Vesicles in Simple Binary Water - non-ionic Surfactant Systems

This work aims at improving our understanding of the equilibrium structure of the vesicular La+ phase in the model system water - non-ionic surfactant C12E4. The results may help the theoretical description of vesicles formed in more complicated, biologically relevant systems. We investigated the mean size and bilayer thickness of the vesicles formed. In order to shed light on the thermodynamic stability of the vesicular phase negative and positive temperature jump experiments from the adjacent single phases were performed. The idea is, should the structures obtained following opposite temperature jumps be in agreement, the vesicular phase should be a thermodynamically stable phase. Dynamic Light Scattering (DLS), Small Angle Neutron Scattering (SANS) and Deuterium Nuclear Magnetic Resonance (2H - NMR) experiments were used as experimental tools. Mutually supporting each other these techniques facilitate the characterization of the equilibrium structure of the La+ phase for a wide range of surfactant concentrations and sample preparation procedures. In some cases in addition Freeze Fracture Electron Microscopy (FFEM) was performed. The results show that regions in the phase diagram denoted by La+ represent a wealth of structural aggregates among which vesicular objects are present. However, in most cases lamellar phase fragments and overlapping network supra-structures are found. Even when vesicles were the dominant feature, they were far from being well-defined in size and number of lamellae. Often uni- and multilamellar vesicles were coexisting having a diameters of order 30nm, 100nm and 1mm, respectively. Thus, a thermodynamic stability of the La+ phase is not supported by this work