Structure of emulsions

An emulsion is a mixture of two immiscible liquids, one of which is uniformly dispersed within the other as small droplets (i.e., droplet diameter in the range O.l-lOOpm) (McClements, 2005). In this chapter, the concern is exclusively with dispersions of oil in water, that is, oil-in-water emulsions. Advances in homogenizer technology have allowed the development of nanoemulsions with droplets far smaller and with more uniform size distributions than commonly seen in manufactured foods (Weiss et al, 2008). There is not a commonly accepted size cut-off for nanoemulsions and different researchers have used different definitions below lOOOnm (Muller et al, 2000), 500mn (Anton et al, 2008), 200nm (Higami et al, 2003 Solans et al, 2005), and lOOmn (Luykx et al, 2008). Emulsions with nano-scale crystalline droplets are sometimes referred to as solid lipid nanoparticles (SLN). 

Emulsions are formed either by using mechanical energy to reduce the particle size of a coarse mixture or, less frequently, by a controlled phase separation (e.g., on dilution of raki 

Emulsions are thermodynamically unstable structures given a degree of kinetic stability by an adsorbed interfacial layer of amphiphilic emulsifiers. The emulsifiers serve to lower the interfacial tension and provide some inter-droplet repulsive forces to stabilize the dispersions (e.g., steric and electrostatic). The interfacial layer is typically between about 1 and lOnm thick for food grade emulsifiers, such as surfactants, phospholipids, proteins, or polysaccharides, and the interfacial concentration is in the order of a few mg per square meter of surface (McClements, 2005 McClements and Decker, 2000). 

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Later we discover another parameter, the phase inversion temperature(PIT), which helps us to predict the structure of emulsions stabilized by nonionic surfactants. The PIT concept is based on the idea that the type of an emulsion is determined by the preferred curvature of the surfactant film. For a modern introduction into the HLB and PIT concepts see Ref. [546],... 

Light Scattering Studies of the Internal Structure of Emulsion Polymer Particles... 

Louden JD, Rowe RC. A quantitative examination of the structure of emulsions prepared using cetostearyl alcohol and cetrimide using Fourier transform infrared microscopy. Int J Pharm 1990 63 219-225. 

Interactions between proteins and polysaccharides give rise to various textures in food. Protein-stabilized emulsions can be made more stable by the addition of a polysaccharide. A complex of whey protein isolate and carboxymethylcellulose was found to possess superior emulsifying properties compared to those of the protein alone (Girard et al., 2002). The structure of emulsion interfaces formed by complexes of proteins and carbohydrates can be manipulated by the conditions of the preparation. The sequence of the addition of the biopolymers can alter the interfacial composition of emulsions. The ability to alter interfacial structure of emulsions is a lever which can be used to tailor the delivery of food components and nutrients (Dickinson, 2008). Polysaccharides can be used to control protein adsorption at an air-water interface (Ganzevles et al., 2006). The interface of simultaneously adsorbed films (from mixtures of proteins and polysaccharides) and sequentially adsorbed films (where the protein layer is adsorbed prior to addition of the polysaccharide) are different. The presence of the polysaccharide at the start of the adsorption process hinders the formation of a dense primary interfacial layer (Ganzelves et al., 2008). These observations demonstrate how the order of addition of components can influence interfacial structure. This has implications for foaming and emulsifying applications. 

Many emulsions have droplets and structural features such as interfacial arrangements on the nanoscale that require electron microscopy, but the examples given here will only deal with structures of emulsions in the micron range. Figure 5.1 shows two-dimensional CLSM images of commercial spreads, where the fat phase is stained to make it fluorescent. The fat phase, therefore, appears bright in the images and the water phase appears dark. 

This chapter will be concerned almost exclusively with liquid 0/W emulsions, mainly because they are the most exhaustively studied and the principles for their behavior are the most thoroughly established, not neeessarily because they are the most important of the emulsions. For example, no description of emulsions in meat products or in bread mixes and cake batters is given, as these are less un-drstood from a fundamental point of view than are the more simple 0/W emulsions. What is attempted here is a description of the structures of emulsion droplets and how these af-feet the properties of the emulsion. 

TABLE VIII Chain Structure of Emulsion Polybutadiene and SBR°... 

When applying the laws of dilute solution physical chemistry, it is generally presumed that the solute is miscible with (soluble in) the solvent in the mixture of interest. If this is not true, then the mixture is heterogeneous (in a phase sense) and must be treated accordingly. In colloid science, solubility plays an important role in the collapse of the colloidal structure of emulsions and dispersions by ripening. t is apparent from these observations that solubility is a fundamental property of all surfactant systems, and because the practical utility of a consumer product is dictated by its physical chemistry and colloid science [1], solubility is therefore also relevant to utility and performance. 

The abihty to capture direct images of double-emulsion globules enables verification of the actual existence of the complex, three-phase system. The importance of visualization of double emulsions is seen in the work of Matsumoto and coworkers (1976) and their early success in developing a technique for preparation of stable double-emulsion systems. Similarly Kavaliunas and Frank (1978) used photomicrography to detect the structure of emulsions prepared with a water, p-xylene, and nonylphenol diethylene glycol ether system. As a result the authors were able to determine conditions favorable for the preparation of multiple emulsions, as shown in Figure 3.1. With polarized microscopy, the authors detected diffuse birefringence in multiple emul-... 

Microscopy. Particle size, shape and structure of emulsion droplets can be visualized by various microscope techniques, such as phase contrast light microscopy, confocal scanning light microscopy (CSLM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray microtomography (XRT), atomic force microscopy (AFM) and imaging techniques. 

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