Use of a fluorescence spectroscopy technique to study the adsorption of sodium dodecylsulfonate on liposomes

The fluorescent probe 2-(p-toluidinyl)-naphthalene-6-sodium sulfonate was used to study the surface adsorption of sublytic concentrations of the anionic surfactant sodium dodecylsulfonate (C-12-SO3) on phosphatidylcholine bilayers. The number of adsorbed molecules was quantified by determination of the electrostatic potential (psi (0)) Of the bilayers. The abrupt decrease in the fluorescence intensity already detected 10 s after the surfactant addition and the slight fluorescence variations with time indicated that the surfactant adsorption was very fast and almost complete. For a given number of monomers adsorbed, a linear dependence between the lipid and C-12-SO3 concentrations was obtained, indicating a similar adsorption mechanism regardless of the surfactant concentration. Hence, a monomeric adsorption is assumed even in systems with a C-12-SO3 concentration above its critical micellar concentration (CMC). In addition, this linear correlation allowed us to determine the surfactant/lipid molar ratios (Re) (inversely related to the C-12-SO3 ability to be adsorbed on liposomes) and the bilayer/aqueous phase coefficients (K). The fact that the lowest values for Re were always reached after 10 s of incubation corroborates the rapid kinetic of the process. The decrease in the C-12-SO3 partitioning (K) when the number of surfactant molecules exceeded 15 000 was possibly due to the electrostatic repulsion between the free and the adsorbed monomers, which could hinder the incorporation of new monomers on the charged surface of liposomes. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.