Interactions in oil/water/oil films stabilized by β-lactoglobulin; role of the surface charge

Abstract

In this work we investigate emulsion films (oil/water/oil), stabilized by β-lactoglobulin (BLG). Isotherms of disjoining pressure versus the film thickness are measured experimentally, at different pH (4.0, 5.2, and 6.5), and ionic strength. The data are fitted successfully with the classic theory of DLVO (assuming superposition of electrostatic repulsion and van der Waals attraction). One adjustable parameter, the surface potential, is determined from the best fit; the results are used to calculate the surface charge density in the films. At the pH of 5.2 (which is the isoelectric point, p I, in the bulk solution), the interface is charged. Possible reason is the conformational change, which the protein undergoes upon adsorption. At bulk pH of 4.0, the BLG-laden oil/water interface is close to isoelectric state (the surface charge density is very low). Under these conditions, there is evidence for long-range steric repulsion, possibly due to favored aggregation at the interface. In some cases, after eventual collapse of the repulsion, we observe formation of spots of very thin Newton Black Films (NBF). Addition of inorganic salt, NaCl, leads to increase of surface charge (up to a certain limit). This effect is derived both from results with thin liquid films, and from zeta-potentials of emulsion drops. At the “natural” pH of 6.5, with 150 mM ionic strength, the extent of charging of adsorbed BLG is considerably lower than that in the bulk aqueous solution. Thus, also at pH 6.5, the charge state of the protein molecules residing on the oil/water interface is significantly influenced by the conformational transformations that accompany the adsorption. The emulsion films with BLG are less stable than the foam films, and rupture without overcoming a barrier (maximum) in the disjoining pressure isotherm. The latter fact implies that certain weakness of the interfacial layer is brought about by the contact with the oil phase (hydrophobization).