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Adapted from Davis 1994b.

Adapted from Davis 1994b.

colloid formed in many food emulsions, we will focus principally on their properties (Dickinson and McClements 1995).

4.5.2. Functional Properties 4.5.2.1. Critical Micelle Concentration

A surfactant forms micelles in an aqueous solution when its concentration exceeds some critical level, known as the critical micelle concentration or CMC (Myers 1988). Below the CMC, the surfactant molecules are dispersed predominantly as monomers, but once the CMC is exceeded, any additional surfactant molecules form micelles, and the monomer concentration remains fairly constant (Hiemenz 1986). Despite the highly dynamic nature of their structure, surfactant micelles have a fairly well-defined average size and shape under a given set of environmental conditions. Thus, when surfactant is added to a solution above the CMC, the number of micelles tends to increase, rather than the size or shape of the individual micelles (although this may not be true at high surfactant concentrations). There is an abrupt change in the physicochemical properties of a surfactant solution when the CMC is exceeded (e.g., surface tension, electrical conductivity, turbidity, and osmotic pressure) (Rosen 1978, Hiemenz 1986). This is because the properties of surfactant molecules dispersed as monomers are different from those in micelles. For example, surfactant monomers are amphiphilic

FIGURE 4.13 Some typical structures formed due to the self-association of surfactant molecules.

Bilayer

FIGURE 4.13 Some typical structures formed due to the self-association of surfactant molecules.

and have a high surface activity, whereas micelles have little surface activity because their surface is covered with hydrophilic head groups. Consequently, the surface tension of a solution decreases with increasing surfactant concentration below the CMC, but remains fairly constant above it (Chapter 5).

4.5.2.2. Cloud Point

When a surfactant solution is heated above a certain temperature, known as the cloud point, it becomes turbid (Myers 1988). This occurs because the hydrophilic head groups of the surfactant molecules become progressively dehydrated as the temperature is raised, which alters their molecular geometry (Section 4.5.3.3) and decreases the hydration repulsion between them (Israelachvili 1992, Evans and Wennerstrom 1994). Above a certain temperature, known as the cloud point, the micelles form aggregates which are large enough to scatter light and therefore make the solution appear turbid. As the temperature is increased further, the aggregates may grow so large that they sediment under the influence of gravity and form a separate phase which can be observed visually. The cloud point increases as the hydropho-bicity of a surfactant molecule increases (i.e., the length of its hydrocarbon tail increases or the size of its hydrophilic head group decreases) (Myers 1988).

4.5.2.3. Solubilization

Nonpolar molecules, which are normally insoluble or only sparingly soluble in water, can be solubilized in an aqueous surfactant solution by incorporation into micelles or other types of association colloids (Elworthy et al. 1968, Mukerjee 1979, MacKay 1987, Christian and Scamehorn 1995). The resulting system is thermodynamically stable; however, it may take an appreciable time to reach equilibrium because of the time taken for molecules to diffuse through the system and because of the activation energy associated with transferring a nonpolar molecule from a bulk phase into a micelle (Dickinson and McClements 1995, Kabalnov and Weers 1996). Micelles containing solubilized materials are referred to as swollen micelles or microemulsions, whereas the material solubilized within the micelle is referred to as the solubilizate. The ability of micellar solutions to solubilize nonpolar molecules has a number of potentially important applications in the food industry, including selective extraction of nonpolar molecules from oils, controlled ingredient release, incorporation of nonpolar substances into aqueous solutions, transport of nonpolar molecules across aqueous membranes, and modification of chemical reactions (Dickinson and McClements 1995). There are three important factors which determine the functional properties of swollen micellar solutions: (1) the location of the solubilizate within the micelles, (2) the maximum amount of material that can be solubilized per unit mass of surfactant, and (3) the rate at which solubilization proceeds. Food manufacturers must therefore select a micellar system which is most appropriate for their particular application.

4.5.2.4. Surface Activity and Droplet Stabilization

Surfactants are used widely in the food industry to enhance the formation and stability of food emulsions (St. Angelo 1989, Dickinson 1992, Bergenstahl 1997). To do this, they must adsorb to the surface of emulsion droplets during homogenization and form a protective membrane which prevents the droplets from aggregating with each other during a collision (Walstra 1993a, 1996a,b). Surfactant molecules adsorb to oil-water interfaces because they can adopt an orientation in which the hydrophilic part of the molecule is located in the water while the hydrophobic part is located in the oil. This minimizes the contact area between hydrophilic and hydrophobic regions and therefore reduces the interfacial tension (Chapter

5). This reduction in interfacial tension is important during homogenization because it facilitates the further disruption of emulsion droplets (i.e., less energy is needed to break up a droplet when the interfacial tension is lowered) (Chapter 6). Once adsorbed to the surface of a droplet, the surfactant must provide a repulsive force which is strong enough to prevent the droplet from aggregating with its neighbors (Chapters 3 and 7). Ionic surfactants provide stability by causing all the emulsion droplets to have the same electric charge and therefore electrostatically repel each other. Nonionic surfactants provide stability by generating a number of short-range repulsive forces which prevent the droplets from getting too close together, such as steric, hydration, and thermal fluctuation interactions. Some surfactants form multilayers (rather than monolayers) at the surface of an emulsion droplet, which greatly enhances the stability of the droplets against aggregation (Friberg and El-Nokaly 1983). In summary, surfactants must have three characteristics to be effective at enhancing the formation and stability of emulsions (Chapter 6). First, they must rapidly adsorb to the surface of the freshly formed emulsion droplets during homogenization. Second, they must reduce the interfacial tension by a significant amount. Third, they must form an interfacial layer that prevents the droplets from aggregating. It should also be noted that the ability of surfactants to form micelles in the continuous phase of an emulsion can have a negative impact on emulsion stability, because they induce depletion flocculation or facilitate the transport of oil molecules between droplets (Dickinson and McClements 1995). The ability of surfactants to regulate the interactions between droplets can also have a pronounced influence on the rheological properties of emulsions (Chapter 8).

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