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 permeabi lity changes. A linear correlation was observed between the surfactant /lipid molar ratio (Re) and the surfactant concentration throughout th e 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 molecul es into liposomes governs the initial interaction steps, leading to th e beginning of bilayer saturation with a free surfactant concentration that was lower than its critical micelle concentration(cmc). Addition al surfactant amounts increased the free surfactant until the cmc is r eached, after which solubilization started to occur. Large unilamellar vesicles began to form as the surfactant exceeded 10 mol % (20% perme ability), the maximum vesicle growth being attained between 30 and 100 % of permeability. A sharp decrease in static light scattering occurre d after the Re for saturation (Re-sat), where TEM observations still s howed 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 t o the solubilization of bilayers.