Hysteresis behaviour in temperature-induced emulsion inversion † Laura Ma ´rquez, 1 Alain Graciaa, 2 Jean Lachaise, 2 Jean-Louis Salager 1 * and Noelia Zambrano 1 1 Laboratorio FIRP, Ingenierı´a Quı´mica, Universidadde Los Andes, Me ´rida, Venezuela 2 Laboratoire Fluides Complexes, Universite ´ de Pau, BP 1155, 64013 Pau Cedex, France Abstract: Emulsion inversion produced by a change in temperature usually takes place reversibly, ie at the same temperature whatever the direction of change. The present study indicates previously unreported hysteresis in a transitional inversion driven by a temperature variation in a non-ionic system containing Tween surfactant mixtures. In both directions of the temperature variation, a delay is exhibited with respect to the standard phase inversion temperature of the equilibrated system. The width of the hysteresis region depends on the rate of temperature change. This behaviour, which does not happen in the presence of alcohol, is attributed to the presence of liquid crystals. # 2003 Society of Chemical Industry Keywords: emulsion; inversion; hysteresis; transitional inversion INTRODUCTION The morphology of an emulsion produced by stirring a surfactant–oil–water (SOW) system is closely linked to its physicochemical formulation and composition. 1–3 A change in any of these variables can induce emulsion inversion. It is well known that variation of the water- to-oil ratio (WOR) generates a so-called catastrophic inversion, 4 which in most cases, exhibits a delayed inversion with respect to the standard inversion, ie the boundary between the regions where the two types of emulsions are attained upon stirring equilibrated systems. 1 Because this delay occurs in both directions of change, the formulation-composition diagram dis- plays a hysteresis zone in which the emulsion type depends on the direction of change. 5–10 However, the so-called transitional inversion resulting from a change in formulation, including temperature, as in the phase inversion temperature (PIT) emulsification pro- cess, 11,12 generally takes place at the so-called opti- mum formulation for three-phase behaviour, whatever the direction of change and without any delay, 13–15 with very few exceptions. 16–18 This absence of delay is generally attributed to the high instability of the emulsion in the neighborhood of optimum formula- tion, 19,20 which has been explained by different mechanisms. 21–23 Because the occurrence of a delay can modify the emulsion properties, as in paints’ or cosmetics’ manufacturing, any exception to the non- delay rule is worth investigating. This paper exposes previously unreported evidence of delay in tempera- ture-driven transitional inversion. 24 EXPERIMENTAL In all experiments, the surfactant–oil–water systems contain 24 ml of a kerosene cut oil (EACN = 8.2) as the oil phase and 29ml of a 1g dl 1 NaCl brine as the aqueous phase. Blends of surfactants (2.8 g) with an hydrophilic lipophilic balance (HLB) value of 11.6 were prepared with surfactants from the Tween series (0.85 wt% Tween 85 with HLB = 11 and 0.15 wt% Tween 80 with HLB = 15 supplied by Sigma Chemical Co). Systems labeled SOW only contain surfactant, oil and water, while SAOW systems also contain 2.5 ml of sec-butanol as co-surfactant. sec-Butanol was used because its influence upon the system can be studied while keeping the physicochemical formulation prac- tically unchanged, because the alcohol effect on the physical chemistry of the system is essentially nil. 25,26 The experiments were carried out in a 4.57 cm internal diameter 100 ml glass emulsification vessel, fitted with four stainless steel baffles. The system was kept under continuous stirring by an electric mixer (Caframo, Model BDC6015) whose three-blade (Received 21 January 2002; revised version received 15 April 2002; accepted 3 May 2002) * Correspondence to: Jean-Louis Salager, Laboratorio FIRP, Ingenierı ´a Quı ´mica, Universidad de Los Andes, Me ´rida, Venezuela E-mail: salager@ula.ve † Poster presentation – Paper presented at the Formula III Conference: New Concepts and Strategies in Formularies, from Laboratory to Industry, 13–16 October, 2001, He ´ rault, France Contract/grant sponsor: CDCHT-ULA Contract/grant sponsor: CONICIT # 2003 Society of Chemical Industry. Polym Int 0959–8103/2003/$30.00 590 Polymer International Polym Int 52:590–593 (2003) DOI: 10.1002/pi.1046