FACTORS CONTROLLING THE PHASE INVERSION OF OIL-IN-WATER EMULSIONS

Many current processes for the commercial manufacture of oil-continuou s emulsions utilise a phase inversion step from an oil-in-water (o/w) to a water-in-oil (w/o) emulsion. Whilst the operators of such process es may have practical experience in the control of inversion, little k nowledge is available regarding the precise mechanisms and the control ling factors. The aim of this study was to examine the complete phase inversion of coarse (> 10 mu m) o/w emulsions which were cooled and sh eared simultaneously. Batch experiments were conducted in a small stir red and jacketed vessel in which an o/w emulsion was prepared. The emu lsion was then cooled with continuous stirring. Measurements of torque and conductivity during the cooling stage allowed monitoring of the p hase inversion process. Factors such as fat level and type,emulsifier level and type, and different cooling regimes, were investigated. In a ddition, we studied the factors controlling oil droplet coalescence to help determine the underlying mechanism in phase inversion and to hel p identify the rate-limiting step in the process. From these studies a nd previously published results a mechanism for phase inversion has be en proposed in which the rate-limiting step is the entrapment of water within the oil phase. Entrapment results from multi-oil droplet colli sions in which film drainage is controlled by the presence of crystals at the interface. A mechanism is proposed in which the fat crystals l imit oil droplet deformation. The primary role of the emulsifier is to position the fat crystals at the interface. A secondary role of some emulsifiers is to impose a steric barrier to film drainage, resulting in longer phase inversion times.