Factors Determining Emulsion Stabilization

If two pure, immiscible liquids, such as benzene and water, are vigorously shaken together, they will form a dispersion, but it is doubtful that one phase or the other will be uniquely continuous or dispersed. On stopping the agitation, phase separation occurs so quickly that it is questionable whether the term emulsion really should be applied to the system. A surfactant component is generally needed to obtain a stable or reasonably stable emulsion. Thus, if a little soap is added to the benzene-water system, the result on shaking is a true emulsion that separates out only very slowly. Theories of 

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Film stability and interfacial forces are important in determining emulsion stability and the likelihood of creaming or complete separation of the phases. Characterization of these interfacial effects is an important factor in determining the fundamental properties that might ultimately determine coalescence kinetics. Some relevant papers and reviews have been published elsewhere (54, 89-96). 

On the other hand, there are two significant differences between macroemulsions and foams (1) The surfactants in the inter-facial film of a foam cannot dissolve in the dispersed (gas) phase, while in a macroemulsion the solubility of the surfactants in the liquid being dispersed is a major factor determining the stability of the emulsion. (2) In macroemulsions, both oil and water can serve as the continuous phase, i.e., both O/W and W/O emulsions are commonly encountered, while in foams, only the liquid acts as the continuous phase. 

Physical Nature of the Interfacial Film The droplets of dispersed liquid in an emulsion are in constant motion, and therefore there are frequent collisions between them. If, on collision, the interfacial film surrounding the two colliding droplets in a macroemulsion ruptures, the two droplets will coalesce to form a larger one, since this results in a decrease in the free energy of the system. If this process continues, the dispersed phase will separate from the emulsion, and it will break. The mechanical strength of the interfacial film is therefore one of the prime factors determining macroemulsion stability. 

Stability of a macroemulsion is an important factor as this determines its extent of usability for particle preparation or various other applications. Instability is basically coalescence of the dispersed phase droplets or Ostwald ripening (growth of large droplets at the expense of much smaller ones). When this process goes on, the emulsion eventually breaks into two layers. Other processes related to stability but considered less important [3] are (a) creaming or sedimentation, the rate of which is dependent on the difference in density between the continuous and dispersed phases, droplet size, viscosity of the continuous phase and interdroplet interaction and (b) flocculation, dependent on colloidal interactions between the droplets [8, 12]. Several factors determine the stability of macroemulsions these are discussed here in brief. This discussion is largely derived from Rosen [3] and some subsequent investigations [e.g. 6, 7, 13-15]. 

An Empirical Approach to Demulsifier Selection. Research into emulsion fundamentals added greatly to our understanding of the factors that determine emulsion stability and the surface-active chemicals that can be used to manipulate those factors. In spite of these advances, the requirement for blending demulsifiers in order to achieve acceptable field performance means that empirical approaches are often required for demulsifier selection. In fact, complete characterization of emulsion properties, including process residence times, temperatures, and product requirements still only provides guidance in the selection of process demulsifiers. The costs and time involved in achieving the level of characterization required for a fundamental approach can also be... 

The above factors determine the "stability" of emulsions. Some stable emulsions may take weeks or months to separate if left alone in a tank with no treating. Other unstable emulsions may separate into relatively clean oil and water phases in just a matter of minutes. 

Rheological Property Determination. The rheology of an emulsion is often an important factor in determining its stability. Any variation in droplet size distribution, degree of flocculation, or phase separation frequently results in viscosity changes. Since most emulsions are non-Newtonian, the cone-plate type device should be used to determine their viscosity rather than the capillary viscometer. 

The immense interfacial area separating dispersed globules from the dispersion phase is of critical Importance in determining their stability. For example, it is estimated that a typical emulsion has approximately 7 X 10 cra interfacial area per liter (3 ). Thus, those factors controlling the properties of the interfacial membrane are extremely Important in determining the stability of the emulsion. 

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. 

The efficiency of any water-removal steps depends upon the size distribution of the dispersed water and the stability of the emulsion. Emulsion formation may be exacerbated by inappropriate pumping speed or other process variables. An evaluation of the chemical and physical factors that determine emulsion size distribution or emulsion bulk properties is essential to optimize emulsion breaking efficiency. 

Emulsion Pipeline Operations. Prediction of pipeline pressure gradients is required for operation of any pipeline system. Pressure gradients for a transport emulsion flowing in commercial-size pipelines may be estimated via standard techniques because chemically stabilized emulsions exhibit rheological behavior that is nearly Newtonian. The emulsion viscosity must be known to implement these methods. The best way to determine emulsion viscosity for an application is to prepare an emulsion batch conforming to planned specifications and directly measure the pipe viscosity in a pipe loop of at least 1-in. inside diameter. Care must be taken to use the same brine composition, surfactant concentration, droplet size distribution, brine-crude-oil ratio, and temperature as are expected in the field application. In practice, a pilot-plant run may not be feasible, or there may be some disparity between pipe-loop test conditions and anticipated commercial pipeline conditions. In these cases, adjustments may be applied to the best available viscosity data using adjustment factors described later to compensate for disparities in operating parameters between the measurement conditions and the pipeline conditions. 

