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The dependence of the (nonretarded and nonscreened) van der Waals interaction between two emulsion droplets on the thickness and composition of an interfacial layer consisting of a mixture of protein and water was calculated using Equation 3.6 and the physical properties listed in Table 3.1 (Figure 3.5). In the absence of the interfacial layer, the attraction between the droplets was about -110 kT at a separation of 1 nm. Figure 3.5 clearly indicates that the interfacial layer causes a significant alteration in the strength of the interactions between the droplets, leading to either an increase or decrease in the strength of the attraction, depending on the protein concentration. At high protein concentrations (>60%), the attraction is greater than that between two bare emulsion droplets, whereas at low protein concentrations (<60%) it is smaller.

FIGURE 3.5 Influence of the composition of an interfacial layer, consisting of water and protein, on the van der Waals interactions between emulsion droplets. The interdroplet pair potential is reported at an outer surface-to-surface separation of 1 nm for 1-^m droplets. The physical properties of the oil, water, and interfacial layer used in the calculations are reported in Table 3.1

FIGURE 3.5 Influence of the composition of an interfacial layer, consisting of water and protein, on the van der Waals interactions between emulsion droplets. The interdroplet pair potential is reported at an outer surface-to-surface separation of 1 nm for 1-^m droplets. The physical properties of the oil, water, and interfacial layer used in the calculations are reported in Table 3.1

3.3.7. General Features of van der Waals Interactions

1. The interaction between two oil droplets (or between two water droplets) is always attractive.

2. The strength of the interaction decreases with droplet separation, and the interaction is fairly long range (w ^ 1/h).

3. The interaction becomes stronger as the droplet size increases.

4. The strength of the interaction depends on the physical properties of the droplets and the surrounding liquid (through the Hamaker function).

5. The strength of the interaction depends on the thickness and composition of the adsorbed emulsifier layer.

6. The strength of the interaction decreases as the concentration of electrolyte in an oil-in-water emulsion increases because of electrostatic screening.

van der Waals interactions act between all types of colloidal particles, and therefore they must always be considered when calculating the overall interaction potential between emulsion droplets (Hiemenz 1986, Israelachvili 1992). Nevertheless, it must be stressed that an accurate calculation of their magnitude and range is extremely difficult, because of the lack of physicochemical data required to perform the calculations and because of the need to simultaneously account for the effects of screening, retardation, and interfacial layers (Hunter 1986). The fact that van der Waals interactions are relatively strong and long range, and that they are always attractive, suggests that emulsion droplets would tend to associate with each other. In practice, many food emulsions are stable to droplet aggregation, which indicates the existence of repulsive interactions that are strong enough to overcome the van der Waals attraction. Some of the most important types of these repulsive interactions, including electrostatic, polymeric steric, hydration, and thermal fluctuation interactions, are discussed in the following sections.

3.4. ELECTROSTATIC INTERACTIONS 3.4.1. Origins of Surface Charge

The droplets in many food emulsions have electrically charged surfaces because of the adsorption of emulsifiers which are either ionic or capable of being ionized (e.g., proteins, polysaccharides, and surfactants) (Chapter 4). All food proteins have acidic (-COOH ^ COO- + H+) and basic (NH2 + H+ ^ NH+) groups whose degree of ionization depends on the pH and ionic strength of the surrounding aqueous phase (Charalambous and Doxastakis 1989, Damodaran 1996, Magdassi 1996, Magdassi and Kamyshny 1996). Some surface-active polysaccharides, such as modified starch and gum arabic, also have acidic groups which may be ionized (BeMiller and Whistler 1996). Ionic surfactants may be either positively or negatively charged depending on the nature of their hydrophilic head group (Linfield 1976, Myers 1988, Richmond 1990). The magnitude and sign of the electrical charge on an emulsion droplet therefore depend on the type of emulsifier used to stabilize it, the concentration of the emulsifier at the interface, and the prevailing environmental conditions (e.g., pH, temperature, and ionic strength). All the droplets in an emulsion are usually stabilized by the same type of emulsifier and therefore have the same electrical charge. The electrostatic interaction between similarly charged droplets is repulsive, and so electrostatic interactions play a major role in preventing droplets from coming close enough together to aggregate.

It is convenient to divide the different types of ions which can influence surface charge into three categories (Hunter 1986, 1989):

1. Potential-determining ions. This type of ion is responsible for the association-dissociation of charged groups (e.g., -COOH ^ COO- + H+). In food emulsions, the most important potential-determining ions are H+ and OH-, because they govern the degree of ionization of acidic and basic groups on many proteins and polysaccharides. The influence of potential-determining ions on surface charge is therefore determined principally through the pH of the surrounding solution.

2. Indifferent electrolyte ions. This type of ion accumulates around charged groups because of electrostatic interactions (e.g., Na+ ions may accumulate around a negatively charged -COO- group). These ions reduce the strength of the electrical field around a charged group principally due to electrostatic screening (Section 3.4.2), rather than causing association-dissociation of the charged group. At high ionic strengths, some "indifferent" electrolyte ions can actually alter the degree of ionization of charged groups. They do this either by altering the dissociation constant of the surface groups (i.e., their pK value) or by acting as potential-determining ions that compete with the H+ or OH- ions (e.g., -COO- + Na+ ^ -COO-Na+). The influence of indifferent electrolyte ions on surface charge is therefore determined principally by the ionic strength of the surrounding solution.

3. Adsorbed ions. Surface charge can also be altered by the adsorption of surface-active ions. In food emulsions, the most important types of surface-active ions are ionic emulsifiers, including many surfactants, proteins, and polysaccharides (Chapter 4). The contribution of adsorbed ions to surface charge is governed mainly by the type and concentration of emulsifiers present in the system and their relative affinities for the droplet surface.

Emulsion scientists are interested in understanding the role that each of these different types of ions play in determining the electrical charge on emulsion droplets, because the magnitude of this charge determines the stability of many food emulsions to aggregation and therefore has a pronounced influence on their appearance, taste, texture, and stability (Chapters 7 to 9).

3.4.2. Ion Distribution Near a Charged Surface

An understanding of the origin and nature of electrostatic interactions between emulsion droplets relies on an appreciation of the way that the various types of ions are organized close to a charged surface. Consider a charged surface which is in contact with an electrolyte solution (Figure 3.6). Ions of opposite charge to the surface (counterions) are attracted toward it, whereas ions of similar charge (co-ions) are repelled from it. Nevertheless, the tendency for ions to be organized in the vicinity of a charged surface is opposed by the disorganizing influence of the thermal energy (Evans and Wennerstrom 1994). Consequently, the concentration of counterions is greatest at the charged surface and decreases as one moves away from the surface until it reaches the bulk counterion concentration, whereas the concentration of co-ions is smallest at the charged surface and increases as one moves away from the surface until it reaches the bulk co-ion concentration (Figure 3.6). The concentration of counterions near a charged surface is always greater than the concentration of co-ions, and so a charged surface can be considered to be surrounded by a cloud of counterions. Nevertheless, the overall system must be electrically neutral, and so the charge on the surface must be completely balanced by the excess charge of the counterions in the electrolyte solution. The distribution of ions close to a charged surface is referred to as the electrical double layer, because it is convenient to assume that the system consists of two oppositely charged layers:

Charged

Surface Electrolyte solution

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