Dispersion emulsions and foams

The study of electrokinetic phenomena has evolved considerably, since the first smdies were carried out by Quincke, Helmholtz, and Lipman in the nineteenth century. In the last 30 years, at least five books have been devoted to the subject [1-5]. Among the parameters which can be deduced from electrokinetic measurements, potential is profoundly related to colloid science because the stability of dispersions, emulsions, and foams depends on its value. 

Study of the dynamics of fluid flow is concerned with the forces acting on the bodies in the fluid. In the earher chapters on soUd dispersions, emulsions, and foams, fluid dynamics was largely ignored in favor of the true colloidal interactions. In aerosols, the nature of the continuous medium makes the subject of fluid dynamics much more important to the understanding of the system, so that the following discussion will introduce a few basic relationships that can be important in the study of aerosols. 

In other applications, proteins are adsorbed by purpose, for example, as immobilized enzymes in biosensors and bioreactors, immunoglobulins in immunoassays, drugs in drug targeting and controlled release systems, and as stabilizers of dispersions, emulsions, and foams in foodstuffs, pharmaceuticals, and cosmetics. 

Uses Wetting, dispersing, emulsitying, and foaming agent for industrial processes, detergent/cleaner formulations... 

Vincent, B. (1999). Dispersion stabilization and destabilization by polymers. In Dickinson, E., Rodriguez Patino, J.M. (Eds). Food Emulsions and Foams Interfaces, Interactions and Stability, Cambridge, UK Royal Society of Chemistry, pp. 19-28. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

What is necessary for a collision to be followed by a coalescence What is the mechanism of coalescing These are questions that have fascinated many scientists and engineers both in the field of the physical chemistry of emulsions and foams and in the engineering field of agitated dispersions. 

Microscopy in Food Science is in an exciting state of flux. Traditional techniques of specimen preparation and observation will continue to give essential data on the structure of foods. However, the emphasis in the future will probably lie in the development of faster methods and in the quantification of individual components, both aiming at definition of structre /function relationships. This will be true of particulates as they relate to sensory scores and to the characterization of dispersed phases in emulsions and foams. At the same time, the use of microchemical methods should become more common as a means of... 

Just as with emulsions and foams, suspensions can exist with additional dispersed phases present. They may contain, in addition to solid particles and a continuous liquid phase (and possibly a stabilizing agent), emulsified droplets and/or gas bubbles. Figure 2.4 (in Section 2.2.1) shows photomicrographs of a practical suspension that contains suspended oil droplets in addition to the particles. The terminology used to describe such systems can become confusing. Consider an aqueous dispersion of solid particles and emulsion droplets. If the solid particles are adsorbed on the emulsion droplets then it is an emulsion that also contains solids. If, however, the particles and droplets are not mutually associated then the system is at once a suspension and an emulsion. Which term is used becomes a matter of choosing the most appropriate context frequently one or the other is considered to be the primary dispersion while the other phase is considered to be an additive or a contaminant. 

In concentrated emulsions and foams the thin liquid films that separate the droplets or bubbles from each other are very important in determining the overall stability of the dispersion. In order to be able to withstand deformations without rupturing, a thin liquid film must be somewhat elastic. The surface chemical explanation for thin film elasticity comes from Marangoni and Gibbs (see Ref. [199]). When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded... 

No-slip is assumed (care is needed for some dispersions such emulsions and foams). 

Suspensions, and to some degree emulsions and foams, play crucial roles in the evolution of the earth s rocks, rivers, streams, lakes, oceans, and soils. Table 9.1 lists some examples. In many cases their role is somewhat disguised in that these colloidal dispersions are the precursors to the ultimate products, the latter having very different final appearances, such as many rocks, sediments, and soils. 

The merging of two or more dispersed species into a single one. Coalescence reduces the total number of dispersed species and also the total interfacial area between phases. In emulsions and foams coalescence can lead to the separation of a macrophase, in which case the emulsion or foam is said to break. The coalescence of solid particles is termed sintering . 

