Intermediate aggregates resulting in the interaction of sodium dodecyl sulphate with phosphatidylcholine liposomes

Abstract

The physicochemical properties of the sodium dodecyl sulphate (SDS)-phosphatidylcholine (PC) aggregates during the overall interaction of the anionic surfactant SDS with PC unilamellar liposomes were investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles and bilayer solubilization as a decrease in the static light scattered by liposome suspensions. A linear relationship was detected in the initial steps of this interaction between the effective molar ratio of surfactant to phospholipid in bilayers ( Re ), the leakage of entrapped CF and the growth 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 with 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 dependence was not observed in the last solubilization steps ( Re between 2.40 and 2.70), possibly owing to the increased formation of mixed micelles in this interval. The fact that the free SDS concentration at sub-solubilizing and solubilizing levels showed values lower than and similar to, respectively, its critical micelle concentration confirms that permeability alterations and solubilization were determined by the action of surfactant monomer and by the formation of mixed micelles, respectively.