INTERMEDIATE AGGREGATES RESULTING IN THE INTERACTION OF SODIUM DODECYL-SULFATE WITH PHOSPHATIDYLCHOLINE LIPOSOMES

The physicochemical properties of the sodium dodecyl sulphate (SDS)-ph osphatidylcholine (PC) aggregates during the overall interaction of th e anionic surfactant SDS with PC unilamellar liposomes were investigat ed. Permeability alterations were detected as a change in 5(6)-carboxy fluorescein (CF) released from the interior of vesicles and bilayer so lubilization as a decrease in the static light scattered by liposome s uspensions. A linear relationship was detected in the initial steps of this interaction between the effective molar ratio of surfactant to p hospholipid in bilayers (Re), the leakage of entrapped CF and the grow th of vesicles (Re up to 0.10), whereas in the Re interval 0.10-0.73 a maximum in the growth of vesicles (413 nm for Re 0.43) co-existed wit h a progressive increase in the CF release. In the Re range 1.10-2.40 (solubilizing level), a linear dependence was also established between the decrease in both the surfactant-PC aggregate size and the static light scattering of these systems and their composition (Re). This dep endence was not observed in the last solubilization steps (Re between 2.40 and 2.70), possibly owing to the increased formation of mixed mic elles in this interval. The fact that the free SDS concentration at su b-solubilizing and solubilizing levels showed values lower than and si milar to, respectively, its critical micelle concentration confirms th at permeability alterations and solubilization were determined by the action of surfactant monomer and by the formation of mixed micelles, r espectively.