Kinetic and structural aspects of the adsorption of sodium dodecyl sulfateon phosphatidylcholine liposomes

The ability of sodium dodecyl sulfate (SDS) to be adsorbed on the surface o f phosphatidylcholine (PC) bilayers was examined by fluorescence spectrosco py using the fluorescent probe 2-(p-toluidinyl)naphthalene-6-sodium sulfona te. The surfactant adsorption was quantified by means of the variations in the surface potential(psi(o)) of liposomes versus incubation time. Very low free SDS concentrations were detected already 10 s after the surfactant ad dition, indicating that the adsorption process was very fast and almost com plete. The correlation between the lipid and SDS concentrations for a given number of monomers adsorbed was always linear, indicating no changes in th e adsorption mechanism. Hence,a monomeric adsorption always occurred even i n systems with a SDS concentration above its critical micelle concentration . Thus, the breaking of the micellar SDS structure into monomers needed for a monomeric adsorption does not seem to be the rate-limiting step of the p rocess. The fact that the lowest values for the surfactant to lipid molar r atio (inversely related to the SDS ability to be adsorbed on liposomes) wer e always reached after 10 s of incubation corroborates the rapid kinetic of the process. The fall in the SDS partitioning for a number of surfactant m olecules exceeding 15 000 was possibly due to the electrostatic repulsion b etween the free and the adsorbed monomers, which hindered the incorporation of new monomers on the charged surface of liposomes.