The electrophoretic mobility of oil droplets, dispersed without any surfactant in the aqueous phase, was measured. Four different oils were studied:  xylene, dodecane, hexadecane, and perfluoromethyldecalin. Special precautions were undertaken to avoid artifacts caused by the presence of surfactant impurities. The results show that the oil droplets are negatively charged and the magnitude of their ζ-potential strongly depends on pH and the ionic strength of the aqueous phase. The electrophoretic mobility is almost independent of the type of specific nonpolar oil. Series of experiments were performed to check different hypotheses about the origin of the spontaneous charging of the oil−water interfaces. The results lead to the conclusion that hydroxyl ions, released by the dissociation−association equilibrium of the water molecules, adsorb at the oil−water interface. The specific adsorption energy was estimated to be 25kT per ion (kT is the thermal energy). The molecular origin and the implications of this phenomenon are discussed. The ζ-potential decreases in magnitude when poly(oxyethylene) chain nonionic surfactants are adsorbed at the interface.

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We propose a simple new method for measuring the surface shear elasticity modulus (μ) together with the dilatational modulus (K) of gel-like protein layers on an air/water boundary. The stress response to compression/expansion of the interface in a Langmuir trough is measured at two different orientations of a Wilhelmy plate, collateral and perpendicular to the movable barrier in the trough. The interfacial tension is a tensorial quantity, whence the measured values depend on the direction of the length along which the stress acts. The fact that the deformation in the trough is uniaxial, i.e., a combination of dilatation and shear, is used to determine the respective two elastic moduli (K, μ). The experiment demonstrates that adsorbed layers of β-lactoglobulin (BLG), when subjected to small deformations, exhibit a predominantly elastic rheological behavior. This proves the existence of the two-dimensional gel, as a result from partial denaturation and unfolding accompanied with entanglement of the protein molecules on the interface. Layers of this kind exhibit finite shear elasticity (μ ≠ 0). Data are reported for systems containing BLG at different concentrations, and for mixtures including low molecular weight nonionic surfactant Tween 20. The elastic moduli are found to increase with rising protein content (at relatively higher concentrations), which is perhaps due to reinforcement of the gel-like structure. It is proved that in all cases the presence of Tween 20 brings about a complete fluidization of the adsorbed layer, in the sense that the shear elasticity disappears and the respective modulus (μ) becomes equal to zero. The frequency dependence of the elastic moduli is discussed in view of possible exchange of protein molecules from the interface with the bulk or with the adjacent subsurface layers.

A novel method for determination of the three-phase contact angle at the surface of a micrometer-sized particle (latex sphere, oil droplet, or biological cell) is described. The particle is entrapped within a liquid film of equilibrium thickness smaller than the particle diameter. Thus a liquid meniscus (a layer of uneven thickness) is formed around the particle. When observed in reflected monochromatic light, this meniscus appears as an interference pattern of concentric bright and dark fringes. From the radii of the interference fringes, one can restore the meniscus shape by using the solution of the Laplace equation of capillarity. In this way the three-phase contact angle of the particle and the capillary pressure can be determined. We demonstrate the applicability of our method to latex spheres from several batch samples (between 1 and 7 µm in diameter) and to oil droplets, stabilized by adsorbed protein layer. The numerical procedures used for contact angle determination are described, and illustrative results are presented and discussed.