General Features of Electrostatic Interactions

1. Electrostatic interactions may be either attractive or repulsive depending on the sign of the charges on the droplets. The interaction is repulsive when droplets have similar charges (which is usually the case), but is attractive when they have opposite charges.

2. The strength of the interaction decreases with droplet separation and may be either long or short range depending on the ionic strength of the electrolyte solution surrounding the droplets. The interaction becomes increasingly short range as the ionic strength increases because of electrostatic screening.

3. The strength of the interaction is proportional to the size of the emulsion droplets.

4. The strength of the interaction depends on the electrical characteristics of the droplet surfaces (e.g., the number of emulsifier molecules adsorbed per unit surface area, the number of ionizable groups per emulsifier molecule, and the concentration of any potential-determining ions in the aqueous phase, for example, H+ or

FIGURE 3.13 Polyvalent ions are capable of forming ion bridges between emulsion droplets.

FIGURE 3.13 Polyvalent ions are capable of forming ion bridges between emulsion droplets.

5. The interaction becomes more difficult to predict when association-dissociation of ionizable groups or adsorption-desorption of ionic emulsifiers occurs, especially at close droplet separations.

6. Ion bridging effects have to be taken into account when polyvalent ions are involved.

In this section, we have seen that under certain conditions repulsive electrostatic interactions may be relatively strong and long range compared to attractive van der Waals interactions (compare Figures 3.3 and 3.11). This suggests that they may be strong enough to prevent droplets from aggregating in certain systems. Indeed, it is widely recognized that electrostatic stabilization plays an important role in determining the aggregation of droplets in many food emulsions and particularly those stabilized by proteins (Friberg and Larsson 1997, Dickinson and Stainsby 1982, Dickinson 1992). It should also be recognized that electrostatic interactions influence various other properties of food emulsions, such as the partitioning of ingredients and the rates of chemical reactions. For example, the partitioning of an ionizable volatile flavor compound, such as butyric acid, between the head space and bulk of an emulsion is influenced by electrostatic interactions (Guyot et al. 1996). The negatively charged flavor component is attracted to positively charged droplets, which causes its volatility to be decreased, thus reducing the aroma. Lipid oxidation in food emulsions is often catalyzed by polyvalent ions, such as Fe3+, that are normally present in the aqueous phase. The rate of iron-catalyzed lipid oxidation in oil-in-water emulsions has been shown to increase when the droplets have a negative charge because the Fe3+ catalyst and oil molecules are brought into close contact (Coupland and McClements 1996, Mei et al. 1998). A knowledge of the factors which determine the magnitude and range of electrostatic interactions is therefore extremely important to food scientists.

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