INTERACTION OF EQUIMOLECULAR MIXTURES OF NONIONIC ANIONIC SURFACTANTSWITH LIPOSOMES/

The mechanisms governing the interaction of equimolecular mixtures of nonylphenol polyethoxylated with 10 mol of ethylene oxide (NP(EO)(10)) and sodium dodecyl sulfate (SDS) with phosphatidylcholine (PC) Liposo mes were investigated. Permeability alterations were detected as a cha nge in 5(6)-carboxyfluorescein (CF) released from the interior of vesi cles and bilayer solubilization was detected as a decrease in the stat ic light scattered by liposome suspensions. At the subsolubilizing lev el, a maximum bilayer/water partitioning of surfactant mixture was rea ched at 30-50% CF release, which correlated with the increased presenc e of SDS in the bilayers. However, transition stages between 60% CF re lease and 100% light scattering corresponded to the increased presence of NP(EO)(10) in these structures. The surfactant mixture showed thro ughout the interaction a much higher affinity with bilayers and a high er ability to saturate and solubilize these structures than that repor ted for the anionic component. A direct dependence was established in the initial interaction steps (effective molar ratio of surfactant to phospholipid in bilayers (R(e)) lower than 0.190) between the growth o f vesicle fluidity and the maximum bilayer/water partitioning of the s urfactant mixture (K). These parameters also depended on the surfactan t saturation of the outer vesicle leaflet. A linear dependence was als o established during solubilization between the decrease in both the s urfactant-PC aggregate size and the scattered light of the system and the composition of these aggregates (R(e)). The fact that the free sur factant concentration at subsolubilizing and solubilizing levels was r espectively lower than and similar to the critical micelle concentrati on (cmc) of the surfactant mixture indicates that permeability alterat ions and solubilization were determined respectively by the action of the surfactant monomer and by the formation of mixed micelles. This fi nding supports the generally admitted assumption, for single surfactan ts, that the concentration of free surfactant must reach the cmc for s olubilization to occur and highlights the influence of the negative sy nergism of this surfactant mixture on the free surfactant concentratio n needed to saturate or solubilize liposomes.