Emulsion instability mechanism

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... 

Creaming or sedimentation. Creaming or sedimentation is one of the principal instability mechanisms seen in emulsions. Emulsion... 

Figure D3.4.6 Comprehensive overview of the principal mechanisms that cause emulsion instability.

In summary, all the recent improvements in the doubleemulsion compositions (emulsifiers and oils), the improved evaluation related to the weight given to any of the possible instability mechanisms, and the better understanding of the instability factors that were achieved in the last 15 years of research work were supposed to solve most of the scientific problems of this technology. Yet, only very limited improvements in the stability of the emulsions and in extending their shelf-life have been recorded. There is practically very limited control of the release of the additives or the active matter. 

Often, in bench studies aimed at understanding emulsion-stabilization mechanisms, a hypothesis is devised. Most often the components of crudes are first separated, and a model emulsion is prepared from various combinations of the components in a model oil and in water of quality similar to that of formation or process water. The stability or instability is traced either by water resolution or by observing flie interfacial film properties under some form of externally applied stress over time. The stress may include temperature increases or solvent changes. The system may then be modified by the demulsifier and the changes in behavior are compared to that without the demulsifier. Deductions are then made about the film mechanics of the system in response to the variables. 

The eventual equilibrium state for emulsions, shown in Figure 4.1(f), is completely separated into the two phases. This process, called breaking, may take from a few minutes to several years to occur. The transition from the initial state (a) to the final state (f) must proceed via one of the mechanisms shown in (c) or (e). However, in practice, a complex set of inter-relationships exists between these instability mechanisms and often all will occur on the way to the final state. Frequently the onset of one of the instability mechanisms will modify the rate at which the others occur. 

Thermal Stress. The Arrhenius equation states that a 10°C increase in the temperature doubles the rate of most chemical reactions. However, this approach is generally only useful to predict a product s shelf life if the instability of the emulsion is due to a chemical degradation process. Furthermore, this degradation must be identical in mechanism but different in rate at the investigated temperatures. Thus, the instability of... 

C.H. ViUa, L.B. Lawson, Y. Li, and K.D. Papadopoulos Internal Coalescence as a Mechanism of Instability in Water-in-Oil-in-Water Double-Emulsion Globules. Langmuir 19, 244 (2003). 

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. 

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. 

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. 

Theoretical aspects of emulsion formation in porous media were addressed by Raghavan and Marsden (51-53). They considered the stability of immiscible liquids in porous media under the action of viscous and surface forces and concluded that interfacial tension and viscosity ratio of the immiscible liquids played a dominant role in the emulsification of these liquids in porous media. A mechanism was proposed whereby the disruption of the bulk interface between the two liquids led to the initial formation of the dispersed phase. The analysis is based on the classical Raleigh-Taylor and Kelvin-Helmholz instabilities. 

Stability is an essential condition for PFC emulsions to be of practical use. The principal mechanism for irreversible droplet growth in submicronic PFC emulsions during storage is molecular diffusion (also known as Ostwald ripening or isothermal distillationj.P Coalescence may contribute to instability when mechanical stress is applied and at higher temperatures, as during heat sterilization. Sedimentation and flocculation are fully reversible and pose no problem. 

One of the main drawbacks to the commercial development of multiple emulsions is their inherent instability. The intention of this paper is to review studies on the stability and mechanism of breakdown of multiple systems and attempts to minimise such instability, for example, by appropriate choice of surfactant, polymerisable surfactants or gelation of the aqueous or oily phases. 

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