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Food Lipids: Chemistry, Nutrition, and Biotechnology

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4. Short-Range Forces<br />

When two emulsion droplets come sufficiently close together, their interfacial layers<br />

start to interact. A number of short-range forces result from these interactions, including<br />

steric (osmotic <strong>and</strong> elastic components), hydration, protrusion, <strong>and</strong> undulation<br />

forces [11,12]. Some progress has been made in developing theories to predict<br />

the magnitude <strong>and</strong> range of short-range forces associated with interfacial layers of<br />

fairly simple geometry. Nevertheless, both magnitude <strong>and</strong> range of these forces are<br />

particularly sensitive to the size, shape, conformation, packing, interactions, mobility,<br />

<strong>and</strong> hydration of the molecules in the adsorbed layer, <strong>and</strong> so it is difficult to predict<br />

their contribution to the overall interaction potential with any certainty. Even so, they<br />

are usually repulsive <strong>and</strong> tend to increase strongly as the interfacial layers overlap.<br />

5. Overall Interaction Potential<br />

It is often difficult to accurately calculate the contribution of each type of interaction<br />

to the overall interdroplet pair potential because information about the relevant physicochemical<br />

properties of the system is lacking. Nevertheless, it is informative to<br />

examine the characteristics of certain combinations of interactions that are particularly<br />

important in food emulsions, for this provides a valuable insight into the factors<br />

that affect the tendency of droplets to aggregate. Consider an emulsion in which the<br />

only important types of droplet–droplet interaction are van der Waals attraction,<br />

electrostatic repulsion, <strong>and</strong> steric repulsion (e.g., an emulsion stabilized by a charged<br />

biopolymer).<br />

The van der Waals interaction potential is fairly long range <strong>and</strong> always negative<br />

(attractive), the electrostatic interaction potential is fairly long range <strong>and</strong> always<br />

positive (repulsive), while the steric interaction is short range <strong>and</strong> highly positive<br />

(strongly repulsive). The overall interdroplet pair potential has a complex dependence<br />

on separation because it is the sum of these three different interactions, <strong>and</strong> it may<br />

be attractive at some separations <strong>and</strong> repulsive at others. Figure 11 shows a typical<br />

Figure 11 The overall interaction potential for an emulsion stabilized by a charged<br />

biopolymer.<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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