Stability of an Emulsion

The stabilization of an emulsion iavolves slowiag the destabilization, primarily the flocculation process. This may be achieved ia two principal manners by reduciag the mobiHty of droplets through enhanced viscosity or by inserting an energy barrier between them (see also Dispersants Flocculating agents). 

The stability of an emulsion mud is an important factor that has to be closely monitored while drilling. Poor stability results in coalescence of the dispersed phase, and the emulsion will separate into two distinct layers. Presence of oil in the emulsion mud filtrate is an indication of emulsion instability. 

The impression that little or nothing is known about emulsion theory and technology, however, is far from the truth. Indeed, a great deal is known— much has been learned from theoretical studies and the Edisonian techniques have made their contributions. In terms of the stability of an emulsion one can predict to a certain extent the effects of radical changes in the types of emulsifier used, changes in pH, addition of certain salts, etc. Qualitatively the effects of changes in viscosity or particle size can also be predicted. 

Because the main point of interest is the stability of an emulsion, a series of phase-type diagrams could be plotted. A hypothetical diagram could have as its ordinate the temperature of mixing and as its abscissa the rate of mixing. A line could be drawn on this diagram indicating that above any given... 

The stability of an emulsion is dependent on the amount of the emulsifying agent at the interface. Briggs Jour. Phys. Ghem.. XIX. 210, 1915) has examined the adsorption of sodium oleate at... 

This is a subjective test to determine the storage stability of an emulsion. A sample of the liquid emulsion before spray-drying is used to fill a 16 oz. tall glass jar. The jar is capped and stored in an oven for 16 hours at 50 C. When storage is complete, the jar is removed from the oven and evaluated. Surface oil layers on the emulsion indicate poor emulsion stability performance by the carrier. 

The stability of an emulsion is dependent on several factors. The size of the dispersed water drops is a measure of (he stability. Figs. 1 and 2. The type and severity of agitation generally determines the drop size. The more shearing action (cutting across sharp edges) applied to the oil-water mixture, the more the water win be divided Into smaller and smaller drops and the more stable the emul-sioa becomes. 

Any conclusion that a low intcrfacial tension per sc is an indication of enhanced emulsion stability is nut reliable. In fact, very low interfacial tensions lead to instability. The stability of an emulsion is inlluenced by the charge at the interface and by the packing of the emulsifier molecules, but the interfacial tension at the levels found in the common emulsion has no influence on stability. 

Before determining the degree of stability of an emulsion and the reason lor this stability, the mechanisms of this destabilization should be considered. When an emulsion starts to separate, an oil layer appears on top. and an aqueous layer appears on the bottom. This separation is the final slate of the destabilization of the emulsion the initial two processes are called flocculation and coalescence. In flocculation, two droplets become attached to each other but are still separated by a thin film of the liquid. When more droplets are added, an aggregate is funned, in which the individual droplets cluster but retain the thin liquid films between them. The emulsifier molecules remain at the surface of the individual droplets during this process. 

Quality attributes of food emulsions, such as appearance, stability, and rheology, are strongly influenced by the size of the droplets that they contain (Friberg and Larsson, 1997 McClements, 1999). For example, the creaming stability of an emulsion decreases as droplet size increases. Analytical techniques that provide quantitative information about droplet size are therefore required to aid in the development and production of high-quality emulsion-based food products. A variety of analytical techniques have been developed to measure droplet size, e.g., laser diffraction, electrical pulse counting, sedimentation techniques, and ultrasonic spectrometry (McClements, 1999). These techniques are used for fundamental research, product development, and quality assurance. This unit focuses on the two most commonly used techniques in the food industry, laser diffraction and electrical pulse counting. 

Once good physical stability of an emulsion is insured, its commercialization mandates chemical stability of the incorporated drug and other essential components for at least 18 months. Key factors that affect the chemical stability of pharmaceutical emulsions include drug stability in oil, drug stability in aqueous media, drug concentration in oil and emulsion, phase volume ratio, droplet size, presence of excipients, and presence of air and/or peroxide radicals. As mentioned earlier, choice of appropriate antioxidant is important. 

Assessment of the stability of an emulsion against coalescence involves droplet counting218. The most unequivocal method (but one which is rather laborious) is to introduce a suitably diluted sample of the emulsion into a haemocytometer cell and count the microscopically visible particles manually. 

As discussed earlier, it is known that the surfactant concentration present during emulsification can affect the particle size of an emulsion. It has also been shown that the stability of an emulsion can be affected in rather unexpected ways by changing the concentration of the surfactant (16). The techniques presented in the last section allow the researcher to follow the full particle-size distribution of the emulsion system rather than just an average diameter. Using several oil/water emulsion systems as examples, we demonstrate the ability of these techniques to determine the effect of emulsifier concentration on the particle-size distribution produced by an inversion method of emulsification. Some of the benefits of obtaining the full distribution will also be discussed. 

