Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.

The rheological behavior of beta-casein adsorption layers formed at the air-water and tetradecane-water interfaces is studied in detail by means of pendant drop tensiometry. First, its adsorption behavior is briefly summarized at both interfaces, experimentally and also theoretically. Subsequently, the experimental dilatational results obtained for a wide range of frequencies are presented for both interfaces. An interesting dependence with the oscillation frequency is observed via the comparative analysis of the interfacial elasticity (storage part) and the interfacial viscosity (loss part) for the two interfaces. The analysis of the interfacial elasticities provides information on the conformational transitions undergone by the protein upon adsorption at both interfaces. The air-water interface shows a complex behavior in which two maxima merge into one as the frequency increases, whereas only a single maximum is found at the tetradecane interface within the range of frequencies studied. This is interpreted in terms of a decisive interaction between the oil and the protein molecules. Furthermore, the analysis of the interfacial viscosities provides information on the relaxation processes occurring at both interfaces. Similarly, substantial differences arise between the gaseous and liquid interfaces and various possible relaxation mechanisms are discussed. Finally, the experimental elasticities obtained for frequencies higher than 0.1 Hz are further analyzed on the basis of a thermodynamic model. Accordingly, the nature of the conformational transition given by the maximum at these frequencies is discussed in terms of different theoretical considerations. The formation of a protein bilayer at the interface or the limited compressibility of the protein in the adsorbed state are regarded as possible explanations of the maximum.

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