Emulsions, formation and stability

Several detailed discussions have described the complex theories of emulsion technology (1, 2, > 1 ) To summarize these theories, emulsifiers are essential for emulsion formation and stabilization to occur these surface-active compounds reduce the surface and interfacial tensions between two immiscible liquids, but this property accounts for only part of the mechanisms at work in emulsification. Three separate mechanisms that appear to be involved in formation of a stable emulsion include ... 

Viscosity is an important physical property of emulsions in terms of emulsion formation and stability (1, 4). Lissant (1 ) has described several stages of geometrical droplet rearrangement and viscosity changes as emulsions form. As the amount of internal phase introduced into an emulsion system increases, the more closely crowded the droplets become. This crowding of droplets reduces their motion and tendency to settle while imparting a "creamed" appearance to the system. The apparent viscosity continues to increase, and non-Newtonian behavior becomes more marked. Emulsions of high internal-phase ratio are actually in a "super-creamed" state. 

Lee CT, Psathas PA, Johnston KP. Water-in-carbon dioxide emulsions formation and stability. Langmuir 1999 15 6781-6791. 

Tadros, T.F. (ed.) (1987) Solid/Liquid Dispersions, Academic Press, London. Tadros, T. (ed.) (2013) Emulsion Formation and Stability, Wiley-VCH Verlag GmbH, Weinheim. 

A higher degree of emulsion stability depends on the structure of the compounds that constitute the protective layer formed on the surface of the droplet. The emulsifier adsorbs on the surface of the droplet and covers it with a preserving layer thereby stabilizing the droplet. This layer prevents droplets from merging with each other (i.e. promotes emulsion formation and stability). 

The crucial aspect of emulsion formation and stability is the attainment of the balance between molecular cooperation and competition among emulsifiers at the... 

Figure 2.4-3 (type A) shows an apparatus for studying emulsion formation and stability at pressures up to 345 bar and temperatures up to 80 °C. The emulsion is formed by introducing a liquid into a C02/surfactant solution with a six-port rotary valve (Valeo) and shearing the solution into small droplets by recirculation through a 100 pm i.d. silica capillary with an HPLC pump. The optical cell for DLS contains three windows at right angles and... 

In its second function, the additive must form some type of film or barrier (monomolecular, electrostatic, steric, or liquid crystalline) at the new L-L interface that will prevent or retard droplet flocculation and coalescence. The process of barrier formation or adsorption must be rapid relative to the rate of drop coalescence or a rather coarse emulsion will result. Also, with the formation of more interface, the adsorption of the emulsifier depletes its bulk concentration, so that attention must be paid to the quantity of the material employed relative to the final result desired, as well as its quality as an emulsifier. As will be seen below, the exact role of an emulsifier in emulsion formation can be quite complex, and is not always completely understood. In any case, its (or, in many cases, their) presence will be vital to successful emulsion formation and stability. 

Even though emulsions as defined have been in use for thousands of years (even longer if natural emulsions are considered), no comprehensive theory of emulsion formation and stabilization has yet been developed that adequately describes, and predicts, the characteristics of many of the complex formulations encountered in practice. Except in very limited and specialized areas, the accurate prediction of such aspects of emulsion technology as droplet size, size distribution, and stability remain more in the realm of art than true science. 

S, F, PIT, etc.) but others will be driven by the three es —economics, environment, and esthetics. The relative importance of the latter factors will depend mostly on price and value-added considerations, legal and functional restrictions, and subjective appeal for each individual system. Here we are concerned with the technical aspects of emulsion formation and stabilization, so other factors will be ignored. 

Clearly, the process of selecting the best surfactant or surfactants for the preparation of an emulsion has been greatly simplified by the development of the more or less empirical but theoretically based approaches exemplified by the HLB, solubility parameter, and PIT methods. Unfortunately, each method has its significant limitations and cannot eliminate the need for some amount of trial-and-error experimentation. As our fundamental understanding of the complex phenomena occurring at oil-water interfaces, and of the effects of additives and environmental factors on those phenomena, improves it may become possible for a single, comprehensive theory of emulsion formation and stabilization to lead to a single, quantitative scheme for the selection of the proper surfactant system. 

