Association and Microstructure in Self-Assembling Catanionic Systems

Abstract: This thesis addresses the phase behavior and phase structure in mixed surfactant-surfactant and surfactant-polymer systems, where the co-solutes have opposite charges (catanionic systems). The initial goal has been to investigate self-assembly aspects in mixtures of single-/double-chained surfactants and bile salt/double-chained surfactants, with the main focus on vesicle assembly in dilute regions. Thus, the phase behavior of the catanionic pairs sodium dodecyl sulphate (SDS) - didodecyldimethylammonium bromide (DDAB) and sodium taurodeoxycholate (STDC) -DDAB have been comprehensively studied. Phase diagrams are available for the entire composition range and for the very dilute region. Regions of vesicle formation and micelle/vesicle coexistence in the SDS-DDAB sytem have been identified and characterized in detail by microscopy and NMR self-diffusion methods. The formation of liquid crystalline phases in several binary bile salt-water systems, and the phase behavior of the STDC-DDAB system, where coacervation and strong dispersions occur, have also been investigated in detail. The influence of preparation metods on vesicle properties, namely size and polydispersity, has also been studied by means of NMR self-diffusion and light microscopy. Another main goal of this work, was the investigation of the effects of addition of a macromolecule to a solution of the catanionic vesicles previously characterized. Both the general phase behavior and microstructural aspects have been addressed. The macromolecules consist of cationic polyelectrolytes, unmodified and modified with hydrophobic chains. The polymer is found to induce considerable structural changes in the vesicle, at low partial charge neutralization. Beyond charge neutralization, gel formation was obtained, where the vesicle structure is preserved.