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 u sed to study the surface adsorption of sublytic concentrations of the anion ic surfactant sodium dodecylsulfonate (C-12-SO3) on phosphatidylcholine bil ayers. 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 ad dition 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 c oncentrations was obtained, indicating a similar adsorption mechanism regar dless of the surfactant concentration. Hence, a monomeric adsorption is ass umed even in systems with a C-12-SO3 concentration above its critical micel lar 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 phas e coefficients (K). The fact that the lowest values for Re were always reac hed 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 surfactan t molecules exceeded 15 000 was possibly due to the electrostatic repulsion between the free and the adsorbed monomers, which could hinder the incorpo ration of new monomers on the charged surface of liposomes. (C) 2001 Elsevi er Science Ireland Ltd. All rights reserved.