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 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 coefficientsK_50%CF,K_SAT andK_SOL were determined. The free surfactant concentrationsS_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 theRe increased as the mole fraction of the SDS rose (X_SDS), theK parameters showed a maximum atX_SDS values of about 0.6, 0.4 and 0.2 forK_50%CF,K_SAT andK_SOL respectively. Thus, the higher the surfactant contribution in surfactant/lipid system, the lower theX_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.Abbreviations PC Phosphatidylcholine - PIPES piperazine-1,4 bis (2-ethanesulphonic acid) - SDS sodium dodecyl sulphate - X_SDS mole fraction of sodium dodecyl sulphate in the mixed system - CF 5(6)-carboxyfluorescein - Re effective surfactant/lipid molar ratio - Re_50%CF effective surfactant/lipid molar ratio for 50% CF release - Re_SAT effective surfactant/lipid molar ratio for bilayer saturation - Re_SOL effective surfactant/lipid molar ratio for bilayer solubilization - S_W surfactant concentration in the aqueous medium - S_{W, 50%CF} surfactant concentration in the aqueous medium for 50% CF release - S_{W, SAT} surfactant concentration in the aqueous medium for bilayer saturation - S_{W, SOL} surfactant concentration in the aqueous medium for bilayer solubilization - S_B surfactant concentration in the bilayers - K bilayer/aqueous phase surfactant partition coefficient - K_50%CF bilayer/aqueous phase surfactant partition coefficient for 50% CF release - K_SAT bilayer/aqueous phase surfactant partition coefficient for bilayer saturation - K_SOL bilayer/aqueous phase surfactant partition coefficient for bilayer solubilization - PL phospholipid TLC-FID, thin-layer chromatography/flame ionization detection system - PI polydispersity index - CMC critical micellar concentration - r² regression coefficient