Short Communication: Mass Balance and Emulsion Stability

Abstract

During emulsification droplet break-up and coalescence proceed simultaneously. To suppress the latter process fast adsorbing emulsifiers and eventually formation of an adsorption layer with optimum (maximum) emulsifier coverage are needed. For a given total emulsifier concentration the surface coverage will depend on degree of adsorption, but also critically on droplet size distribution which determines the interfacial area. Besides viscosity of the liquid phases and dispersion intensity droplet size distribution is determined by coalescence rate (and rate of Ostwald ripening), i.e. surface coverage and coalescence rate are interdependent. If full coverage of the droplet is achieved surplus emulsifier can lead to emulsion destabilization through depletion flocculation when a critical concentration is exceeded. In case of surfactants this critical concentration is above the cmc. As known high concentration of micelles induce depletion flocculation in a similar way as dissolved polymers. Increasing ionic surfactant concentration, however, also increases overall electrolyte concentration, leading to decreasing electrostatic repulsion by screening of charges. This has two effects – favouring adsorption and increasing packing density of surfactant molecules in the adsorption layer, but also destabilization of emulsion by flocculation. In the following it should be shown that despite rather strong assumptions made one can get a deeper understanding of the dominant destabilizing factors for emulsification processes using a mass balance of emulsifier adsorbed and dissolved in continuous phase.