The surface tension and dilatational visco-elasticity isotherms for β-casein determined at pH 7 and 9 are essentially the same, but differ remarkably from those measured at the isoelectric point (i.e.p.) of the protein at pH 5. A recently developed thermodynamic model is applied to the experimental data, which were not only obtained at equilibrium, but also under quasi-equilibrium conditions. It turned out that such a model can be adequately applied to data obtained not too far from the equilibrium state of a protein adsorption layer. The change in the model parameters allows to understand slow changes in the structure of the adsorption layer. Even at pH 5, where β-casein is most hydrophobic and in its most compact conformation, the data point to the fact that conformational changes may happen at the interface upon adsorption.

This chapter is dedicated to the surface properties of mixed protein/surfactant adsorption layers, formed by two different experimental approaches, i.e. by sequential and simultaneous adsorption, respectively. A special modification of a drop profile analysis tensiometer, consisting of a coaxial double capillary, provides a unique protocol for studies of mixed surface layers formed by sequential adsorption of the individual components in addition to the traditional simultaneous adsorption from their mixed solution. A CFD simulation allowed to optimize the drop exchange process performed with the special double capillary arrangement.The experiments show that properties of sequentially formed layers differ significantly from those formed simultaneously, which can be explained by the different nature and structure of the complexes formed at the two different locations. The nature of the interface, water/air or water/oil, influences strongly the adsorption behavior of the protein molecules and consequently the mixed layers due to their different degree of polarity and hydrophobicity. Washing out experiments are performed in order to support the proposed mechanism of the protein displacement process from mixed surface layers, i.e., to check how many protein molecules are left in the adsorption layer. Based on the experimental studies of the milk protein beta-casein (beta CS) mixed with the anionic surfactant sodium dodecyl sulfate (SDS) at water/air and water/hexane interfaces, the results are discussed according to different mechanisms describing the different location of interaction.

Models for the thermodynamics, adsorption kinetics and dilational rheology are presented for the quantitative analysis of adsorption layer formed from mixed solutions containing proteins and surfactants. The models are applied to experimental data obtained from experiments from drop profile analysis and capillary pressure tensiometry. A particular modification of drop profile tensiometry is presented, namely the real-time bulk exchange of the drop volume by using a co-axial double capillary, which allows for the adsorption studies of different components in a sequential way. The results indicate that proteins form complexes with surfactants in the bulk and at the interface. The properties of these complexes strongly depend on the composition of the solution in the bulk and the location of their formation. In particular, the dilational rheology is very sensitive for changes in mixed adsorption layers.

The adsorption of mixtures of β-casein and the anionic surfactant sodium dodecyl sulphate and the cationic dodecyl trimethyl ammonium bromide were studied at the water/hexane interface by drop profile analysis tensiometry and compared with those obtained for the water/air interface. The obtained interfacial tension isotherms suggest a gradual replacement of the protein molecules from both liquid interfaces with increasing surfactant concentration and a fixed amount of protein. The mechanical properties of the interfacial layers were probed by dilational rheological investigations. The results show, that although the equilibrium state is described by the existing theoretical models adequately, the quantitative analysis of the dilational visco-elasticity data requires further refinement of the corresponding model.

The present study deals with dilational and shear rheological properties of adsorption layers of the milk protein beta-casein (BCS) mixed with the nonionic dodecyl dimethyl phosphine oxide (C12DMPO) and the positively charged dodecyl trimethyl ammonium bromide (DoTAB), respectively. The drop profile analysis tensiometer PAT-1 was applied for the dilational rheological studies at low frequency harmonic relaxations. A special modification of the setup, consisting of a coaxial capillary combined with a double dosing system, provides exchange of the drop volume during experiments. This arrangement offers a unique protocol for studies of mixed surface layers, formed by sequential adsorption of the individual compounds. The dilational viscoelastic modulus and the dilational viscosity of the mixed layers, built-up in the two different ways, were investigated and compared. The features of the mixed surface layers drawn from the dilational rheology are qualitatively confirmed by the shear rheological parameters measured by torsion shear rheometry ISR-1. Recently derived theoretical models were used for a quantitative description of the equilibrium state and dilational rheology of the surface layers formed by the single components and their mixtures.

