So far, we have only considered the molecular characteristics of an interface that separates two pure liquids. In practice, food emulsions contain various types of surface-active mol ecules which can accumulate at the interface and therefore alter its properties (e.g., proteins, polysaccharides, alcohols, and surfactants) (Dickinson and Stainsby 1982, Dickinson 1992).
188.8.131.52. Surface Activity and the Reduction of Interfacial Tension
The surface activity of a molecule is a measure of its ability to accumulate at an interface. A molecule tends to accumulate at an interface when the free energy of the adsorbed state is significantly lower than that of the unadsorbed state (Hiemenz 1986). The difference in free energy between the adsorbed and unadsorbed states (AGads) is determined by changes in the interaction energies of the molecules involved, as well as by various entropy effects (Shaw 1980). The change in the interaction energies which occurs as a result of adsorption comes from two sources, one associated with the interface and the other with the emulsifier molecule itself. First, by adsorbing to an oil-water interface, an emulsifier molecule is able to "shield" the oil molecules from the water molecules. The direct contact between oil and water molecules is replaced by contacts between the nonpolar segments of the emulsifier and oil molecules and between the polar segments of the emulsifier and water molecules (Israelachvili 1992). These interactions are less energetically unfavorable than the direct interactions between oil and water molecules. Second, emulsifier molecules usually have both polar and nonpolar segments, and when they are dispersed in bulk water, some of the nonpolar segments come into contact with water, which is energetically unfavorable because of the hydrophobic effect. By adsorbing to an interface, they are able to maximize the number of energetically favorable interactions between the polar segments and water while minimizing the number of unfavorable interactions between the nonpolar segments and water (Figure 5.2). The major driving force favoring the adsorption of an amphiphilic molecule at an interface is therefore the hydrophobic effect. Nevertheless, various other types of interaction may also contribute to the surface activity, which may either favor or oppose adsorption (e.g., hydration repulsion, electrostatic interactions, steric interactions, and hydrogen bonding).
The entropy effects associated with adsorption are mainly due to the fact that when a molecule adsorbs to an interface, it is confined to a region which is considerably smaller than the volume it would occupy in a bulk liquid and that its molecular rotation is restricted. Both of these effects are entropically unfavorable, and so a molecule will only adsorb to an interface if the energy gained by optimizing the interaction energies is sufficiently large to offset the entropy lost. When the adsorption energy is large compared to the thermal energy (i.e., -AGads >> kT), a molecule "binds" strongly to the surface and has a high surface activity. When the adsorption energy is small compared to the thermal energy (i.e., -AGads << kT), a molecule tends to be located mainly in the bulk liquid and has a low surface activity. When
FIGURE 5.3 The decrease in interfacial tension caused by surface-active molecules depends on how effectively they "shield" the oil molecules from the water molecules. The interfacial tension of the system shown on the left is lower than that on the right, because the contact between the polar and nonpolar molecules is shielded more effectively.
the free energy of adsorption is highly positive (i.e., AGads >> kT), there is a deficit of solute in the interfacial region, which is referred to as negative adsorption (Shaw 1980).
The decrease in the free energy of a system which occurs when a surface-active molecule adsorbs to an interface manifests itself as a decrease in the interfacial tension (i.e., less energy is required to increase the surface area between the oil and water phases). The extent of this decrease depends on the effectiveness of the molecule at "shielding" the direct interactions between the oil and water molecules, as well as on the strength of the interactions between the hydrophilic segments and water, and between the hydrophobic segments and oil (Israelachvili 1992). The more efficient an emulsifier is at shielding the interaction between oil and water, the lower the interfacial tension (Figure 5.3).
The ability of surfactant molecules to shield direct interactions between two immiscible liquids is governed by their optimum packing at an interface, which depends on their molecular geometry (Chapter 4). When the curvature of an interface is equal to the optimum curvature of a surfactant monolayer (i.e., optimum packing is possible), the interfacial tension is ultralow because direct interactions between the oil and water molecules are effectively eliminated (Section 184.108.40.206). On the other hand, when the curvature of an interface is not at its optimum, the interfacial tension increases because some of the oil molecules are exposed to the polar regions of the surfactant or some of the water molecules come into contact with the hydrophobic part of the surfactant. Surfactants can usually screen the interactions between the oil and water phases more efficiently than biopolymers, which means they are more effective at reducing the interfacial tension. This is because biopolymers cannot pack as efficiently at the interface and because they often have some nonpolar regions on their surface exposed to the water phase and some polar regions exposed to the oil phase (Damodaran 1996).
