Determination of isotherms for binding of surfactants to vesicles using a surfactant-selective electrode

Mixtures of the nonionic lipid glycerol monooleate with four cationic surfactants have been investigated. The surfactants were cetyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, and dodecyltrimethylammonium chloride, and one with a partially fluorinated chain, N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride. With a low addition of the cationic surfactant, GMO easily forms vesicles. Using a surfactant selective electrode, binding isotherms of the surfactants to a suspension of preformed vesicles were determined. The mole fractions of surfactant at which the micelle-vesicle coexistence region and the micelle region were reached, as obtained from the binding isotherms, were compared to the location of the corresponding phase borders in the phase diagrams of the respective mixtures. The different stages of the solubilization process were directly followed using cryo-TEM. Instead of a micelle-vesicle coexistence region, perforated vesicles were observed during the solubilization with C(16)TAC. The binding isotherm was nevertheless similar to those of the other surfactants. On the basis of this result, the formation of pores in the bilayer is regarded as a cooperative process, similar to micelle formation. The edges of the pores are richer in surfactant than the intact part of the bilayer. Increasing the surfactant concentration does not change the composition but results in an increased number of pores, similar to a micelle-vesicle phase separation.