INTERACTION OF ANIONIC/NONIONIC SURFACTANT MIXTURES WITH PHOSPHATIDYLCHOLINE LIPOSOMES

The mechanisms governing the interaction of mixtures of sodium dodecyl sulfate (SDS) and nonylphenol oxyethylenated with 10 mol of ethylene oxide (NP(EO)(10)) with phosphatidylcholine liposomes were investigated, Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles and bilayer solubilization as a decrease in the static light scattered by liposome suspensions. Three parameters were described as the effective surfactant/lipid molar ratios (Re) at which the surfactant system (a) resulted in 50% of CF release (Re-50%CF), (b) saturated the liposomes (Re-SAT), and (c) led to a complete solubilization of these structures (Re-SOL). From these parameters the corresponding surfactant partition coefficients (K-50%CF), K-SAT, and K-SOL) were determined. Despite the fact that Re increased as the mole fraction of the SDS rose (X(SDS)), the K parameters showed maximum values at X(SDS) 0.6 and 0.2 for K-50%CF and K-SAT, respectively, the K-SOL reaching the highest value in the absence of SDS X(SDS) = 0). Thus, the higher the surfactant contribution in surfactant/lipid system, the lower the X(SDS) at which the maximum bilayer/water partitioning of mixed surfactant systems added took place. The free surfactant concentrations S-W were lower than the mixed surfactant CMCs at subsolubilizing level, whereas it remained similar to these values during saturation and solubilization of bilayers in all cases. (C) 1995 Academic Press,Inc.