Applications of emulsions

Emulsions are formed when two immiscible liquids are mixed with each other. The most familiar forms are oil-in-water emulsions (0/W emulsions), which consist of oil droplets in water, and water-in-oil emulsions (W/O emulsions), where an aqueous solution is emulsified in an outer oil phase. In specific cases, two immiscible liquids form transparent systems, which are termed microemulsions. According to lUPAC, a microemulsion is a thermodynamically stable emulsion. This definition applies to a few systems made up of oil, water and emulsifiers, which spontaneously form transparent mixtures. Over and above this narrow definition, however, in patents and scientific literature the name microemulsion is applied to other transparent to translucent systems, which are not created spontaneously but by special production procedures. 

Emulsions are not a human invention. In living nature they play an especially important role in the absorption of fats with nutrients. The earliest known use of an emulsion by humans is certainly the exploitation of milk and milk products such as cream, butter and cheese for nutritional purposes. With increasing prosperity, the advanced civilizations of antiquity began to use emulsions for cosmetic purposes. Nowadays these ancient uses have been joined by countless others, primarily in a wide variety of technical processes, so that emulsions play a role in connection with almost all everyday products and processes. What is it that makes emulsions so attractive  

In this overview a detailed explanation of the connection between the structure and use of an emulsion is given. A look at the various fields of application in the cosmetics and technology sectors shows how powerfully the specific profile of requirements concerning the technical properties of an emulsion determine physicochemical characteristics such as emulsion type, particle size, rheological properties and manufacturing process. The current state of development of emulsions in the various fields of application is described, as are trends and future developments. 

This refers to the process of thinning and disruption of the liquid film between the droplets, with the result that two or more droplets fuse into a larger droplet. The limiting case for coalescence is the complete separation of the emulsion into two distinct liquid phases. The driving force for coalescence is the surface or film fluctuations this results in a close approach of the droplets whereby the van der Waals forces are strong and prevent their separation. 

This refers to the process when an exchange occurs between the disperse phase and the medium. For example, an O/W emulsion may, with time or change of conditions, invert to a W/O emulsion. In many cases phase inversion passes through a transition state whereby multiple emulsions are produced. 

Several industrial systems involve emulsions, of which the following are worthy of mention. Food emulsions include mayonnaise, salad creams, deserts, and beverages, while personal care and cosmetics emulsions include hand creams, lotions, hair sprays, and sunscreens. Agrochemical emulsions include self-emulsifiable oils that produce emulsions on dilution with water, emulsion concentrates with water as the continuous phase, and crop oil sprays. Pharmaceutical emulsions include anaesthetics (O/W emulsions), hpid emulsions, and double and multiple emulsions, while paints may involve emulsions of alkyd resins and latex. Some dry-cleaning formulations may contain water droplets emulsified in the dry cleaning oil that is necessary to remove soils and clays, while bitumen emulsions are prepared stable in their containers but coalesce to form a uniform fihn of bitumen when apphed with road chippings. In the oil industry, many crude oils (e.g.. North sea oil) contain water droplets that must be removed by coalescence followed by separation. In oil slick dispersion, the oil spilled from tankers must be emulsified and then separated, while the emulsification of waste oils is an important process for pollution control. 

The above-described importance of emulsion in many industries justifies the extensive research that has been carried out to understand the origins of emulsion instabihty and methods to prevent their breakdown. Unfortunately, fundamental research with emulsions is not easy, as model systems (e.g., with monodisperse droplets) are difficult to produce. In many cases, theories on emulsion stabiHty are not exact and semi-empirical approaches must be used. 

The area of emulsion applications is very large and cannot be described in the space available here. However, some important areas will be highlighted to indicate the basic criteria for emulsion technology (Friberg, 1976 Gitis and Sivamani, 2004). 

---

M Chappat. Some applications of emulsions. Colloids Surfaces A Physicochem Eng Aspects 9 57-77, 1994. 

S.S. Davis, J. Hadgraft, and K.J. Pahn In P. Becher (ed) Medical and Pharmaceutical Applications of Emulsions. Encyclopedia of Emulsion Technology, Vol. 2. Marcel Dekker New York (1985). 

The trick in using emulsions is that both water and oil (the latter is insoluble in water) can be applied simultaneously. Further, other molecules may be included that may be soluble in either phase (water or oil). This obviously leads to the observation of thousands of applications of emulsions. It is very important to mention here that, actually, nature uses this trick in most of the major biological fluids. The most striking example is milk. The emulsion chemistry of milk has been found to be the most complex, and still not very well investigated. 

