Kinetic and structural aspects of the adsorption of sodium dodecyl sulfate on phosphatidylcholine liposomes

The ability of sodium dodecyl sulfate (SDS) to be adsorbed on the surface of phosphatidylcholine (PC) bilayers was examined by fluorescence spectroscopy using the fluorescent probe 2-(p-toluidinyl)naphthalene-6-sodium sulfonate. 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 addition, indicating that the adsorption process was very fast and almost complete. The correlation between the lipid and SDS concentrations for a given number of monomers adsorbed was always linear, indicating no changes in the adsorption mechanism. Hence,a monomeric adsorption always occurred even in 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 process. The fact that the lowest values for the surfactant to lipid molar ratio (inversely related to the SDS ability to be adsorbed on liposomes) were 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 molecules exceeding 15 000 was possibly due to the electrostatic repulsion between the free and the adsorbed monomers, which hindered the incorporation of new monomers on the charged surface of liposomes.