Alterations in phospholipid bilayers caused by sodium dodecyl sulphate triton X-100 mixed systems

The mechanisms governing the subsolubilizing and solubilizing interaction of sodium dodecyl sulphate (SDS)/Triton X-100 mixtures and phosphatidylcholine liposomes were investigated. Permeability alterations were detected as a change in 5(6)-carboxy-fluorescein (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 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); 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. The free surfactant concentrations S-W were lower than the mixed surfactant CMCs at subsolubilizing level, whereas they remained similar to these values during saturation and solubilization of bilayers in all cases. Although the Re increased as the mole fraction of the SDS rose (X(SDS)), the K parameters showed a maximum at X(SDS) values of about 0.6, 0.4 and 0.2 for K-50%CF, K-SAT and K-SOL respectively. Thus, the higher the surfactant contribution in surfactant/lipid system, the lower the X(SDS) at which a maximum bilayer/water partitioning of mixed surfactant systems added took place and, consequently, the lower the influence of the SDS in this maximum bilayer/water partitioning.