CHANGES IN PHOSPHOLIPID-BILAYERS CAUSED BY SODIUM DODECYL SULFATE NONIONIC SURFACTANT MIXTURES/

The interaction of mixtures of sodium dodecyl sulfate (SDS) and oxyeth ylenated nonylphenol (30 mol of ethylene oxide) [NP(EO)(30)] with phos phatidylcholine liposomes was investigated. Permeability alterations w ere detected as a change in 5 (6)-carboxyfluorescein (CF) released fro m the interior of vesicles, and bilayer solubilization was measured as a decrease in the static light scattered by liposome suspensions. Thr ee parameters were described as the effective surfactant/lipid molar r atios (Re) at which the surfactant system: (i) resulted in 50% CF rele ase (Re-50%CF); (ii) saturated the liposomes (Re-SAT); (iii) led to co mplete solubilization of these structures (Re-SOL). The corresponding surfactant partition coefficients (K-50%CF, K-SAT, and K-SOL) were det ermined from these parameters. The free surfactant concentrations S-W were lower than the mixed surfactant critical micellar con centration at subsolubilizing levels, whereas they remained similar to these valu es during saturation and solubilization of bilayers. Although the Re v alues increased linearly as the mole fraction of the SDS rose (X(SDS)) , the K parameters showed maximum values at X(SDS) 0.6 for K-50%CF and approximately at X(SDS) 0.2 for values at X(SDS) 0.2 for K-SAT and K- SOL, respectively. Thus, the lower the surfactant contribution in the surfactant/lipid system, the higher the X(SDS) at which the maximum bi layer/water partitioning of added mixed surfac tant systems occurred. As a consequence, the influence of SDS in this partition appears to be more significant at the sublytic level (monomeric effect), whereas th e influence of NP(E0)(30) seems to be greater during saturation and so lubilization of liposomes via formation of mixed micelles.