Effects of sodium dodecyl sulfate on mixed liposome solubilization

It is well-known that harshness and skin irritation are related to surfactant-skin lipid interactions. Delipidization of the stratum corneum is caused by surfactant penetration into extracellular lipid bilayers, disrupting the lipid microstructure and, thereby, impairing skin-barrier properties. The main objective of this work was to investigate the mechanisms of interactions of sodium dodecyl sulfate (SDS) with liposomes as model membranes. Vesicle-to-micelle structural transitions that occur as a result of the interaction of SDS with liposomes (1: 1 phosphatidylcholine/phosphatidic acid) were studied by monitoring the optical density, surface tension, and monomer concentration. Two inflections appeared on the turbidity curve, which allowed the determination of surfactant concentrations corresponding to the onset and completion of the solubilization of liposomes. The effective surfactant-to-lipid mole ratios in the surfactant-saturated lipid bilayers (R-sat) and in the lipid-saturated mixed micelle (R-sol) were estimated from the excess surfactant monomer left in the supernatant. The monomer concentration of the surfactant in the supernatant of the liposome/surfactant suspension was estimated by ultrafiltration followed by a titration technique. The changes in the surface-tension data enabled us to elucidate the mechanisms by which surfactant solubilizes the liposomes and finally reaches the critical micelle concentration of mixed micelles at the end of the solubilization process.