SUBLYTIC INTERACTIONS OF OCTYLPHENOL SURFACTANTS WITH LIPOSOMES MODELING THE STRATUM-CORNEUM LIPID-COMPOSITION

The subsolubilizing interactions of a series of octylphenol polyethoxy lated surfactants (average ethylene oxide units (EO) between 8.5 and 2 0) with liposomes modeling the stratum corneum lipid composition was i nvestigated, Liposomes were formed from a lipid mixture containing 40% ceramides, 25% cholesterol, 25% palmitic acid, and 10% cholesteryl su lfate, The surfactant/lipid molar ratios (R(e)) and the bilayer/aqueou s phase partition coefficients (K) were determined at two sublytic lev els (50% and 100% CF release) by monitoring the increase in the fluore scence intensity of liposomes due to the 5(6) carboxyfluorescein (CF) released from the interior of vesicles, Given that the free surfactant concentrations were always lower than the critical micelle concentrat ions, we may assume that the SC liposomes/surfactant sublytic interact ions were mainly ruled by the action of surfactant monomers, At the tw o interaction levels studied the octylphenol with 8.5 EO units showed the highest ability to alter the release of the CF trapped into bilaye rs (lowest Re values), whereas that with 9.5 EO showed the highest deg ree of partitioning into liposomes or affinity with these structures ( highest K values), The octylphenol with 20.0 EO units showed the lowes t tendencies. Different trends in the interaction of these surfactants with SC and phosphatidylcholine (PC) liposomes were observed when com paring the present Re and K parameters with those reported for PC lipo somes, Thus, although SC liposomes were more resistant to the action o f octylphenols, the degree of partitioning of these surfactants into S C bilayers was always greater than that reported for PC liposomes, (C) 1996 Academic Press, Inc.