Mixed protein–surfactant adsorption layers formed in a sequential and simultaneous way at water–air and water–oil interfaces

Abstract

Mixed protein–surfactant adsorption layers can be built up in two different ways. The classical way is when all components adsorb simultaneously from a mixed solution. Alternatively, the components adsorb one after another, i.e. in a sequential way. In the present work, the formation of such surface layers has been studied with the random coil protein β-casein in the presence of added anionic surfactant SDS and compared for two different interfaces: the water–air (W–A) and water–hexane (W–H) interfaces. The used experimental technique is a drop profile analysis tensiometer PAT-1 specially equipped with a coaxial double capillary for drop volume exchange during the experiments. The results are supported by dynamic light scattering measurements, which describe the size of the protein–surfactant complexes formed in the solution bulk. There are remarkable differences in the behavior of mixed adsorption layers for the two ways of the protein–surfactant complex formation, which are either formed in the bulk in case of simultaneous adsorption or in the surface layer in case of sequential adsorption. The dynamics of desorption of the mixed layers into the bulk solution also deviate significantly, which is explained by the different nature and structure of the two types of complexes formed in two different locations. Compared to the W–A interface, at the W–H interface a different conformation of the pre-adsorbed protein molecules that penetrates more into the oil phase stabilizes the complexes adsorbed at the interface. In contrast, in the competitive adsorption, the surfactants due to their higher affinity to the W–H interface, adsorb strongly at the interface with the complexes formed already in the bulk. This leads to an increasing presence of freely adsorbed surfactant molecules at the interface, however, only at higher SDS concentrations.