Probing of Competitive Displacement Adsorption of Casein at Oil-in-Water Interface Using Equilibrium Force Distance...

The equilibrium force distance measurement is employed for the first time to probe the competitive and displacement adsorption of casein at an oil-water (O/W) emulsion interface that was initially adsorbed with either a diblock polymer or an anionic surfactant. A significant change in the force-distance profile was observed under the competitive displacement adsorption of casein, which is further confirmed from the hydrodynamic diameter and zeta potential measurements. A decrease in the onset of repulsion and decay length are observed on competitive adsorption of smaller size casein molecules at O/W interface. With addition of casein in PVA-vac diblock polymer stabilized emulsion, the onset of repulsion decreases from 88 to 48 nm whereas the magnitude of force increases from 1 to 19 nN. The force decay length is reduced from 10.5 to 4.5 nm upon addition of casein. Our results suggest the complete replacement of adsorbed diblock polymers by casein molecules. The hydrodynamic diameter and zeta potential measurements corroborate the casein mediated polymer displacement and the competitive adsorption of casein at the O/W interface. In the case of anionic surfactant covered O/W interfaces, casein molecules weakly associate at the interface without displacing the smaller size surfactant molecules where no significant changes in the onset repulsion and force profiles are observed. These results suggest that the casein molecules are effective displacers for replacement of adsorbed macromolecules from formulations, which has several important practical applications.