In the application of emulsions for cleaning and polishing, a water phase and an oil phase (a selected halogenated solvent) are needed. The emulsions are used in cleaning and polishing the surfaces of metals and other hard surfaces (glass, etc.). 

In the pharmaceutics literature, an application-triggered drug release from an O/W emulsion recipe has been reported. The application of emulsions in the pharmaceutical industry is very important, and a large number of references are found in the current literature. 

Chem CS (2008) Principles and applications of emulsion polymerization. Wiley, Chichester... 

Doris, S. S., J. Handgratt, and K.J. Palin. 1985. Medical and pharmaceutical applications of emulsions. In Encyclopedia of Emulsion Technology. Vol. 2, pp. 159-237. New York Marcel Dekker, Inc. 

Fig. 2.5 Application of emulsion to skin and formation of protective membrane...

The wide range of practical and industrial applications of emulsions, foams, and surfactants in particular, has led to the adoption of a wide range of technical terms, some quite specific to particular industries. This chapter provides brief explanations for about 500 significant terms in the science and engineering of industrial emulsions, foams, and suspensions. In addition, cross-references for selected synonyms, abbreviations, and closely-related terms are included. 

Forster, T. von Rybinski, W. Applications of Emulsions in Modem Aspects of Emulsion Science, Binks, B.P. ( Ed.), Royal Society of Chemistry Cambridge, 1998, Chapter 12. 

An emulsion is a dispersed system where one liquid phase is finely subdivided as globules or droplets and uniformly distributed in the other liquid phase. The practical application of emulsions and their technology applies to pharmaceutical and cosmetic formulations. The usual globular or droplet sizes range from 0.1 to 10 pm. 

Other recent applications of emulsion formulation involve mucosal gene and vaccine delivery [185-187] and the preparation of polymeric microspheres by the w/o emulsification solvent extraction technique [188],... 

Obtaining highly stabie emuisions efficientiy entaiis optimizing a number of variabies. Because the applications of emulsions are mainly industrial, processes are first optimized at the laboratory scale and then scaled up for commercial production. The optimization procedure can also be used in analytical applications involving an emulsification operation. 

Lieberman, R., Moghissi, A.A., Low-level counting and liquid scintillation—II Applications of emulsions in tritium counting. Int. J. Appl. Radiat. Isot., 21 (1970) 319-327. 

Kasaini H, Nakashio F, and Goto M. Application of emulsion bquid membranes to recover cobalt ions from a dual-component sulphate solution containing nickel ions. J Membr Sci 1998 146 159-168. 

Cahn RP and Li NN. Commercial applications of emulsion liquid membranes. In Li NN, Calo JM, eds. Separation and Purification Technology, New York Marcel Dekker, 1992 195-212. 

Decker and Flock (74) investigated the application of emulsion injection for steam-flooding processes. In laboratory models, emulsions containing 5 vol% crude oil were effective in blocking channels created by steam injection during subsequent steam-injection cycles. Oil droplets in the emulsion were predominantly in the 1-2- xm range, but droplets as large as 10 fxm were observed. 

Chapters 5 and 6 begin the progression into more practical emulsion considerations by describing the fiow properties of emulsions in pipelines and in porous media. Armed with the necessary tools, the reader is next introduced to some petroleum industry applications of emulsions. Chapters 7 and 8 cover some important areas in which emulsification is a desirable process in some enhanced oil recovery processes and in petroleum transportation via emulsion pipelining. 

Israelachvih, J. The science and applications of emulsions— an overview. Colloids Surf. A Physicochem. Eng. Aspects 1994, 91, 1-8. 

Cahn, R. P., and Li, N. N., "Commercial Applications of Emulsion Liquid Membranes, presented at The Third Chemical Congress of North America, Toronto, Ontario, Canada, June 5-10, 1988. 

PVA is not suitable as the base of packaging materials since its physical and mechanical properties are impaired abruptly above the glass-transition temperature (28°C). It is used, nevertheless, along with polyvinyl butyral (PVB), polyvinyl alcohol (PVAL), alcohol-soluble polyamides and polyacrylates as a layer-carrier of Cl in multilayered films. The layer is formed via application of emulsions, suspensions or solutions of named polymers containing Cl onto the base film [23-26]. 

Chern C-S. Principles and Applications of Emulsion Polymerization. New Jersey John Wiley Sons 2008. 

J>) reported on the application of emulsion liquid membrane technology to the recovery of uranium from wet process phosphoric acid. 

---