Emulsions and foams

A classical and important application of stirring methods is the formation of foams and emulsions. The most common examples are in food processing. To prepare whipped cream, a beater is used to incorporate air into a liquid, in the form of microbubbles. The preparation of a mayonnaise sauce is somewhat subtler. First, a small quantity of oil is incorporated into an aqueous liquid (the egg yolk and mustard). The oil is dispersed by the beater in the form of droplets. When the level of oil exceeds a certain volume fraction, an inversion occurs. The oil becomes the continuous phase, while the water becomes the dispersed phase, because the volume of water is no longer sufficient to contain all the oil droplets. 

The term emulsion refers to a mixture of two non-miscible liquids, one of which is dispersed within the other in the form of droplets. Foam is a mixture of microbubbles in a liquid. The physico-chemical properties of the products are important for stabilizing emulsions and foams. If the emulsion is not stabilized, the droplets coalesce when the stirring ceases and the emulsion gradually subsides. It is the function of the egg yolk to stabilize the mayonnaise. Anyone who ever prepared a mayonnaise or whisked egg whites will have understood that the beater is a precious utensil for achieving an emulsion or a foam/mousse. It is for this reason that we discuss this issue here. 

8 More precisely, surface tension refers to a gas/liquid pair, and interfacial tension to a pair of non-miscible liquids. 

It is necessary to anticipate the resnlts of Chapter 11 in order to evaluate the turbulent velocity u d). It will be established (equations [11.18] and [11.19] of Chapter 11) that the average kinetic energy per imit volume in a tirrbirlent eddy of dimension a is fixed by the rate, e, of dissipation per imit mass of the turbirlent kinetic energy  

Equations [9.26] and [9.27], therefore, indicate that a bubble of radius a can be broken by turbulence if  

Milk comes from different types of mammals, cow s milk being most common. Over the centuries, it has been used in various ways. This has resulted in the development of a number of milk products, such as cream, butter, cheese, and yogurt. 

Milk is an emulsion, a suspension of tiny fat droplets in water that further contains proteins (predominantly occurring as casein micelles), lactose, and some minerals. The fat droplets are coated with an adsorbed layer of proteins that helps to keep the droplets suspended. 

Prodnction of yogurt and cheese involves destabilization of the milk emulsion with the assistance of microbial activity. In yogurt, the milk is coagulated and sonred by lactic acid produced by bacteria. Cheese making starts with an enzymatic modification of the casein micelles allowing them to co-precipitate with fat droplets, thus forming the cheese curd. 

Cream is obtained by collecting the fatty fraction of the milk in a fat-rich phase leaving the skimmed milk behind. Like milk, cream is still a fat-in-water emulsion. When the cream is whipped, air bubbles are incorporated into the cream. The emulsion is transformed into a foam. The bubbles are stabilized by a layer of denatured milk proteins and surrounded by fat globules. Further whipping results in collapse of the foam structure and destruction of the fat globules. Fat becomes the continuous phase in which water droplets are dispersed. The emulsion, as it was in milk and cream, has inverted butter is formed. 

or any other apolar liquid, and water mix poorly if they mix at all. Also, gases usually have a limited solubility in liquids. However, in various systems in nature as well as in technological products, we find these compounds intimately blended in emulsions and foams. Emulsions are fine dispersions of droplets of one type of liquid in another that forms the continuous phase. In foams, gas bubbles are finely dispersed in a liquid that, thereafter, may be solidified. 

An emulsion is a dispersed system in which the phases are immiscible or partially miscible liquids. The globules of the dispersed liquid in the usual type of emulsion (sometimes now called a macroemulsion) are usually between 0.1 fim and 10 fim in diameter, and so tend to be larger than the particles found in sols. 

1 Suggest a Gibbs-energy argument for why a liquid drop is spherical. 

2 What happens to the surface tension at the gas-liquid critical point  

3 Why should the Langmuir adsorption isotherm be more reliable, at high gas pressures, for chemisorption than for physical adsorption  

4 Colloidal particles of the same charge immersed in an electrolyte solution attract each other by van der Waals forces and repel each other by Debye screened interactions (see Eq. 16.70). Why does the ease of coagulation increase rapidly with increasing solution ionic strength  

5 Describe the roles of both the inner and outer portions of the micelle in the action of soap. 

---

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

Microemulsions are treated in a separate section in this chapter. Unlike macro- or ordinary emulsions, microemulsions are generally thermodynamically stable. They constitute a distinctive type of phase, of structure unlike ordinary homogeneous bulk phases, and their study has been a source of fascination. Finally, aerosols are discussed briefly in this chapter, although the topic has major differences from those of emulsions and foams. 

The surface active properties of aHphatic amine oxides were discovered ia the 1930s and the wetting, detergent, emulsion, and foam stabilizing properties were published shortiy thereafter (42). However, the use of amine oxides was not significant until Procter and Gamble started usiag them ia household products around 1960 (43—46). 

E. Dickinson, ed., food Emulsions and foams. Royal Society of Chemistry, London, 1987. 

Foam Production This is important in froth-flotation separations in the manufac ture of cellular elastomers, plastics, and glass and in certain special apphcations (e.g., food products, fire extinguishers). Unwanted foam can occur in process columns, in agitated vessels, and in reactors in which a gaseous product is formed it must be avoided, destroyed, or controlled. Berkman and Egloff (Emulsions and Foams, Reinhold, New York, 1941, pp. 112-152) have mentioned that foam is produced only in systems possessing the proper combination of interfacial tension, viscosity, volatihty, and concentration of solute or suspended solids. From the standpoint of gas comminution, foam production requires the creation of small biibbles in a hquid capable of sustaining foam. 

