Control of strength and stability of emulsion gels by a combination of long- and short-range interactions

This paper discusses the change in phase behavior and mechanical properties of oil-in-water emulsion gels brought about by variation of long- and short-range attractive interactions. The model system studied consisted of oil droplets stabilized by the protein beta-lactoglobulin (beta-1g). A long-range depletion attraction was obtained by addition of dextran. At short distances, the interaction is dominated by electrostatic repulsion between the adsorbed layers of beta-1g. This interaction was varied by addition of Ca2+ ions and by changing the NaCl concentration. Combination of long- and short-range attraction resulted in a substantial decrease in the rate of serum separation and an increase in the emulsion gel modulus at small deformations compared to depletion attraction alone. The flocculation process and the morphology of the floes were investigated by diffusing wave spectroscopy and confocal scanning laser microscopy. Above a minimum concentration, dextran induced fast depletion flocculation, leading to a network of emulsion droplets. This network quickly collapsed due to gravity. Addition of Ca2+ ions above a minimum concentration induced slow flocculation, and the floes creamed before a network was formed. Addition of both dextran and Ca2+ ions resulted in a two-step mechanism of emulsion gel formation. A network is quickly formed by depletion flocculation and subsequently the bonds between the emulsion droplets are reinforced by Ca2+ ions. Due to this reinforcement, rearrangements of this network were suppressed resulting in a smaller rate of serum separation.