Physical instability of 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. 

Droplet aggregation is said to occur when droplets stay together for a time much longer than they would in the absence of colloidal interactions, (i.e., than can be accounted for by collisions due to Brownian motion) (Walstra, 2003). Mechanisms responsible for the physical instability of droplets through aggregation are flocculation, coalescence or partial coalescence. 

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Four major phenomena are associated with the-physical instability of emulsions flocculation, creaming, coalescence, and breaking (Fig. 8) [144]. 

Instability of an emulsion may be physical or chemical in nature. Chemical instability, which results in an alteration in the chemical structure of the lipid molecules due to oxidation or hydrolysis (McClements, 1999), will not be considered in this chapter for more information, the reader is referred to Chapters 11 and 12. Physical instability results in an alteration in the spatial distribution or structural organization of the globules (i.e., the dispersed phase of the emulsion). A number of important mechanisms responsible for the physical instability of emulsions, as depicted in Figure 5.1, can be divided into two categories gravitational separation and droplet aggregation. 

The physical instability of emulsions involves creaming, flocculation, coalescence, or breaking, whereas the chemical instability can be a result of hydrolysis of the stabilizing moieties. In order to assess the stability of the emulsion, heating and freezing cycles and centrifugation and steam sterilization can be employed. 

The term emulsion stability is broadly used to describe the ability of an emulsion to resist changes in its properties with time. The properties of an emulsion may evolve over time due to a variety of physical, chemical, or biochemical processes. From a technological standpoint, it is important to identify the dominant processes occurring in the system of interest because effective strategies can then be rationally designed to overcome the problem. A number of the most important physical mechanisms responsible for the instability of emulsions are shown schematically in Figure 5. 

Multiple emulsions are unique in that a true liquid phase is maintained separate from an external aqueous phase. This may be especially important for bioactive molecules that cannot be appropriately stabilized in the solid state. In addition, the separation of aqueous phases enables highly specialized environments, conducive to protein activity, to be prepared. The physical instability of conventional systems remains a major factor limiting their wider application. Attempts to improve the physical stability of the aqueous dispersions through interfacial complexation and the use of microemulsions are improving the short-term stability. As an alternative approach, solid-state emulsions attempt to store the multiple emulsion as a solid. Although solid-state emulsions appear to have the potential to be useful protein delivery systems, a substantial experimental data base has yet to be generated. 

The following sections discuss the various instability mechanisms that result in the breakdown of emulsions. Because most of these instability mechanisms are driven by droplet-droplet interactions that occur on the colloidal level, the physical bases of colloidal interactions should be understood as well. Such a detailed discussion is, however, beyond the scope of this unit and interested readers are... 

Physical stability. As indicated earlier, conventional emulsions are inherently unstable from a physical standpoint. Poor physical stability is ultimately exhibited by phase separation, which can be visually monitored. Certain properties of the emulsion will start to change long before this separation is visually apparent. An increase in particle size is particularly indicative of physical instability, since this monitors the coalescence or Locculation that is part of the process involved in ultimate phase separation. Increases in viscosity (due to Locculation) and changes in zeta potential (arising from a decrease in droplet surface area) are both indicative of poor physical stability. The presence of drug and cosolvents can potentially hasten the phase separation. 

In most cases, physical instabilities are consequences of previous chemical instabilities. Physical instabilities can arise principally from changes in uniformity of suspensions or emulsions, difficulties related to dissolution of ingredients, and volume changes [6], For instance, some cases where physical stability has been affected are cloudiness, flocculence, film formation, separation of phases, precipitation, crystal formation, droplets of fog forming on the inside of container, and swelling of the container [8],... 

Davies and Smith suggest that the effect of addition of small amounts of hexadecane on stability may be due to a prevention of emulsion degradation by molecular diffusion. This approach to emulsion instability was first presented by Higuchi and Misra (12), and was based on the fact that small droplets will demonstrate deviations in physical properties as compared to larger droplets or plane surfaces. 

A stable emulsion is considered to be one in which the dispersed droplets retain their initial character and remain uniformly distributed throughout the continuous phase for the desired shelf life. There should be no phase changes or microbial contamination on storage, and the emulsion should maintain elegance with respect to odor, color, and consistency. Instabilities of both chemical and physical origins can occur in emulsion formulations. Chemical instabilities, such as the development of rancidity in natural oils due to oxidation by atmospheric oxygen, the depolymerization of macromolecular emulsifiers by hydrolysis, or... 

The chapter should allow an appreciation of the factors leading to emulsion stability and physical instability, including flocculation and coalescence. Approaches to the formulation of emulsions to provide vehicles for drug delivery and parenteral nutrition (the main uses in pharmacy) should be understood. 

Emulsions. Emulsions have much higher solvent capacity than micelles for hydrophobic materials. However, emulsions are metastable colloids that will phase separate over a period of time and form a two-phase system (i.e., oil phase and aqueous phase). Because of its physical instability, large energy input (usually mechanical mixing) is required to form an emulsion. 

To achieve the above criteria complex multipheise systems are formulated (i) Oil-in-Water (0/W) emulsions (ii) Water-in-Oil (W/0) emulsions (iii) solid/liquid dispersions (suspensions) (iv) emulsions-suspension mixtures (suspoemulsions) (v) nanoemulsions (vi) nanosuspensions (vii) multiple emulsions. All these disperse systems require fundamental understanding of the interfacial phenomena involved, such as the adsorption and conformation of the various surfactants and polymers used for their preparation. This will determine the physical stability/instability of these systems, their application and shelf-life. 

Emulsion instability is manifested in changes in the physical properties of the dispersion such as its droplet size distribution, its rheological properties or other parameters which are a consequence of the coalescence of globules or their flocculation, that is, of the alteration in the real or effective mean globule diameter, respectively. Flocculation, which is often the precursor of coalescence can affect the appearance of both liquid and solid emulsions. It accelerates the rate of creaming or settling which in itself is regarded as a form of instability. 

Lipid peroxides are also able to react with other components of parenteral nutrition admixtures (trace elements), causing a drop in pH with the subsequent potential for physical-chemical instability [29]. Table 11 shows the peroxide value and the pH drop in a pure lipid emulsion and a lipid-containing AlO admixture stored in EVA bags under different conditions of temperature and light exposure in the presence and absence of trace elements. 

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