From single gemini surfactants in water to catanionic mixtures with the bile salt sodium taurodeoxycholate: Extensive micellar solutions, coacervation and liquid crystal polymorphism as revealed by phase behavior studies

Abstract

Gemini surfactants and bile acid salts are interesting amphiphiles owing to their unconventional and versatile molecular structure, and consequent unusual aggregation properties. In this work, we investigated the phase behavior of binary surfactant/water systems for the cationic bis-quat gemini surfactants 12-2-12 (short spacer) and 12-6-12 (intermediate spacer) and subsequently catanionic mixtures of each gemini surfactant and the bile salt sodium taurodeoxycholate, STDC. The phase diagrams and phase structure were unraveled by polarized light microscopy, differential scanning calorimetry, deuterium NMR and small angle X-ray scattering. The 12-2-12 and 12-6-12 gemini form a micellar-lamellar-cubic and micellar-hexagonal-cubic phase sequence, respectively, denoting the strong influence of the spacer length on the self-assembly properties. Both the catanionic 12-2-12/STDC/water and 12-6-12/STDC/water systems display a rich mesomorphic behavior. One of the most striking features is that the STDC micelles are able to incorporate enormous amounts of gemini surfactant, forming an extremely wide solution region of mixed micellar structures. By contrast, the gemini micelles can only solubilize limited amounts of STDC. The liquid crystalline phases from the binary systems incorporate sizeable amounts of the counterpart surfactant, forming mixed catanionic aggregates at high concentration. Interestingly, the dilute region of both systems is dominated by liquid-liquid phase separation implying the formation of aqueous two-phase micellar systems. The observed features are qualitatively rationalized in terms of packing and electrostatic effects between the surfactants at play.