Phase inversion in p-xylene/water emulsions with the non-ionic surfactant pair sorbitan monolaurate/polyoxyethylene sorbitan monolaurate (Span 20/Tween 20)

Phase inversion behaviour of a model oil–water emulsion, p-xylene–water, with Span 20/Tween 20 as a surfactant set was investigated. The surfactant hydrophilic-lipophilic balance (HLB), p-xylene/water ratio, and the addition policy were varied for different surfactant concentrations. The morphology of the resulting emulsions was recorded, and maps for different surfactant concentrations were developed. The ‘transitional’ inversion was irreversible and the inversion boundary was found to be discontinuous and was only observed at a high dispersed-phase ratio. No transitional inversion was recorded for the middle range of dispersed phase ratio whatever the type of emulsion was. The transitional inversion boundaries became wider with increasing surfactant concentration. An interaction between catastrophic and transitional inversion boundaries was found for the Span 20/Tween 20 system. The transitional inversion occurred only if, according to the catastrophic inversion boundaries, there is only one morphology possible for the inverted emulsion. The discontinuity in the transitional inversion boundary and the fact that its existence depended on the direction of HLB change and water/oil ratio are uncharacteristic of conventional transitional inversions. However, the drop size and interfacial-tension varied with the surfactant HLB and both decreased to a minimum value at the potential inversion boundary, even though the inversion did not occur, similar to the trend generally observed for a conventional transitional phase inversion. The results suggest that a minimum in drop size, together with a minimum in the interfacial tension, does not necessarily lead to a phase inversion.