Theories of Emulsion Stabilization

It is quite clear, first of all, that since emulsions present a large interfacial area, any reduction in interfacial tension must reduce the driving force toward coalescence and should promote stability. We have here, then, a simple thermodynamic basis for the role of emulsifying agents. Harkins [17] mentions, as an example, the case of the system paraffin oil-water. With pure liquids, the inter-facial tension was 41 dyn/cm, and this was reduced to 31 dyn/cm on making the aqueous phase 0.00 IM in oleic acid, under which conditions a reasonably stable emulsion could be formed. On neutralization by 0.001 M sodium hydroxide, the interfacial tension fell to 7.2 dyn/cm, and if also made O.OOIM in sodium chloride, it became less than 0.01 dyn/cm. With olive oil in place of the paraffin oil, the final interfacial tension was 0.002 dyn/cm. These last systems emulsified spontaneously—that is, on combining the oil and water phases, no agitation was needed for emulsification to occur. 

One may rationalize emulsion type in terms of interfacial tensions. Bancroft [20] and later Clowes [21] proposed that the interfacial film of emulsion-stabilizing surfactant be regarded as duplex in nature, so that an inner and an outer interfacial tension could be discussed. On this basis, the type of emulsion formed (W/O vs. O/W) should be such that the inner surface is the one of higher surface tension. Thus sodium and other alkali metal soaps tend to stabilize O/W emulsions, and the explanation would be that, being more water- than oil-soluble, the film-water interfacial tension should be lower than the film-oil one. Conversely, with the relatively more oil-soluble metal soaps, the reverse should be true, and they should stabilize W/O emulsions, as in fact they do. An alternative statement, known as Bancroft s rule, is that the external phase will be that in which the emulsifying agent is the more soluble [20]. A related approach is discussed in Section XIV-5. 

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Schulman and Cockbain (see Appendix) have recently put forward a new theory of emulsion stability and inversion. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

The question of emulsion stability has already been raised in the context of emulsion preparation. However, the preparation process is very dynamic and represents a complicated combination of events that is not easily analyzed. Once prepared, however, and left at rest to do its own thing, the fates of individual droplets become more readily determined and some semblance of understanding can be extracted from the initial chaos. That is not to say, however, that there exists a good general theory of emulsion stability that one can apply under all, or even most, circumstances. 

Theory of emulsion stability Emulsions can show instability by creaming, by inversion and by breaking (demulsification). 

Eccleston and Florence [3] discussed the theories of emulsion stability. The concepts of HLB, flocculation, coagulation, interface, and stability are commented on from a thermodynamic point of view. 

On this basis, one question is unanswered. How much can the lifetime be modified by the presence, number and efficiency of these impurities, when compared to the mean field prediction Answering this question will allow us to model heterogeneous growths (bimodal or demixtion) reliably and very much extend the knowledge of emulsion stability since these types of destruction are the most frequently encountered. However, we believe the mean field description which is considered here, and its ability to descibe homogeneous growths, gives a firm basis to the theory of emulsion stability. 

Early practical theories of emulsion stability recognized the importance of additives such as surfactants, polymers, and particulates to the processes of emulsion preparation, the type of emulsion produced, and the overall stability of the final system. However, a reasonably sound theoretical picture began to evolve only once an understanding of the concepts and principles of interfaces and monolayers began to become clear. Studies of oriented amphiphilic monolayers at interfaces led to the conclusion that such structures, in which each portion of the adsorbed molecules showed a strong preference for association with one of the two liquid phases, offered the best explanation for observed experimental results. As a result, it became possible to schematically represent the emulsion droplet as shown in Figure 9.2d. 

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

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 ... 

Detailed description of theory, measurements, and applications in emulsion science covering all aspects of emulsion stability. 

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

Muller et. al. [421] have studied the behaviour of emulsion Newton bilayer films and compared it to that of foam films. They determined the dependence of the lifetime on surfactant concentration of emulsion films stabilised with 22-oxythylated dodecyl alcohol (see Section 3.4.1). Experimental data for both kinds of films proved to be in conformity with the theory of bilayer stability (see Section 3.4). The values of the equilibrium concentrations Ce calculated for emulsion films were higher (Ce 10 3 mol dm 3) than those for foam films (Ce 3 1 O 5 mol dm 3). It is worth noting that Ce value of foam films from certain surfactants is lower than CMC (C < CMC) while for emulsion films - Ce > CMC. That is why it is impossible to obtain thermodynamically stable films in the latter case. This result is of particular importance for the estimation of stability of aqueous emulsions with bilayer films between the drops of the organic liquid. 

