Colloids and Surfaces A: Physicochem. Eng. Aspects 288 (2006) 151–157 Emulsion inversion from abnormal to normal morphology by continuous stirring without internal phase addition Effect of surfactant mixture fractionation at extreme water–oil ratio Marianna Rond´ on-Gonz´ alez a , V´ eronique Sadtler a , Lionel Choplin a , Jean-Louis Salager b,∗ a Centre de G´ enie Chimique des Milieux Rh´ eologiquement Complexes (GEMICO), Ecole Nationale Sup´ erieure des Industries Chimiques (ENSIC), Institut National Politecnique de Lorraine (INPL), Nancy, France b Laboratorio FIRP, Ingenier´ ıa Qu´ ımica, Universidad de Los Andes, M´ erida, Venezuela Received 28 September 2005; received in revised form 2 March 2006; accepted 13 March 2006 Available online 2 May 2006 Abstract 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. © 2006 Elsevier B.V. All rights reserved. Keywords: Catastrophic inversion; Multiple emulsion; Phase behavior; Partitioning; Liquid crystal 1. Introduction An emulsion is a liquid–liquid dispersion stabilized by a sur- factant. The classical emulsion preparation process consists in dissolving the surfactant in the suitable continuous phase (in which it has to be soluble), and to add the dispersed phase under appropriate agitation [1,2]. In the last decade, the interest in emulsion inversion, i.e. the change in the morphology from oil-in-water (O/W) to water-in- oil (W/O) or vice versa, has increased as it became a method to produce fine and concentrated emulsions at a low energy expense even with very viscous oils [3–5]. Depending on the changed variable, there are two types of emulsion inversion: the transitional inversion which is induced by the change of the surfactant affinity for the liquid phases, and the catastrophic inversion which is produced by a change in the ∗ Corresponding author. Tel.: +58 274 2402954; fax: +58 274 2402957. E-mail address: salager@ula.ve (J.-L. Salager). composition or physical factors of the system, e.g. the water- to-oil ratio, the viscosity of the phases, or the stirring protocol [6]. The catastrophic inversion is usually triggered by the contin- uous addition of the internal phase of the emulsion [7,8], but it could be also generated by the continuous stirring of an abnormal emulsion [9–11], i.e. an emulsion type that does not correspond to the normal curvature requirement of the interface, according to the Brancoft’s rule [12,13], Langmuir’s wedge theory [14] or Winsor’s interaction energy analysis [15,16]. Abnormal emulsions frequently exhibit a multiple morphol- ogy in which the dispersed phase drops contain some tiny droplets of the continuous phase; the presence of these droplets satisfies Bancroft’s rule and as a consequence, this inner emul- sion is quite stable. Fig. 1 illustrates such a situation for a w/O/W multiple emulsion in which the low case “w” refers to the droplet phase which is inserted in the “O” drops [1]. When an abnormal emulsion is submitted to a continuous stirring, the external phase is continuously included as droplets in the dispersed phase drops and the process goes on indefinitely 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.03.034