Surface Properties and Foam Stability of Protein/Surfactant Mixtures:  Theory and Experiment

This research work deals with the surface behavior of mixed protein/surfactant systems and its relation with foam stability. In particular, a novel procedure of analysis of the experimental data is proposed, which provides a direct correlation between both phenomena. Accordingly, the results presented might constitute a promising tool in the control of foam stability, which has a crucial technological application. The surface characterization is performed by studying the surface pressure isotherms of mixed systems. The qualitative information extracted from the experimental curves is importantly verified by the application of a thermodynamic model to the mixed system, which was previously probed with the individual components. Moreover, the model provides the concrete composition of the surface-adsorbed layer in which the surfactant seems to displace the protein from the surface. Subsequently, the surface tension arises as a key factor in determining which species dominate the surface, and the findings appear in agreement with the theoretical conclusions. Also, the displacement of the protein by the surfactant, previously inferred experimentally and theoretically, is observed in situ by means of the evolution of the surface pressure in the sequential adsorption. Finally, this structural characterization of the surface is correlated with the measurements of the foam stability. This last feature provides a direct connection between fundamental properties and industrial magnitudes and constitutes a main achievement of this work.