Aggregation behaviour of mixtures of phosphatidylcholine and polyoxyethylene sorbitan monoesters in aqueous solution

Photon correlation spectroscopy and electron microscopy have been used to determine the nature of aggregates formed using the thin film method of preparation by mixtures of phospholipids (specifically dimyristoyl-, dipalmitoyl-, distearoyl- and dioleoylphosphatidylcholine––DMPC, DPPC, DSPC and DOPC, respectively) and nonionic surfactants (specifically polyoxyethylene sorbitan monolaurate, mixtures of monopalmitate and monostearate, monooleate––Tween 20, 40, 60 and 80) at molar ratios of up to 50 mol% surfactant. In all cases the mixed phospholipid–Tween preparations were subjected to 5 freeze–thaw cycles in order to determine the stability of the aggregates formed in the mixed systems. Regardless of the phospholipid present, mixtures of up to 20 mol% Tween contained as their major aggregate, vesicles. In a number of instances larger vesicles were formed in the presence of lower amounts of Tween than were formed when the phospholipids were used alone, presumably due to the incorporation of at least some of the Tween into the vesicle bilayers. This result suggests that, under the conditions of the test, it was easier to form multilamellar vesicles in the presence of Tween. At the highest concentrations of Tween, micelles were frequently the most commonly encountered aggregate. However the longer diacyl chain phospholipids (i.e. DPPC and DSPC) were found to be less likely to form mixed-micelles with Tween than the shorter diacyl chain phospholipid, DMPC. The unsaturated phospholipid, DOPC did not readily form mixed-micelles over the surfactant concentration ranges studied, forming instead surfactant-containing vesicles.