Very often, the microstructure and the macroscopic states of dispersions are determined by kinetic and thermodynamic considerations. While thermodynamics dictates what the equilibrium state will be, kinetics determine how fast that equilibrium state will be determined. While in thermodynamics the initial and final states must be determined, in kinetics the path and any energy barriers are important. The electrostatic and the electrical double-layer (the two charged portions of an inter cial region) play important roles in food emulsion stability. The Derjaguin-Landau-Verwey-Oveibeek (DLVO) theory of colloidal stability has been used to examine the factors affecting colloidal stability. 

All of the petroleum emulsion applications or problems just discussed have in common the same basic principles of colloid science that govern the nature, stability, and properties of emulsions. The widespread importance of emulsions in general and scientific interest in their formation, stability, and properties have precipitated a wealth of published literature on the subject. This chapter provides an introduction and is intended to complement the other chapters in this book on petroleum emulsions. A good starting point for further basic information is one of the classic texts Becher s Emulsions Theory and Practice (4) or Sumner s Claytons Theory of Emulsions and... 

Interrelationship of Emulsion Stability and Interfacial Viscosity in Improved Oil Recovery," paper presented at the Engineering Foundation Conference on Theory, Practice... 

Correspondingly, EDM may be accomphshed by combining experiment and theory (1) determination of coalescence and fragmentation kernels with die use of emulsion stability experiments at low-density contrast (l.d.c.) and s.d.e., because this permits the omittance of creaming and... 

We end this section by summarizing the areas where we feel that the NMR diffusion method will prove important in future studies of emulsions and refer to a more detailed account presented in Chapter 10 of this book. As theories describing emulsion stability become more refined, there will be a need for data on droplet size distribution and also on total emulsion droplet area and how these quantities evolve with time. As outlined above, NMR is eapable of providing sueh data. Another important question pertains to the mi-crostrueture of the continuous phase, which can be studied both in the emulsion phase and also in the phase-separated systems which yield the emulsion. Finally, we note that one important class of emulsions, namely, multiple emulsions, is practically virgin territory with regard to NMR studies. In the characterization and understanding of important features of these systems NMR will most likely play an important role. 

Casein or egg-yolk proteins are used as emulsifiers in another category of O/W food emulsions [34,126]. A key difference here is that in these caseinate-stabilized oil emulsions, the casein forms essentially monolayers and there are no casein micelles or any calcium phosphate. Such emulsions are thought to be stabilized more by electrostatic repulsive forces and less by steric stabilization [126]. Similarly, mayonnaise, hollandaise, and beamaise sauces, for example, are O/W emulsions mainly stabilized by egg-yolk protein [34,129], Here, the protein-covered oil (fat) droplets are stabilized by a combination of electrostatic and steric stabilization [129]. Perram et al. [130] described the application of DLVO theory to emulsion stability in sauce beamaise. 

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. 

In the context of emulsion stabilization, it is important that a surfactant molecule have a strong interaction with both the water and oil phases. If water interactions are too dominant (i.e., c(ab) is too large), the molecule will tend to be too soluble in that phase and will lose effectiveness at the oil-water interface. If c(ab) is too small, the opposite effect will result (Fig. 11.10). The goal, then is to balance the cohesive interaction of the surfactant tail with that of the oil phase and that of the head with the aqueous phase. That end can (in theory) be accomphshed in one of two ways (1) by adjusting the structure of the surfactant or (2) by adjusting the composition (e.g., the polarity) of one or both phases. In many critical apphcations (e.g., pharmaceuticals, cosmetics, foods) the choice of surfactant is limited by law and/or activity, so that it may be more feasible to adjust the characteristics of one or both liquid phases. 

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. 

Theory of steric stabilization of emulsions the role of the relative ratio of adsorbed layer thickness to the droplet radius. 

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