An important conclusion of the above approximate Analysis is that the effective kinematic viscosity of the emulsion is the dominant factor determining both the initial growth of instabilities and the most sensitive wave number. Thus, prediction of the effect of system properties on bubble stability depends on prediction of the effect on the effective kinematic... 

The stability factor, W, is determined to give clear insight into the emulsion stability with the help of the equation used elsewhere (17) ... 

Emulsions are characterized in terms of dispersed / continuous phase, phase volume ratio, droplet size distribution, viscosity, and stability. The dispersed phase is present in the form of microscopic droplets which are surrounded by the continuous phase both water-in-oil (w/o) and oil-inwater (o/w) emulsions can be formed. The typical size range for dispersed droplets which are classified as emulsions is from 0.25 to 25 p (6). Particles larger than 25 p indicate incomplete emulsification and/or impending breakage of the emulsion. Phase volume ratio is the volume fraction of the emulsion occupied by the internal (dispersed) phase, expressed as a percent or decimal number. Emulsion viscosity is determined by the viscosity of the continuous phase (solvent and surfactants), the phase volume ratio, and the particle size (6). Stroeve and Varanasi (7) have shown that emulsion viscosity is a critical factor in LM stability. Stability of... 

The determination of the molecular orientation and the thickness of this surfactant film is very important in understanding the basic factors which affect emulsion stability (32). 

To determine the influence of the ingredients of a recipe on the stability of an emulsion of a cosmetic cream, an experimental plan is proposed with three factors (two levels) the nature of the cream (oil in water for a positive effect and water in oil for a negative one), an emulsifier (dilute or very dilute), and fatty acid concentration (high or low). The indices of the emulsion stabilities obtained are 38, 37, 26, 24, 30, 28, 19, and 16, with an experimental error of 2. Lamine... 

The first term on the right-hand side of equation (8.7) is the contribution of the head group repulsion, while the second is the interfacial energy contribution where Ahg is the total surface area of the head groups and (Tmic is the interfacial tension. Within the framework of the Gouy-Chapmann theory, the dressed micelle model allows the estimation of values, which are for sodium dodecyl sulfate (SDS), sodium octyl sulfate, and teradecyltrimethylammonium bromide, 15-16, 11 and 11-14 mN m , respectively (15). Note that these values are up to a factor of 3 lower than those of the pure monomers (cf. Table 8.2). A further decrease of or is possible in the case of emulsions of organic liquids where the interface is saturated with stabilizer. For example, a value of about 4 mN m was determined for a toluene emulsion stabilized with potassium lau-rate (16). 

Encounters between particles in dispersion can occur frequently due to any of the Brownian motion, sedimentation, or stirring. The stability of the dispersion depends upon how the particles interact when this happens. Table 4.2 lists some factors involved in determining the stability of emulsions. The main causes of repulsive or barrier forces may be electrostatic, steric, or mechanical. The main attractive forces are the van der Waals forces between objects. 

Some Factors Involved in Determining the Stability of Emulsions... 

The processes of flocculation and coalescence in the context of emulsion stability will be treated in a bit more detail below. At this point it is useful to point out their role in the determination of the final nature of the emulsion. The process leading to emulsion formation usuahy begins with the production of preliminary large drops, probably of both hquid phases. The continuous phase-to-be will be determined by many factors, to be outlined below. In any case, droplets of that phase must disappear rapidly during the process through flocculation and coalescence. The ultimate dispersed phase, on the other hand, must maintain (or reduce) its droplet size during and after processing. 

It is of interest to try to relate the adsorption characteristics of a surfactant to the stability of an emulsion stabilized solely by an adsorbed monomolecular film. The toM number of molecules that can be adsorbed in a given interfacial area wiUbe controlledmainly by the effective area per molecule of the adsorbing species. That is, how many of the molecules can fit into the limited space of the interface For most normal surfactant species, the area per molecule is determined primarily by the hydrophilic group and its hydration layer. The relative solubility of the surfactant in the two phases will also affect the result, but that factor is difficult to determine and is most often ignored. A few representative molecular areas at the oil-water interface are given in Table 11.2. 

Casein has been found to be an excellent candidate to produce oil-in-water emulsions that have both high physical and oxidative stability. The differences in the physical properties and oxidative stability of com oil-in-water emulsions stabilized by casein, WPI, or SPI at pH 3.0, have been investigated. Emulsions have been prepared with 5 per cent com oil and 0.2-1.5 per cent protein. Physically stable, monomodal emulsions have been prepared with 1.5 per cent casein, 1.0 or 1.5 per cent SPI, and 2=0.5 per cent WPI. The oxidation stability of the different protein-stabilized emulsions was in the order of casein > WPI > SPI, as determined by monitoring both lipid hydroperoxide and headspace hexanal formation. The degree of positive charge on the protein-stabilized emulsion droplets was not the only factor involved in the inhibition of lipid oxidation, because the charge of the emulsion droplets... 

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