The kinetics of adsorption of several positively and negatively charged proteins at the air/water and triolein/water interfaces have been studied. It is shown that adsorption of proteins at these interfaces is not simply diffusion-controlled but is strongly influenced by the energetics of interaction of proteins with the interfaces. Generally, positively charged proteins experience an energy barrier for adsorption at the air-water interface and therefore exhibit adsorption rates an order of magnitude slower than their respective bulk diffusivities. In contrast, the negatively charged proteins exhibit an attraction toward the air/water interface and therefore their adsorption rates are either slightly higher or 1.5-2 times lower than their bulk diffusivities. At the triolein-water interface however, all proteins, except phosvitin, adsorbed at rates 1-2 orders of magnitude faster than their bulk diffusivities. The differences between adsorptivities of positively and negatively charged proteins at the air-water interface, and the differences between adsorptivities of all proteins at the air/water and triolein/water interfaces can be explained convincingly in terms of the energetics of interaction of proteins with the interfaces.

The adsorption kinetics of β-casein, BSA, and lysozyme at the air−water interface were compared with those at the triolein−water interface. The rates of adsorption of β-casein, BSA, and lysozyme to the oil−water interface were consistently higher by an order of magnitude, compared to the respective bulk diffusion coefficients. The rates of adsorption of β-casein and BSA to the air−water interface were, however, closer to the respective bulk diffusion coefficients, while that of lysozyme to the air−water interface was an order of magnitude smaller than its bulk diffusivity. Also, the equilibrium concentrations of the three proteins at the oil−water interface were several times higher than those at the air−water interface. The remarkable differences in the rates of adsorption of the three proteins, particularly lysozyme, at these interfaces were unequivocally explained by differences in the potential energies of interaction of the proteins with these two interfaces. Potential energy calculations showed that dispersion interactions between proteins and the oil−water interface is attractive, whereas those between proteins and the air−water interface were generally repulsive. Thus, the slower rate of adsorption of proteins to the air−water interface may be attributed to the dominating repulsive dispersion interactions. In addition, the existence of an energy barrier in the potential energy profile of lysozyme as it approached the air−water interface conclusively explained its unusually slow adsorption rate and the presence of a long lag period (60 min) for its adsorption to commence at the air−water interface.

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Lateral phase separation in two-dimensional mixed films of soy 11S/beta-casein, acidic subunits of soy 11 (AS11S)/beta-casein, and alpha-lactalbumin/beta-casein adsorbed at the air-water interface has been studied using an epifluorescence microscopy method. No distinct lateral phase separation was observed in the mixed protein films when they were examined after 24 h of adsorption from the bulk phase. However, when the soy 11S/beta-casein and AS11S/beta-casein films were aged at the air-water interface for 96 h, phase-separated regions of the constituent proteins were evident, indicating that the phase separation process was kinetically limited by a viscosity barrier against lateral diffusion. In these films, beta-casein always formed the continuous phase and the other globular protein the dispersed phase. The morphology of the dispersed patches was affected by the protein composition in the film. In contrast with soy 11S/beta-casein and AS11S/beta-casein films, no lateral phase separation was observed in the alpha-lactalbumin/beta-casein film at both low and high concentration ratios in the film. The results of these studies proved that proteins in adsorbed binary films exhibit limited miscibility, and the deviation of competitive adsorption behavior of proteins at the air-water interface from that predicted by the ideal Langmuir model (Razumovsky, L.; Damodaran, S. J. Agric. Food Chem. 2001, 49, 3080-3086) is in fact due to thermodynamic incompatibility of mixing of the proteins in the binary film. It is hypothesized that phase separation in adsorbed mixed protein films at the air-water and possibly oil-water interfaces of foams and emulsions might be a source of instability in these dispersed systems.