The reduction of the interfacial tension by the presence of an emulsifier is referred to as the surface pressure: n = yo/w - Yemulsifier, where yo/w is the interfacial tension of a pure oil-water interface and Yemulsifier is the interfacial tension in the presence of the emulsifier (Hiemenz 1986).
220.127.116.11. Adsorption Kinetics of Emulsifiers to Interfaces
The tendency of an emulsifier to exist in either the bulk or interfacial regions is governed by thermodynamics; however, the rate at which an emulsifier adsorbs to an interface is determined by various mass transport processes (e.g., diffusion and convection) and energy barriers associated with the adsorption process (e.g., availability of free sites, electrostatic repulsion, steric repulsion, hydrodynamic repulsion, and micelle dynamics). The major factors which influence the adsorption kinetics of an emulsifier at an interface are discussed in some detail in Section 5.7.
The conformation and orientation of molecules at an interface are governed by their attempt to reduce the free energy of the system (Evans and Wennerstrom 1994). Amphiphilic molecules arrange themselves so that the maximum number of nonpolar groups are in contact with the oil phase, while the maximum number of polar groups are in contact with the aqueous phase (Figure 5.4). For this reason, small-molecule surfactants tend to have their polar head groups protruding into the aqueous phase and their hydrocarbon tails protruding into the oil phase (Myers 1988). Similarly, biopolymer molecules adsorb so that predominantly nonpolar segments are located within the oil phase, whereas predominantly polar segments are located within the water phase (Dickinson 1992, Damodaran 1996, Dalgleish 1996b). Biopolymer molecules often undergo structural rearrangements after adsorption to an interface in order to maximize the number of favorable interactions. In aqueous solution, globular proteins adopt a three-dimensional conformation in which the nonpolar amino acids are predominantly located in the hydrophobic interior of the molecule so that they can be away from the water (Dill 1990). When they adsorb to an oil-water interface, they are no longer completely surrounded by water, and so the protein can reduce its free energy by altering its conformation so that more of the hydrophobic amino acids are located in the oil phase and more of the polar amino acids are located in the water phase (Dalgleish 1996b). The rate at which the conformation of a biopolymer changes at an oil-water interface depends on its molecular structure (Dickinson 1992). Flexible random-coil molecules can rapidly alter their conformation, whereas rigid globular molecules change more slowly because of various kinetic constraints. Immediately after adsorption to an interface, a globular protein has a conformation that is similar to that in the bulk aqueous phase. With time, it alters its conformation so that it can optimize the number of favorable interactions between the nonpolar amino acids and the oil molecules. An intermediate stage in this unfolding process is the exposure of some of the nonpolar amino acids to water, which is energetically unfavorable because of the hydrophobic effect, and so there is an energy barrier which must be overcome before unfolding can occur. In this case, the rate of any conformational changes will depend on the height of the energy barriers compared to the thermal energy.
The configuration of emulsifier molecules at an interface can have an important influence on the bulk physicochemical properties of food emulsions (Dalgleish 1996b). The coalescence stability of many oil-in-water emulsions depends on the unfolding and interaction of protein molecules at the droplet surface. When globular proteins unfold, they expose reactive amino acids that are capable of forming hydrophobic and disulfide bonds with their neighbors, thus generating a highly viscoelastic membrane that is resistant to coalescence (Dickinson and Matsumura 1991, Dickinson 1992). The susceptibility of certain proteins to enzymatic hydrolysis depends on which side of the adsorbed molecule faces toward the droplet surface (Dalgleish 1996a,b). The susceptibility of surfactants with unsaturated hydrocarbon tails to lipid oxidation depends on whether their tails are perpendicular or parallel to the droplet surface, the latter being more prone to oxidation by free radicals generated in the aqueous phase.
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