Solubilization of lecithin vesicles by C 12E 8: Structural transitions and temperature effects

Abstract

The structural transitions induced by addition of the nonionic surfactant C 12E 8 (octaethylene glycol n-dodecyl monoether) to small unilamellar lecithin vesicles has been studied by means of static and dynamic light scattering as well as with cryo-transmission electron microscopy. At low surfactant concentration a slight swelling of the vesicles, due to the incorporation of surfactant monomers into the vesicle membrane, can be seen. Large unilamellar vesicles begin to form as the surfactant concentration exceeds 30 mol%. The vesicle size increases with C 12E 8 concentration until about 40 mol% has been added. At this point the intensity of light scattered at 90° starts to decrease and dynamic light scattering indicates bimodal size distributions. The cryo-TEM micrographs show cylindrical micelles in coexistence with bilayer sheets. At C 12E 8 concentrations above 70 mol%, the lipid vesicles are completely solubilized into mixed micelles that gradually become spherical and decrease in size with surfactant concentration. Depending on the C 12E 8 concentration the formation of the lipid/surfactant mixed aggregates may take from less than a second to more than an hour. Increasing the temperature shifts the lipid/surfactant molar ratios at which the different amphiphilic structures start to appear toward lower values.