium bromide), and anionic surfactant, sodium dodecyl sulfate. Various mixed nano- and microaggregates (micelles, vesicles, thin lamellar sheets, and tubules) were formed depending on bulk composition and total surfactant concentration. Two types of aggregates were found in precipitate, the tubules as prevailing aggregates on the gemini-rich side, and vesicles as prevailing aggregates on the SDS-rich side. The tubules formation was ascribed to mutual influence of specific structure of cationic dimeric surfactant and electrostatic interactions at the bilayer/solution interface. The proposed mechanism involved the formation of lamellar sheets, which rolled-up into tubules.

Keywords:

• Catanionic surfactant mixtures
• gemini surfactants
• sodium dodecyl sulfate
• tubules
• vesicles

Acknowledgments

The authors thank to Professor Nikola Ljubešić for assistance in electron microscopy. This work was supported the Ministry of the Science, Education and Sports of the Republic of Croatia (Grant 098-0982915-2949).