Sr

Indirectly

No

Note: A = attractive, R = repulsive, S = strong, W = weak, SR = short range (<10 nm), LR = long range (>10 nm), and SD = system dependent.

fluctuation

Note: A = attractive, R = repulsive, S = strong, W = weak, SR = short range (<10 nm), LR = long range (>10 nm), and SD = system dependent.

primary minimum that exists at closer separations. On the other hand, if it is not large compared to the thermal energy, then the droplets tend to fall into the primary minimum, w(h°min), which would lead to droplet coalescence. An example of this type of system would be two electrically charged oil droplets suspended in pure water in the absence of any adsorbed emulsifier.

Electrostatically stabilized emulsions are particularly sensitive to the ionic strength and pH of the aqueous phase (Figure 3.23). At low electrolyte concentrations, there may be a sufficiently high energy barrier to prevent the droplets from coming close enough together to aggregate into the primary minimum (Figure 3.23). As the ion concentration is increased, the screening of the electrostatic interaction becomes more effective (Section 3.4.3), which reduces the height of the energy barrier. Above a certain electrolyte concentration, often

5 10

FIGURE 3.22 Typical interdroplet pair potential for an electrostatically stabilized emulsion, where the only interactions are electrostatic repulsion and van der Waals attractions (i.e., DLVO theory).

h/nm

FIGURE 3.23 The stability of emulsion droplets to aggregation decreases as the ionic strength of the intervening medium increases because of electrostatic screening effects.

h/nm

FIGURE 3.23 The stability of emulsion droplets to aggregation decreases as the ionic strength of the intervening medium increases because of electrostatic screening effects.

referred to as the critical aggregation concentration or CAC, the energy barrier is no longer high enough to prevent the droplets from falling into the deep primary minimum, and so the droplets tend to aggregate. This accounts for the susceptibility of many electrostatically stabilized food emulsions to droplet aggregation when salt is added to the aqueous phase (Hunt and Dalgleish 1994, 1995; Demetriades et al. 1997a). The electrical charge of many food emulsifiers is sensitive to the pH of the aqueous phase. For example, the droplet charge of protein-stabilized emulsions decreases as the pH tends toward the isoelectric point of the proteins, which reduces the magnitude of the electrostatic repulsion between the droplets. This accounts for the tendency of protein-stabilized emulsions to become aggregated when their pH is adjusted to the protein isoelectric point (Demetriades et al. 1997a).

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