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 (TNS) was used to study the surface adsorption of sublytic concentrations of the anionic surfactant sodium dodecylsulfonate: (C-12-SO3) on phosphatidylchol ine (PC) bilayers. The number of adsorbed molecules was quantified by deter mination of the electrostatic potential (psi (o)) of the bilayers. The abru pt decrease in the fluorescence intensity detected even 10 s after the surf actant addition and the slight fluorescence variations with time indicated that the surfactant adsorption was very fast and almost complete. For a giv en number of monomers adsorbed a lineal dependence between the lipid and C- 12-SO3 concentrations was obtained, indicating 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 CMC. In addition, this linear correlation allowed us to determine the surfactant:li pid molar ratios (Re) (inversely related to the C-12-SO3 ability to be adso rbed on liposomes) and the bilayer/aqueous phase coefficients (li). The fac t that the lowest values for Re were always reached after 10 s of incubatio n corroborates the rapid kinetics of the process. The decrease in the C-12- SO3 partitioning (K) when the number of surfactant molecules exceeded 15 00 0 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 Published by Elsevier Science Ir eland Ltd. All rights reserved.