The formation of mixed protein/surfactant adsorption layers is studied by the drop profile analysis tensiometry equipped with a special tool for drop volume exchange during experiments. This arrangement allows investigating in the traditional way by simultaneous adsorption from a mixed solution and also by a subsequent adsorption of the protein followed by surfactant. The experiments are performed for β-casein as the protein in the presence of different amounts of the non-ionic surfactant C12DMPO. The surface layers formed via the two routes show similar equilibrium surface properties. However, the dynamics of desorption of the protein complexes into the pure buffer solution deviate significantly, which is explained by the different locations of the protein/surfactant interaction. Although in both cases the complex formation is based on hydrophobic interaction, the accessibility of the hydrophobic parts of pre-adsorbed proteins due to unfolding is more favourable by the surfactant than in the solution bulk. Therefore, the amount desorbed from surface layers formed from mixed solutions is significantly less as compared to the displacement of proteins by subsequently injected surfactants interacting at the surface.

Surfactants appear in multiphase fluid systems in which the interface and the adjacent bulk phase have been removed from equilibrium. Here, a new method is described for the measurement of rate constants of desorption of surface-active materials from fluid/fluid interfaces and the extent to which adsorption is reversible: the coaxial capillary pendant drop experimental technique.Kinetic constants are determined by desorption experiments in pendant drops in which the interface adjacent to a surfactant solution is removed from equilibrium by replacing the subphase of the drop with pure water. Further, we demonstrate that although the rate of subphase exchange is comparatively slow with respect to the desorption timescale, it is possible to resolve desorption processes which occur under local equilibrium with the adjacent bulk phase from those that are determined in part by sorption kinetics. Experiments which measure the desorption kinetic coefficient, alpha, using a homologous series of n-alkyl (C-8, C-10, C-12, C-14) dimethyl phosphine oxides are presented. (c) 2007 Elsevier B.V. All rights reserved.

Ellipsometry and surface profile analysis tensiometry were used to study and compare the adsorption behavior of beta-lactoglobulin (BLG)/C10DMPO, beta-casein (BCS)/C10DMPO and BCS/C12DMPO mixtures at the air/solution interface. The adsorption from protein/surfactant mixed solutions is of competitive nature. The obtained adsorption isotherms suggest a gradual replacement of the protein molecules at the interface with increasing surfactant concentration for all studied mixed systems. The thickness, refractive index, and the adsorbed amount of the respective adsorption layers, determined by ellipsometry, decrease monotonically and reach values close to those for a surface covered only by surfactant molecules, indicating the absence of proteins from a certain surfactant concentration on. These results correlate with the surface tension data. A continuous increase of adsorption layer thickness was observed up to this concentration, caused by the desorption of segments of the protein and transforming the thin surface layer into a rather diffuse and thick one. Replacement and structural changes of the protein molecules are discussed in terms of protein structure and surface activity of surfactant molecules. Theoretical models derived recently were used for the quantitative description of the equilibrium state of the mixed surface layers.

The interrelations between adsorption layer properties and surface forces in thin foam films of protein/surfactant mixtures were investigated. The adsorption from β-casein/dodecyl dimethyl phosphine oxide (C12DMPO) mixed solutions was studied by Profile Analysis Tensiometry (PAT). The adsorption of protein/surfactant mixtures at the air/water interface is of competitive nature. The obtained adsorption isotherms suggest a gradual displacement of the protein molecules from the interface with increasing surfactant concentration. Foam films were studied by microinterferometry, originally introduced by Scheludko and Exerowa. The obtained experimental results show that the composition of the mixed adsorption layers changes with increasing amount of added surfactant. At a certain mixing ratio the corresponding foam film thickness decreases dramatically. This change happens above the surfactant concentration where the surface tension of the protein–surfactant mixtures approaches the values obtained for the respective pure surfactant solutions. The thickness, refractive index and adsorbed amount of the respective adsorption layers were determined by ellipsometry. These results correlate with the above-mentioned PAT and foam film data. The refractive index and adsorbed amount decrease monotonically and reach values close to those for pure surfactant solutions. On the contrary, a continuous increase can be observed in the layer thickness up to the above-mentioned concentration, indicating the formation of a more diffuse layer at the air–water interface formed by β-casein and C12DMPO molecules upon increase of surfactant concentration. For foam films the electrolyte concentration can change the equilibrium thickness of the films and the Π(h)-isotherms significantly.