Generally speaking, for a stable emulsion a densely packed surfactant film is necessary at the interfaces of the water and the oil phase in order to reduce the interfacial tension to a minimum. To this end, the solubility of the surfactant must not be too high in both phases since, if it is increased, the interfacial activity is reduced and the stability of an emulsion breaks down. This process either can be undesirable or can be used specifically to separate an emulsion. The removal of surfactant from the interface can, for example, be achieved by raising the temperature. By this measure, the water solubility of ionic surfactants is increased, the water solubility of non-ionic emulsifiers is decreased whereas its solubility in oil increases. Thus, the packing density of the interfacial film is changed and this can result in a destabilisation of the emulsion. The same effect can happen in the presence of electrolyte which decreases the water solubility mainly of ionic surfactants due to the compression of the electric double layer the emulsion is salted out. Also, other processes can remove surfactant from the water-oil interface - for instance a precipitation of anionic surfactant by cationic surfactant or condensing counterions. 

The stability of an emulsion denotes its ability to resist changes in its properties over time (i.e., higher emulsion stability implies slower change in emulsion properties). When considering the stability of an emulsion, it is of major importance to distinguish between thermodynamic stability and kinetic stability. Thermodynamics predict whether or not a process will occur, whereas kinetics predict the rate of the process, if it does occur. All food emulsions are thermodynamically unstable and thus will break down if left long enough. 

For nonionic surfactants, an optimization of the process was achieved by using a similar approach to the so-called Cohesive Energy Ratio (CER) concept developed by Beerbower and Hill for the stability of classical emulsions (H). Its basic assumption is that the partial solubility parameters of oil and emulsifier lipophilic tail and of water and hydrophilic head are perfectly matched. Thus, the Vinsor cohesive energy ratio Ro, which determines the nature and the stability of an emulsion, is directly related to the emulsifier HliB (hydrophile-lipophile balance) by... 

The size of the droplets in an emulsion has a strong influence on many of its physicochemical and sensory properties, e.g., shelf life, appearance, texture, and flavor (1,2, 4). For example, the stability of an emulsion to gravitational separation or droplet aggregation can be greatly improved by decreasing the droplet size. This is because the velocity of sedimentation is proportional to the square of the droplet size. The size of the droplets in an emulsion is largely determined by the emulsifier type and concentration, the physicochemical properties of the component phases, and the homogenization conditions (4). A food manufacturer normally specifies a preestablished desirable droplet size distribution for a particular product. If the product does not meet this specification, it typically must be reprocessed or even discarded. 

Demulsification (Emulsion Breaking). The stability of an emulsion is often a problem. Demulsification involves two steps. First, agglomeration or coagulation of droplets must occur. Then, the agglomerated droplets must coalesce. Only after these two steps can complete phase separation occur. Either step can be rate determining for the demulsification process. A typical W/O petroleum emulsion from a production well might contain 60-70% water. Some of this (free water) will readily settle out. The rest (bottom settlings ) requires some kind of specific emulsion treatment. 

The characterization techniques that will be discussed here are used in field situations, on-line, and in the laboratory. In order to characterize an emulsion, it is necessary to determine the amount of each phase present, the nature of the dispersed and continuous phases, and the size distribution of the dispersed phase. The stability of an emulsion is another important property that can be monitored in a variety of ways, but most often, from a processing point of view, stability is measured in terms of the rate of phase separation over time. This phenomenological approach serves well in process situations in which emulsion formation and breaking problems can be very site specific. However, emulsion stability is ultimately related to the detailed chemistry and physics of the emulsion components and their interactions, and these details cannot be completely ignored. 

An emulsion is stable within a given environment. Altering the environment may affect the stability of an emulsion and thus allow separation of the phases. 

Water Percentage. The relative proportion of oil and water affects the stability of an emulsion. In a regular emulsion, the maximum stability of an emulsion will occur at a set ratio of water to oil. Typically this maximum is found at low water percentages as these droplets have a much smaller chance of colliding with other water droplets and coalescing. Increasing the water percentage may destroy the stability of an emulsion. 

One of the greatest concerns for emulsions is the question of their stability. A very typical example of the different requirements on the stability of an emulsion is their application in Liquid-Membrane-Permeation (Figure 1) (1,2). In this process, a water-in-oil emulsion is dispersed by stirring in a bulk water phase containing metal-ions. Under certain conditions these ions will permeate through the oil-phase of the emulsion into the inner water phase of the emulsion. During this time, the emulsion should be very stable but after the permeation, the emulsion is to be separated from the bulk water and has to be broken that mean that at this step the emulsion is required to be unstable. 

Emulsification mainly depends on the water/oil IFT. The lower the IFT, the easier the emulsification occurs. The stability of an emulsion mainly depends on the film of the water/oil interface. The acidic components in the crude oil could reduce IFT to make emulsification occur easily, whereas the asphaltene surfactants adsorb on the interface to make the film stronger so that the stability of emulsion is enhanced. The extracted oil cannot be easily emulsified with alkaline solution because of the high IFT. However, the externally added surfactants can reduce the IFT between the extracted oil and mixed solution to a low value so that the emulsification can occur. 

The interference phase contrast technique combined with high resolution optical sectioning needs to be developed further to measure the thickness of the film surrounding oil droplets in caustic systems. The film thickness and the molecular packing in the film need to be correlated with the stability of an emulsion system. Preliminary results obtained to date are quite encouraging. 

Emulsion polymerization is known to be a method of carrying out polymerization in a disperse system in which water is usually the dispersion medium. In order to ensure the stability of an emulsion containing 30-60% of the monomer, emulsifiers are used. They are compounds of diphilic type surfactants which decrease the surface tension at the hydrocarbon-water interface. This decrease facilitates the emulsification of the monomer in water and favours the stabilization of the emulsion. 

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