Stability of Concentrated Emulsions Containing Monomers

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], 

Stability of emulsions refers to the resistance to the formation of two separate phases [13,14]. Coalescence of the droplets is responsible for the phase separation. Ostwald ripening constitutes an additional mechanism by which the large droplets grow in size at the expense of the smaller ones, which decrease in size. 

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. 

Regarding the chemical natures of the dispersed and continuous phases, experiment indicates that the higher the hydrophobicity of one of the phases 

The nature of the surfactant and its concentration is expected to play a role. To achieve a mechanically strong interfacial film, which can ensure the stability of the emulsion, the interfacial film of adsorbed surfactant molecules should be condensed in order to have strong lateral intermolecular interactions. A blend of two surfactants with different areas of head groups rather than an individual surfactant can more easily generate a close-packed and mechanically strong interfacial film. 

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