While a great deal of information has been pubhshed over the years on the theoretical and practical aspects of emulsion formation and stabilization, until recently little has been said about more complex systems generally referred to as multiple emulsions. Multiple emulsions, as the name implies, are composed of droplets of one liquid dispersed in larger droplets of a second liquid, which is then dispersed in a final continuous phase. Typically, the internal droplet phase will be miscible with or identical to the final continuous phase. Such systems may be w/o/w emulsions as indicated in Figure 11.13, where the internal and external phases are aqueous or o/w/o, which have the reverse composition. Although known for almost a century, such systems have only recently become of practical interest for possible use in cosmetics,... 

Regardless of the final coating layer, the synthesis of such nanoreactor materials requires considerable knowledge about emulsion formation and stability, size clarification, precipitation seeding processes, and understanding of function. The synthesis involves three stages. 

The effect of HLB number on nano-emulsion formation and stability was investigated by using mixtures of Q2EO4 (HLB = 9.7) and Q2EO4 (HLB = 11.7). Two surfactant concentrations (4 and 8 wt%) were used and the O/W ratio was kept at 20/80. Eigure 9.14 shows the variation of droplet radius with HLB number. This figure shows that the droplet radius remain virtually constant in the HLB range... 

P. IzQuiERDO, Thesis Studies on Nabo-Emulsion Formation and Stability, University of Barcelona, Spain, 2002. 

PC forms a lamellar layer in the interface between oil and water, different to the reversed hexagonal phase of PE or the hexagonal phase of lysophospholipids (Figure 10.4) [25]. This knowledge is used for the successful application of lecithins with adapted, modified or fractionated phospholipid composition. The different phase structures at the interface influence the emulsion formation and stability. 

It has been repeatedly pointed out that one of the most significant results of the physical phenomenon of surface activity is the preferential adsorption of amphiphilic molecules at interfaces, resulting in potentially dramatic changes in the characteristics of those interfaces. The ability to reproducibly control such adsorption and interfacial modifications is of immeasurable technological importance, not to mention the fact that our very existence as living organisms would be impossible had such a phenomenon not been a direct consequence of natural laws as we understand them. This chapter is concerned with one of the most important overall areas of impact of surfactants on our technological existence emulsion formation and stabilization. 

Modern attempts to formulate a quantitative theory of emulsions and emulsion stability have looked most closely at the nature of the interfacial region separating the two immiscible phases, especially the chemical and physical nature of the adsorbed film, the role of mixed films and complex formation, interfacial rheology, and steric and electronic factors at the interface. The theoretical foundations for current ideas concerning emulsion formation and stability are presented in several of the references cited in the Bibliography. A few of the most basic ideas, however, are presented below. 

Several references were made above to the term phase inversion temperature. With the exceptions of Eqs. (9.17) and (9.18), however, no specific reference was made to the effect of temperature on the HLB of a surfactant. From the discussions in Chapter 4, it is clear that temperature can play a role in determining the surface activity of a surfactant, especially nonionic amphiphiles in which hydration is the principal mechanism of solubilization. The importance of temperature effects on surfactant solution properties, especially the solubility or cloud point of nonionic surfactants, led to the evolution of the concept of using that property as a tool for predicting the activity of such materials in emulsions. Since the cloud point is defined as the temperature, or temperature range, at which a given amphiphile loses sufficient solubility in water to produce a normal surfactant solution, it was assumed that such a temperature-driven transition would also be reflected in the role of the surfactant in emulsion formation and stabilization. 

In this study, the phase behavior of microemulsions consisting of alkyl polyglycosides and ethoxylates as hydrophilic emulsifiers, a lipophilic coemulsifier, an oily component, and water is evaluated in terms of micro-emulsion formation and stability. Parameters such as temperature, oil polarity, and composition of the surfactant mixture are discussed. It was shown that both the concentration range and the temperature stability could be extended by using suitable mixtures of emulsifiers and coemulsifiers. 

In this chapter, the characteristic properties of nano-emulsions and relevant applications have been described. A great deal of research effort in recent years has been focused toward the conditions required for nano-emulsion formation. Low interfacial tension values and the presence of lamellar liquid crystalline phases are among the factors that have been shown to be important for their formation. However, it has also been shown that the kinetics of the emulsification process plays a key role. Comprehensive knowledge of the fundamental aspects related to nano-emulsion formation and stability will allow improvement of established applications, such as those described in this chapter, and development of new ones. 

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