Water/oil/[P 6,6,6,14 ][NTf 2 ] phase equilibria Sara Lago, Borja Rodríguez-Cabo, Alberto Arce, Ana Soto ⇑ Chemical Engineering Department, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain article info Article history: Received 20 January 2014 Received in revised form 19 February 2014 Accepted 20 February 2014 Available online xxxx Keywords: LLE Water n-Dodecane Ionic liquid abstract Phase equilibria data for complex systems (more than three components or two phases, high pressures, complex fluids...) are needed to develop thermodynamic models capable of correlating and predicting their behaviour. Along these lines, systems composed of water, oil and surfactant exhibit an interesting conduct. In this work, the (liquid + liquid) equilibria of a ternary system comprising water, n-dodecane and a surfactant ionic liquid, trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide, have been determined at temperatures (298.15 or 348.15) K and atmospheric pressure. Winsor type III sys- tems have been found without the need of adding any co-surfactant. The stability of the triphasic system with temperature has been tested. Some physical properties (density, viscosity and interfacial tension) for phases involved have been also determined. As expected, a minor surfactant character has been found for this ionic liquid rather than for the homologous with the chloride anion. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The knowledge of the behaviour of substances in phase equilib- rium is a critical factor not only in the design of separation pro- cesses but also for understanding the phase coexistence in a natural system. There are several useful equations that relate prop- erties in different phases that are in equilibrium as a function of temperature, pressure and composition. If a multiphasic system is in equilibrium, besides temperature and pressure, the fugacity of each species must be equal in all phases. Using the different definitions for the fugacities, two differ- ent approaches can be derived for the description of phase equilib- ria [1]. The first uses the fugacity coefficients and they can be calculated by means of equations of state and the corresponding mixing rules. The second approach uses activity coefficients de- scribed through excess Gibbs free energy models. These models have been shown to be successful for the treat- ment of a large number of phase equilibria data available in the lit- erature for binary and ternary systems involving two phases. Nonetheless, the difficulties increase when systems in equilibrium are more complex (higher number of components or phases, spe- cial species as polymers or ionic liquids, etc.). A lack of data for this kind of systems complicates the development of adequate models for the representation of their equilibria. However, many of these systems appear in separations of industrial interest. This is the case of Winsor Type III systems. When mixing water, oil and a surfactant at a fixed temperature and pressure, three kind of basic phase diagrams can be found according Winsor [2,3]. Type I is characterised by the presence of a biphasic area in the region of low concentration of surfactant, and a monophasic area. For this type, the affinity of the surfactant for the aqueous phase is greater than that for the oil. According to tie-lines, any mixture within the interior of the immiscible area will split into an aqueous micro- emulsion in equilibrium with an excess phase of oil. In the type II diagram (with also a monophasic and a biphasic region), the affinity of the surfactant for the oil phase is the dominating one. Therefore, the biphasic system corresponds to an oily micro- emulsion in equilibrium with an excess phase of water. The type III diagram comprises a monophasic region and a 3-phase region surrounded by three biphasic regions. Systems with global compo- sition lying within any of the biphasic regions will split as in the previous cases. With regard to systems with global composition ly- ing in the 3-phase region, they will split into three phases in equi- librium: an aqueous phase and an oily phase containing essentially water and oil respectively, plus an intermediate phase with density between those of the other two phases. These last systems are of interest in enhanced oil recovery since the formation of three phase behaviour, with a liquid crystal or micro-emulsion middle phase when the interactions are equal, leads to a minimum inter- facial tension so that the crude oil trapped in a reservoir can be dis- placed by a surfactant flooding method [4]. Due to the favourable properties of ionic liquids, such as negli- gible vapour pressure, the ability to be designed for a specific pur- pose, the wide liquid state range or thermal stability among others, http://dx.doi.org/10.1016/j.jct.2014.02.012 0021-9614/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +34 881816760; fax: +34 881816702. E-mail address: ana.soto@usc.es (A. Soto). J. Chem. Thermodynamics xxx (2014) xxx–xxx Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct Please cite this article in press as: S. Lago et al., J. Chem. Thermodyn. (2014), http://dx.doi.org/10.1016/j.jct.2014.02.012