Calorimetric study of the interactions between small-molecule surfactants and sodium caseinate with reference to the foaming ability of their binary mixtures

Mixing calorimetry was used to determine the enthalpy of the interactions of sodium caseinate with a series of commercially important water dispersible small-molecule surfactants, which represent generally the mixture of the esters of the stearic and palmitic acids with either a citric acid (CITREM) or a lactic acid (SSL), or polyglycerol (PGE). A combination of the temperature (from 20 to 50 °C) and pH (7.2 and 5.5.) dependencies of the measured enthalpies gives a more insight into the predominant nature of the interactions of the specific surfactants with the protein in an aqueous medium and makes possible to identify the key structural parameters of the surfactants, controlling the character of their interactions with sodium caseinate. Moreover, the concentration dependencies of the enthalpies of the interactions between protein and surfactants normalized to the surfactant concentrations suggest some changeover from the monomolecular state of the surfactants to the micelle-like clusters in the interior and/or at the protein surface. In addition, the distinctions were elucidated between the nature and the intensity of the interactions of sodium caseinate with the studied surfactants in their monomolecular and micellar states. Besides, based on a combination of the data from static and dynamic laser light scattering, the less extent of the protein association followed by the formation of the more compact structure of the protein nanoparticles is the distinctive feature of the interaction between sodium caseinate and the micelles of the surfactants. As a result of the protein-surfactant interactions, the foams stabilized by the protein+surfactant mixtures show an increase in their time stability in the case of PGE and SSL at pH 5.5, as opposed to pH 7.2, and vice versa, for CITREM in a comparison with the pure caseinate foams. These effects were more pronounced at the surfactant concentrations that are above their critical micelle concentrations (cmc).