Emulsion thermodynamically unstable

Emulsions are thermodynamically unstable systems. However, using appropriate emulsifying agents... 

As mentioned earlier, ordinary emulsions as prepared by mixing oil, water, and emulsifier are thermodynamically unstable. That is, such an emulsion may be stable over a length of time, but it will finally separate into two phases (the oil phase and the aqueous phase). They can also be separated by centrifugation. These emulsions are opaque, which means that the dispersed phase (oil or water) is present in the form of large droplets (more than a micrometer and thus visible to the naked eye). 

Another associated issue was the possibility of inactivating the LRES (lym-phoreticuloendothelial system). By analogy with other injectable systems, it could also be deduced that the injectable emulsion system needed to be sterile and apy-rogenic and free of acute or chronic toxicities from components or their associated degradation products. It also followed that the injectable system required to be stable, although how stability was to be determined and, more to the point, measured, has remained an issue to the present day. This is mainly because emulsions are thermodynamically unstable although their stability can be extended by formulation. As a result emulsion products are now available that are submicron in diameter, sterile, and stable for several years after preparation. In major part this has been due to the use of phospholipids as stabilizers and emulsifiers, in particular the mixed products identified as the lecithin of commerce. 

Emulsions are two-phase systems and —because of the free energy associated with the oil-water interface —are thermodynamically unstable with respect to separation into oil and water layers. 

Unlike micelles, an emulsion is a liquid system in which one liquid is dispersed in a second, immiscible liquid, usually in droplets, with emulsiLers added to stabilize the dispersed system. Conventional emulsions possess droplet diameters of more than 200 nm, and are therefore optically opaque or milky. Conventional emulsions are thermodynamically unstable, tending to reduce their total free energy by reducing the total area of the two-phase interface. In contrast, microemulsions with droplet diameters less than 100 nm are optically clear and thermodynamically stable. Unlike conventional emulsions that require the input of a substantial amount of energy, microemulsions are easy to prepare and form spontaneously on mixing, with little or no mechanical energy applied (Lawrence and Rees, 2000). 

An emulsion can be deLned as a mixture of two immiscible phases (namely, water and oil) with an emulsiLer added to stabilize the dispersed droplets (Davis et al., 1987). As conventionally deLned, emulsions will have droplet diameters of more than 100 nm (up tprfijft and thus are opaque or milky in appearance. In addition, they are thermodynamically unstable by nature, that is, on standing they will eventually separate into two phases. However, proper choice of emulsiLer (generally 1-5%) and preparation conditions can delay this separation and thus lead to nominal shelf lives of more than 2 years, as typically required for pharmaceutical products. An emulsion can be characterized as oil-in-water (o/w) (containing up to 40% oil) or water-in-oil (w/o), depending on the identity of the dispersed and continuous phases. Multiple (e.g., w/o/w) emulsions can also be prepared, but these are less widely used in pharmaceutical applications. 

Concerning thermodynamically unstable emulsions, the creation of new interfaces from the disruption of the disperse phase increases the free energy of the system, which tends to return to the original two separate systems. Therefore, the use of emulsifier is necessary not only to reduce the interracial tension, but also to avoid the coalescence and the formation of macroaggregates thanks to electrostatic repulsion between adsorbed emulsifier. 

Apart from microemulsions, all types of emulsions are thermodynamically unstable and their stability is solely a kinetic issue. The relevant timescale can vary between seconds and years. The following mechanisms are responsible for the breakdown of an emulsion. They are depicted schematically in Figure 3.27. 

Upon mixing two immiscible liquids, one of the two liquids (i.e., the dispersed phase) is subdivided into smaller droplets. The surface area and the interfacial free energy increase, and the system is then thermodynamically unstable. Without continuous mixing, the droplets will be stabilized throughout the dispersion medium by dissolving the surface-active agent. There are several theories for the stabilization of emulsions but a single theory cannot account for the stabilization of all emulsions. 

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. 

An emulsion is a thermodynamically unstable system it has a tendency to separate into two phases. 

The structural and rheological properties of emulsions, blends, and foams are of great importance in the food, cosmetics, oil-field, and packaging industries. By definition, such fluids are thermodynamically unstable or at best metastable. Hence, conditions of preparation are of extreme importance to both the scientific study and the engineering of these fluids. 

In summary, lyophobic emulsions are thermodynamically unstable but may be relatively stable in a kinetic sense. Stability must be understood in terms of a clearly defined process. 

It is because of the subdivision of matter in colloidal systems that they have special properties. The large surface-to-volume ratio of the particles dispersed in a liquid medium results in a tendency for particles to associate to reduce their surface area, so reducing their contact with the medium. Emulsions and aerosols are thermodynamically unstable two-phase systems which only reach equilibrium when the globules have coalesced to form one macro-phase, for which the surface area is at a minimum. Many pharmaceutical problems revolve around the stabilisation of colloidal systems. 

This book focuses on chemical EOR processes, including alkaline (A), surfactant (S), polymer (P), and any combination of these processes. We discuss emulsion whenever it relates to any chemical processes. In addition, we briefly describe foam when presenting an application of ASP with foam. Emulsion and foam are more related to mobility control. These two processes are not discussed in detail because they are thermodynamically unstable processes quite different from the stable processes we deal with here. Rather, we discuss the general mobility control requirement in EOR processes in Chapter 4. 

In most cases AA/i2 T AS °, which means that AG °" is positive that is, the formation of emulsions is nonspontaneous and the system is thermodynamically unstable. In the absence of any stabilisation mechanism, the emulsion will break by flocculation, coalescence, Ostwald ripening, or a combination of all these processes. This is illustrated in Figure 10.4, which shows several pathways for emulsion breakdown processes. 

The common nonionic surfactants are often soluble in both water and oil phases. In the practice of emulsion preparation, the surfactant (the emulsifier) is initially dissolved in one of the liquid phases and then the emulsion is prepared by homogenization. In such a case, the initial distribution of the surfactant between the two phases of the emulsion is not in equilibrium therefore, surfactant diffusion fluxes appear across the surfaces of the emulsion droplets. The process of surfactant redistribution usually lasts from many hours to several days, until finally equilibrium distribution is established. The diffusion fluxes across the interfaces, directed either from the continuous phase toward the droplets or the reverse, are found to stabilize both thin films and emulsions. In particular, even films, which are thermodynamically unstable, may exist several days because of the diffusion surfactant transfer however, they rupture immediately after the diffusive equilibrium has been established. Experimentally, this effect manifests itself in phenomena called cyclic dimpling and osmotic swelling. These two phenomena, as well as the equilibration of two phases across a film,568.569 3j.g described and interpreted below. 

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