Inter-actions of oxyethylenated nonylphenols with liposomes mimicking the stratum corneum lipid composition

The interactions of oxyethylenated nonylphenols (ethylene oxide units (EO) average between 5 and 30) with liposomes modeling the stratum corneum lipid composition were investigated. Liposomes were formed by a lipid mixture containing 40% ceramides, 25% cholesterol, 25% palmitic acid and 10% of cholesteryl sulfate. The surfactant/lipid molar ratios (Re) and the bilayer/aqueous phase partition coefficients (K) were determined at three interaction levels: 50% release of the 5 (6) carboxyfluorescein (CF) trapped into the vesicles, saturation, and complete liposome solubilization. The fact that the free surfactant concentrations for each surfactant tested were at sublytic and lytic levels lower than and similar to its critical micelle concentration (in the experimental working medium) indicated that these interactions were mainly governed by the action of surfactant monomers and the formation of mixed micelles respectively. The nonylphenols with 15 EO units always showed the highest activity on SC bilayers (lowest Re values), whereas that with 5 EO units exhibited the highest degree of partitioning into liposomes or affinity with these bilayer structures (highest K values). Different trends in the interaction of nonylphenols with SC and phosphatidylcholine (PC) liposomes were observed when comparing the present Re and K parameters with those reported for PC ones. Thus, whereas SC liposomes were more resistant to the action of nonylphenols, the degree of partitioning of these surfactants into SC bilayers was always greater than that reported for PC ones. (C) 1998 Elsevier Science B.V. All rights reserved.