Interfacial properties of mixed beta-lactoglobulin-SDS layers at the water/air and water/oil interface.

The adsorption behavior of the beta-lactoglobuline has been studied in the presence of the anionic surfactant sodium dodecylsulfate (SDS) and compared for two different interfaces, water/air and water/hexane. The fitting of experimental data (adsorption isotherms) by a mixed adsorption model and the determination of structural parameters such as the molecular area occupied by the protein-surfactant complex and the surfactant molecules at the interface allowed to have a better understanding of the composition and as a consequence the behavior of the mixed interfacial layer. The parameters obtained for the mixtures are similar to those obtained separately for the single components, but the comparison of the both interfaces has shown significant differences. Much higher concentration of complex is found at the water/hexane interface, which is the result of a better affinity of the protein for this interface. A higher penetration of the protein into the oil phase and the presence of interactions between protein-surfactant complexes and free surfactant molecules stabilize the interface preventing its replacement by the SDS molecules. Rheological experiments show a decrease of the visco-elastic modulus at both interfaces with increasing SDS concentration. But at the water/oil interface, contrary to the water/air interface at which the replacement of the protein has been clearly observed, this decrease is attributed to changes of complex properties. At high SDS concentrations, an increase of the hydrophilic character due to hydrophobic interactions with the surfactant molecules leads to an increase in the mobility of the complex, which favors its desorption upon increased competition by the surfactant.