Depletion Interactions 361 Origin of Depletion Interactions

Many food emulsions contain small colloidal particles that are dispersed in the continuous phase which surrounds the droplets (Figure 3.17). These colloidal particles may be surfactant

Depletion Interaction
FIGURE 3.17 An attractive depletion interaction arises between emulsion droplets when they are surrounded by small nonadsorbing colloidal particles.

micelles formed when the free surfactant concentration exceeds some critical value (Aronson 1992, Bibette 1991, McClements 1994), individual polymer molecules (Sperry 1982, Seebergh and Berg 1994, Dickinson et al. 1995, Smith and Williams 1995, Jenkins and Snowden 1996), or aggregated polymers (Dickinson and Golding 1997a,b). The presence of these colloidal particles causes an attractive interaction between the droplets which is often large enough to promote emulsion instability (Jenkins and Snowden 1996). The origin of this interaction is the exclusion of colloidal particles from a narrow region surrounding each droplet (Figure 3.17). This region extends a distance approximately equal to the radius (r) of a colloidal particle away from the droplet surface. The concentration of colloidal particles in this depletion zone is effectively zero, while it is finite in the surrounding continuous phase. As a consequence, there is an osmotic potential difference which favors the movement of solvent molecules from the depletion zone into the bulk liquid, so as to dilute the colloidal particles and thus reduce the concentration gradient. The only way this process can be achieved is by two droplets aggregating and thereby reducing the volume of the depletion zone, which manifests itself as an attractive force between the droplets (Figure 3.17). Thus there is an osmotic driving force that favors droplet aggregation and which increases as the concentration of colloidal particles in the aqueous phase increases.

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