Coalescence of an emulsion

Obviously the lowest free energy is given by the most stable state for a system at constant pressure and, therefore, in due course an emulsion shall break spontaneously to the two-layered system. However, the breaking of an emulsion could be relatively a rather slow phenomenon. There are a number of factors which may be responsible for the slow-coalescence of an emulsion, namely ... 

Altering the Ratio of Solvents Coalescence of an emulsion may also be achieved either by altering the ratio of the prevailing dispersed phase or even by partial evaporation of the solvent,... 

In the absence of any flocculation, the coalescence of an emulsion results in a reduction of its viscosity. At any given volume fraction of oil, an increase in droplet size will result in a reduction of viscosity, and this is particularly the case for concentrated emulsions. Thus, by following the decrease in emulsion viscosity with time, information may be obtained on its coalescence. However, care should be exercised when applying simple viscosity measurements, particularly if flocculation occurs simultaneously (as this results in an increased viscosity). It is possible - at least in principle - to predict the extent of viscosity reduction on storage by combining the results of droplet size analysis (or droplet number) as a function of time with the reduction in viscosity during the first few weeks. 

In principle, it should then be possible to predict the stability of an emulsion system from the interfacial rheology of the continuous phase. Figure 1.5 shows the relative stability to coalescence of an emulsion system stabilized by a protein (beta-lactoglobulin) with increased concentrations of non-ionic surfactant (Tween 20). In this case the presence of surfactants has entirely destabilized the protein emulsion. 

Before determining the degree of stabiUty of an emulsion and the reason for this stabiUty, the mechanisms of its 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 state of the destabilization of the emulsion the initial two processes are called flocculation and coalescence (Fig. 5). In flocculation, two droplets become attached to each other but are stiU separated by a thin film of the Hquid. When more droplets are added, an aggregate is formed, ia which the iadividual droplets cluster but retain the thin Hquid films between them, as ia Figure 5a. The emulsifier molecules remain at the surface of the iadividual droplets duiing this process, as iadicated ia Figure 6. 

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. 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

The viscosity of an emulsion can be of crucial importance for its stability, especially the viscosity of the external phase. A high viscosity reduces creaming and also lessens the tendency of particles to coalescence and produce phase separation. Examples of the widely used viscosity-imparting agents are alginates, bentonite, carboxymethylcellulose, polyvinyl pyrrolidone, hydroxypropylcellulose, and carbomer. 

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

Breaking of an Emulsion (LeCoalescence) Following are the various techniques invariably used so as to break an emulsion or to achieve coalescence, namely ... 

Sudden Cooling of Emulsion (Thermal Shock) Sudden temperature drop or freezing i.e., giving a thermal shock) of an emulsion mostly enhances the interfacial tension between the two immiscible phases thereby causing coalescence. 

Another characteristic property of many biopolymers (proteins, modified starch, chitosan, etc.) which is useful for the encapsulation of bioactive molecules is their ability to adsorb at the oil-water interface and to form adsorbed layers that are capable of stabilizing oil-in-water (OAV) emulsions against coalescence (see Table 2.2). It is worthwhile to note here that the formation of an emulsion is one of the key steps in the encapsulation of hydrophobic nutraceuticals by the most common technique used nowadays in the food industry (spray-drying). The adsorption of amphiphilic biopolymers at the oil-water interface involves the attachment of their hydrophobic groups to the surface of the oil phase (or even their slight penetration into it), whilst their hydrophilic parts protrude into the aqueous phase providing a bulky interfacial layer. 

The use of an emulsion of PDMS particles is attractive because its low viscosity makes it suitable for preimpregnation of fibers to produce composite materials. A possible drawback is the emulsion stability which has to be controlled in order to avoid particle coalescence. 

Probably the most important physical property of an emulsion is its stability. The term emulsion stability can be used with reference to three essentially different phenomena - creaming (or sedimentation), coagulation and a breaking of the emulsion due to droplet coalescence. 

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. 

Figure 5.16 Illustration of creaming, aggregation, and coalescence in an emulsion, foam or suspension.

Flocculation versus Coalescence. The breaking of an emulsion is a two step process requiring the coalescence of the droplets after they are in contact.(17) If the system flocculates but is resistant to coalescence, the system will not phase separate. Over a period... 

Flocculation. Flocculation means an aggregation of emulsion droplets but, in contrast to coalescence, the films of the continuous phase between the droplets survive. Hence, the process may be partially reversible. Both processes, flocculation and coalescence, speed up the creaming of an emulsion due to the increase of the drop size. The process of flocculation is even more important for dispersions of solids than for emulsions because in this case a coalescence is not possible. 

The coalescence or complete separation of an emulsion occurs when two particles approach each other and no barrier exists between them. This process is avoided by producing a strong condensed mixed monolayer film or a multilayer coating around the droplets. A stable W/O emulsion is produced from surfactants or a mixture of surfactants that have very long hydrocarbon chains. 

When two or more emulsion droplets come into contact, a thin film of the liquid continuous phase forms between them. Coalescence is the process whereby liquid droplets merge to form a single larger droplet as a result of the rupture of both this liquid film and the interfacial membrane of the droplets. Coalescence moves an emulsion towards a thermodynamically stable state, because it reduces the contact area between the phases (Tadros and Vincent, 1983 Walstra, 1996 McClements, 1999). 

Liquid emulsions are inherently unstable to a varying degree. It is important to understand, therefore, the mechanisms that contribute to emulsion stability. Before the solidification step, instability of an emulsion can arise due to either phase separation or phase inversion (Mulder and Walstra, 1974). It is evident that the likelihood of phase inversion will increase as the fraction of dispersed phase increases. The vast majority of literature references are concerned with the stability to phase separation as coalescence or creaming in oil-in-water emulsions (Hailing, 1981 Jaynes, 1983). In addition, a method for determining the stability of water-in-oil emulsions to inversion has not been reported. It is usually assumed that certain aspects of oil-in-water emulsion theory apply in reverse to water-in-oil emulsions. 

One of the most important properties of an emulsion is its stability. Emulsions normally break by one of three different processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation have their origin in density differences between the two phases, and emulsions often break by a mixture of the three main processes. The time it takes for an emulsion to break can vary from seconds to years. 

A variety of interaction behaviours can be observed between liquid/liquid interfaces based on the types of colloidal forces present. In general, they can be separated into static and dynamic forces. Static forces include electrostatic, steric, van der Waals and hydrophobic forces, relevant to stable shelf life and coalescence of emulsions or dispersions. Dynamic forces arise ftom flow in the system, for instance during shear of an emulsion or dispersion. EHrect force measurements tend to center on static force measurements, and while there is a large body of work on the study of film drainage between both liquid or solid interfaces, there are very few direct force measurements in the dynamic range between liquid interfaces. Below are general descriptions of some of the types of force observed and brief discussions of their origins. 

Experiments have shown that the smallest droplet size that can be achieved using a high-pressure valve homogenizer increases as the disperse phase volume fraction increases (52). There are a number of possible reasons for this, (1) increasing the viscosity of an emulsion may suppress the formation of eddies responsible for breaking up droplets, (2) if the emulsifier concentration is kept constant, there may be insufficient emulsifier molecules present to completely cover the droplets, and (3) the rate of droplet coalescence is increased. 

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