Figure 2. Results for emulsion and foam cell size in PS foam RSM experiment.

The basic problems involved in emulsion and foam chemistry are discussed with emphasis on how to solve them and how to correlate the information thus obtained for future use and interpretation. A blending of theoretical and practical knowledge has to be used, and examples are given to illustrate the methods of solving the industrial problems involved. 

T he industrial importance of emulsions and foams today cannot be overestimated. There is every indication that this field will be of even greater utility and importance in the near future. 

In the operations of a consulting laboratory, one is constantly confronted with a variety of problems which involve the technology of emulsions and foams. These problems cover a diversity of interests such as foods, cosmetics, adhesives, polymers, and others. 

In emulsion and foam technology much is known concerning the properties and behavior of systems which involve only two or three components. Given a particular system and data concerning concentration, temperature, and manner of mixing, we can today predict fairly well the properties of the comparatively simple emulsion or foam. However, most emulsions and foams of importance are multicomponent systems and in these systems the predictability of the action or the properties of the emulsion or foam on a theoretical basis often becomes small. 

It is hardly possible to predict that a general theory will ever be evolved which will cover all possible types of industrial emulsions and foams. However, it seems that if the information described were gathered in a systematic fashion, the data could be classified in a logical order for different types of emulsions. Hence, cosmetic-type emulsions, salad dressing, and mayonnaise-type emulsions would each fall into its own individual category. The suggestion therefore is made that a repository of such information be made available to all who are interested. 

Phospholipids are amphiphilic compoimds with high surface activity. They can significantly influence the physical properties of emulsions and foams used in the food industry. Rodriguez Patino et al. (2007) investigated structural, morphological, and surface rheology of dipalmitoylpho-sphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers at air-water interface. DPPC monolayers showed structural polymorphisms at the air-water interface as a function of surface pressure and the pH of the aqueous phase (Fig. 6.18). DOPC monolayers showed a... 

The first theoretical considerations concerning n (p) and G (p) of concentrated 3-D emulsions and foams were based on perfectly ordered crystals of droplets [4,5,15-18]. In such models, at a given volume fraction and applied shear strain, all droplets are assumed to be equally compressed, that is, to deform affinely under the applied shear thus all of them should have the same shape. Princen [15,16] initially analyzed an ordered monodisperse 2-D array of deformable cylinders and concluded that G = Qiox(p < (/), and that G jumps to nearly the 2-D Laplace pressure of the cylinders at the approach of ( > = 100%, following a ( — dependence. 

Fuel system corrosion inhibitors must have a low tendency toward emulsification with water and toward foam enhancement in turbulent systems. These properties are especially critical whenever inhibitors are used in jet fuel. The sensitivity of jet fuel pumping and injection systems requires that fuel be free of emulsions and foam. 

The purpose of this section is to give a brief overview of the rheology of highly concentrated emulsions and foams, in as simple terms as possible. For comprehensive reviews in this field, see Refs. 16 and 50. 

Damodaran, S. (2005). Protein stabilization of emulsions and foams. Journal of Food... 

Semenova, M., Antipova, A., Belyakova, L., Dickinson, E., Brown, R., Pelan, E., Norton, I. (1999). Effect of pectinate on properties of oil-in-water emulsions stabilized by asi-casein and p-casein. In Dickinson, E., Rodriguez Patino, J.M. (Eds). Food Emulsions and Foams Interfaces, Interactions and Stability, Cambridge, UK Royal Society of Chemistry, pp. 163-175. 

There is now a solid body of available knowledge to indicate that the general features of biopolymer self-assembly in bulk aqueous solutions can account for various detailed aspects of the stability, rheology and microstructure of oil-in-water emulsions (and foams) stabilized by the same kinds of biopolymers (Dickinson, 1997, 1998 Casanova and Dickinson, 1998 Dickinson et al., 1997, 1998 Semenova et al., 1999, 2006 van der Linden, 2006 Semenova, 2007 Ruis et al., 2007). In particular, the richness of the self-assembly and surface-active properties of the... 

Darling, D.F., Birkett, R.J. (1987). Food colloids in practice. In Dickinson, E. (Ed.). Food Emulsions and Foams, London Royal Society of Chemistry, pp.1-29. 

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. 

Another optical technique, called the back-light scattering (Kossel-diffraction) method can also be used to investigate structure in food emulsions and foams. In this method, the emulsion (or foam) in a transparent vessel is illuminated by a collimated laser beam (See... 

New experimental techniques and several of their applications were presented which help in the understanding of structure, texture and stability of food systems. For future research, the mechanism of film stability by the microlayering of colloid particles seems to be the most promising - especially in food emulsions and foams. Work is in progress in our laboratory to calculate the oscillatory disjoining pressure inside liquid films containing microlayers [30],... 

Krog, N., Barfod N.M. and Buchheim W., Protein-fat-surfactant interactions in whippable emulsions, in "Food Emulsions and Foams , E. Dickinson (Ed.), Royal Society of Chemistry, London (1987) 144. 

---