Emulsion inversion from abnormal to normal morphology by continuous stirring without internal phase addition: Effect of surfactant mixture fractionation at extreme water–oil ratio

When the emulsion inversion from abnormal to normal morphology is induced by continuous stirring only, it takes place through the formation of a multiple emulsion in which the external phase is continuously included as droplets in the dispersed phase drops. In the studied case the system exhibits an initial O/W morphology, then a w/O/W multiple one before inverting into a W/O emulsion. The general trend reported in the literature is that the higher the initial water content, the longer the incorporating process to swell the drops in order to attain the critical dispersed phase volume that triggers inversion. The present study reports, for nonionic systems with a given hydrophilic–lipophilic balance (HLB = 6) and 7 wt.% of surfactant concentration, that this trend is found to be unexpectedly reversed when the initial water content increases beyond 80%. This anomalous result is attributed to an indirect increase in the lipophilicity of the formulation due to the variation of the interfacial surfactant mixture with the water/oil ratio, and to the formation of liquid crystals at high water contents.