Note: z is the valence, | is the dipole moment, e is the electronic charge, and ^ is the angle between the charges.

Note: z is the valence, | is the dipole moment, e is the electronic charge, and ^ is the angle between the charges.

FIGURE 2.3 Dependence of the intermolecular pair potential on intermolecular separation for (a) electrostatic, (b) van der Waals, and (c) steric overlap interactions.

3. Their strength increases as the center-to-center separation of the charged species decreases. Thus, interactions between small ions or molecules (which can get close together) are stronger than those between large ions or molecules of the same charge.

4. The range of ion-ion (1/s) interactions is longer than that of ion-dipole interactions (1/s2), which is longer than that of dipole-dipole interactions (1/s3).

5. Their strength depends on the nature of the material separating the charges (via tR): the higher the relative dielectric constant, the weaker the interaction (Table 2.2). Electrostatic interactions between two charged species in water (eR= 80) are

FIGURE 2.3 (continued)

therefore much weaker than those between the same species in oil (eR = 2), which accounts for the much higher solubility of salts in water than in nonpolar solvents (Israelachvili 1992).

6. Their strength depends on the orientation of any dipoles involved, being strongest when partial charges of opposite sign are brought close together. When the electrostatic interaction between a dipole and another charged species is much stronger than the thermal energy (Section 2.5), the dipole becomes permanently aligned so as to maximize the strength of the attraction. This alignment of dipoles is responsible for the high degree of structural organization of molecules in bulk water and the ordering of water molecules around ions in aqueous solutions (Chapter 4).

The ionization of many biological molecules depends on the pH of the surrounding aqueous phase, and so electrostatic interactions involving these molecules are particularly sensitive to pH (Nakamura 1996; Damodaran 1989, 1994, 1996, 1997). For example, a protein may exist as an individual molecule in solution when the pH is sufficiently far from its isoelectric point because of strong electrostatic repulsion between the protein molecules, but it may precipitate when the pH of the solution is close to its isoelectric point because the electrostatic repulsion between the molecules is no longer strong enough to prevent them from aggregating (Kinsella 1982, Kinsella and Whitehead 1989, Damodaran 1996). The strength of electrostatic interactions between molecules suspended in an aqueous solution is also sensitive to the type and concentration of electrolyte present (Bergethon and Simons 1990, de Wit and van Kessel 1996). A charged molecule tends to be surrounded by oppositely charged ions (counterions), which effectively "screen" (reduce) the electrostatic interaction between other molecules of the same type (Chapter 3).

Electrostatic interactions play an extremely important role in determining the overall properties of food emulsions because many of the major constituents of these products are either ionic or dipolar (e.g., water, sugars, salts, proteins, polysaccharides, surfactants, acids, and bases) (Fennema 1996a). The unique physiochemical properties of water are governed

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