VESICLE-MICELLE STRUCTURAL TRANSITIONS OF PHOSPHOLIPID-BILAYERS AND SODIUM DODECYL-SULFATE

The transitional stages resulting in the interaction of sodium dodecyl sulfate (SDS)/phosphatidylcholine liposomes were studied by means of transmission electron microscopy (TEM), light scattering, and permeability changes. A linear correlation was observed between the surfactant/lipid molar ratio (Re) and the surfactant concentration throughout the process. However, the bilayer/aqueous phase partition coefficient (K) showed a maximum for 30% bilayer permeability (beginning of bilayer saturation). Hence, a preferential incorporation of surfactant molecules into liposomes governs the initial interaction steps, leading to the beginning of bilayer saturation with a free surfactant concentration that was lower than its critical micelle concentration(cmc). Additional surfactant amounts increased the free surfactant until the cmc is reached, after which solubilization started to occur. Large unilamellar vesicles began to form as the surfactant exceeded 10 mol % (20% permeability), the maximum vesicle growth being attained between 30 and 100% of permeability. A sharp decrease in static light scattering occurred after the Re for saturation (Re-sat), where TEM observations still showed vesicles although with traces of smaller structures. From 60 mol % the size curves began to show a bimodal distribution. TEM pictures for 60-68 mol % showed a gradual vesicle disintegration with formation of tubular structures. SDS concentrations greater than 74 mol % led to the solubilization of bilayers.