Polysaccharides may exercise a protective action in an emulsion and foam as a thin film at liquid-liquid (emulsion) and liquid-air (foam) interfaces. The hydrophile-lipophile balance in the macromolecules as well as <(>, determines whether or not the emulsion is an oil-in-water or water-in-oil dispersion (Void and Void, 1983 Dickinson, 1992). 

Fine solid particles adsorb at interfaces and can provide long-term kinetic stability of emulsions and foams.1"5 For more effective stabilization the particles must be much smaller than the dispersed droplets.1,2 For the production of microemulsions, nano-sized particles are therefore of particular interest. 

Refs. [i] Evans ED, Wennerstrom H (1999) The colloidal domain. Wiley-VCH, New York, pp 217 [ii] Hunter RJ (2004) Foundations of colloid science, 2nd edn. Oxford University Press, Oxford, pp 539 [iii] Hamaker HC (1937) Physics 4 1058 Morrison ID, Ross S (2002) Colloidal dispersions. Suspensions, emulsions, and foams. Wiley Interscience, New York, pp 355... 

For mechanistic studies, ambient pressure experiments on emulsions and foams often offer significant experimental advantages over high-pressure experiments. However, high-pressure measurements are also needed since the phase behavior, physical properties of the fluids, and dispersion flow may all depend on pressure. Experiments under laboratory conditions that closely match reservoir conditions are particularly important in the design of projects for specific fields. Chapter 19, by Lee and Heller, describes steady-state flow experiments on CO2 systems at pressures typical of those used in miscible flooding. The following chapter, by Patton and Holbrook,... 

An important common feature of macroion solutions is that they are characterized by at least two distinct length scales determined by the size of macroions (an order up to lOnm in the case of ionic micellar solutions) and size of the species of primary solvent (water molecules and salt ions, i.e. few Angstroms). Considering practical colloidal macro-dispersions, like foams, gels, emulsions, etc., usually we are dealing with as many as four distinct length scales molecular scale (up to lnm) that characterizes the species of the primary solvent (water or simple electrolytes) submicroscopic or nano scale (up to lOOnm) that characterizes nanoparticles or surfactant aggregates called micelles microscopic or mesoscopic scale (up to lOO m) that encompasses liquid droplets or bubbles in emulsion and foam systems as well as other colloidal suspensions, and macroscopic scale (the walls of container etc). 

Then mesoscopic aspects are treated. Chapter 9 gives a general introduction on disperse or particulate systems. It concerns properties that originate from the division of a material over different compartments, and from the presence of a large phase surface. Two chapters give basic theory. Chapter 10 is on surface phenomena, where the forces involved primarily act in the direction of the surface. Chapter 12 treats colloidal interactions, which primarily act in a direction perpendicular to the surface. Two chapters are concerned with application of these basic aspects in disperse systems Chapter 11 with emulsion and foam formation, Chapter 13 with the various instabilities encountered in the various dispersions foams, emulsions, and suspensions. 

B. L. Wedzicha. Distribution of low-molecular-weight food additives in dispersed systems. In E. Dickinson, G. Stainsby, eds. Advances in Food Emulsions and Foams. Elsevier, London, 1988, pp. 329-371. 

Emulsions and foams are dispersions of two fluids, which implies that the interface between the phases is deformable. This makes the breakup of one of the materials into small particles far easier, which does not imply that quantitative understanding of the phenomena involved is easy. Some aspects are discussed in this chapter. 

Stable foams may be formed by surfactant solutions. Thin liquid films separate gas bubbles, which can be colloidal but are usually much larger. Once formed, gravity eventually drains the liquid tmtil the films break. Viscous additives can slow drainage and increase bubble hfetime significandy. Solid emulsions and foams are less common, the dispersing phase being sohd while a liquid or gas phase is dispersed. 

Morrison, Ian D., and Ross, Sydney (2002). Colloidal Dispersions Suspensions, Emulsions, and Foams. New York Wiley Interscience. 

Stabilization and partial destabilization of emulsions and foams by controlling the state of dispersion and agglomeration of oil droplets or fat globules... 

---