Stability of Concentrated Emulsions

In this chapter, we review some results in the field of emulsion metastability, emphasizing the destruction of concentrated emulsions (droplet volume fraction p 70%) through coalescence. The review concerning Oswald ripening (Section 5.2) is more concise, as this mechanism is fairly well understood and has been extensively documented in the literature. So far, the destruction of concentrated emulsions through coalescence is much less understood and has motivated many recent studies and developments that we summarize (Sections 5.3 to 5.6). 

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A.J. Webster and M.E. Cates Osmotic Stabilization of Concentrated Emulsions and Eoams. Langmuir 17, 595 (2001). 

The objective of this paper is to illustrate the efficacy of inferring the interdroplet forces in a concentrated protein stabilized oil-in-water emulsion from the knowledge of the equilibrium profile of continuous phase liquid holdup (or, dispersed phase faction) when the emulsion is subjected to a centrifugal force field. This is accomplished by demonstrating the sensitivity of continuous phase liquid holdup profile for concentrated oil-in-water emulsions of different interdroplet forces. A Mef discussion of the structure of concentrated oil-in-water emulsion is presented in the next section. A model for centrifugal stability of concentrated emulsion is presented in the subsequent section. This is followed by the simulation of continuous phase liquid holdup profiles for concentrated oil-in-water emulsions for different centrifugal accelerations, protein concentrations, droplet sizes, pH, ionic strengths and the nature of protein-solvent interactions. 

Phase Behavior and Stability of Concentrated Emulsions of Hydrocarbons in Water... 

If the oil phase is replaced in an oil in water (o/w) emulsion by a hydrophobic monomer, or the water phase in a water in oil (w/o) emulsion by a hydrophilic monomer, an emulsion is obtained that can be employed as a precursor for the preparation of polymer latexes [10, 11]. Similarly, if both phases are replaced, the oil phase by a hydrophobic phase containing a monomer and the water phase by a hydrophilic phase containing a monomer, the generated emulsion could be employed as a precursor in the preparation of polymer composites [12]. However, concentrated emulsions that are generated and stable at room temperature may become unstable at the polymerization temperature. To be suitable for the preparation of polymers and polymer composites, the concentrated emulsion must first form and, subsequently, it must remain stable at the temperature at which polymerization takes place. The scope of the present section is to investigate the factors that influence the formation and stability of concentrated emulsions at the preparation and polymerization temperatures in order to identify the physico-chemical conditions that ensure their stability [9],... 

The stability of concentrated emulsions is affected by the chemical natures of the dispersed and continuous phases as well as that of the surfactant, the viscosities of the continuous and dispersed phases, the temperature and the volume fraction of the dispersed phase. 

In Fig. 10 the interfacial tension and the stability of concentrated emulsions containing styrene and an aqueous sodium chloride solution are plotted against the concentration of sodium chloride. The w/o concentrated emulsions are stable for both Span 20 and Span 80. When SDS was used as surfactant, the o/w concentrated emulsions were more unstable at 50 °C than the above w/o concentrated emulsions because the double layer repulsion between cells is shielded by the high ionic strength. With SDS, concentrated emulsions did not form at room temperature above a salt concentration of 1.2 moll-1 because of the salting-out effect. The o/w concentrated emulsion did not form at all at 25 °C when Span 20 was employed as surfactant. 

Table 1. Effect of the HLB of the surfactant on the formation and stability of concentrated emulsions. The concentrated emulsion contains styrene and water as the two phases and the volume fraction of the dispersed phase is 0.9. The concentrated emulsion was prepared at room temperature and its stability test was conducted by heating the emulsion at 50 °C for 3 h and 24 h, respectively...

Fig. 4. Effect of sodium dodecylsulfonate concentration on fast elastic deformation 9 (I), and stability of concentrated emulsions T (II).

Taubman, A. B. and S. A. Nikitina. 1962. Structural and mechanical properties of the surface layers of the emulsifier and mechanism of stabilization of concentrated emulsions. Kolloidnyi Zh. 24 633-666. 

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