Elsevier

Fluid Phase Equilibria

Volume 405, 15 November 2015, Pages 124-131
Fluid Phase Equilibria

Liquid–liquid–liquid equilibria for water + [P6 6 6 14][DCA] + dodecane ternary system

https://doi.org/10.1016/j.fluid.2015.07.022Get rights and content

Abstract

In this work the phase diagrams for the ternary system water + trihexyltetradecylphosphonium dicyanamide + n-dodecane were determined at 298.15 K and 348.15 K and atmospheric pressure. A triphasic region surrounded by two biphasic regions has been found. Due to the surfactant character of the ionic liquid, the system can be classified as Winsor Type III. The liquid–liquid and liquid–liquid–liquid equilibrium data of the corresponding immiscible regions were obtained. Focusing the work on surfactant flooding applications, the interfacial tensions between equilibrium phases of the binary and ternary systems were determined in presence or absence of salt. A comparison with other ionic liquids previously tested has been done on the basis of the position of the apex of the three-phase tie-triangle and the measured interfacial tensions. Other trihexyltetradecylphosphonium ionic liquids, with chloride or bis-(2,4,4-trimethylpentyl)phosphinate anions, showed more promising results. However, the knowledge of the phase diagram is of interest for processes that involve the co-existence of water, oil and ionic liquid, as triphasic catalysis or microemulsions applications.

Introduction

There is an important number of applications that involve three immiscible liquid phases. Among them the most studied, due to its important economic implications, is related to the use of surfactants in Enhanced Oil Recovery (EOR). The high oil/water interfacial tension is responsible for the entrapment of the oil into the pores, so it cannot be pushed out of the reservoir by water flooding. One of the main mechanisms of surfactant-related EOR is reducing interfacial tension, which is closely linked to water and oil solubilisation (phase behaviour) [1]. In optimal conditions, the addition of a surfactant to an oil/water mixture can produce a three-phase system (water, oil and a microemulsion) associated to an ultralow interfacial tension [2], [3]. Winsor classified these systems as Type III [4].

Another three-liquid-phase application that currently is gaining attention refers to a traditional extraction with an organic and an aqueous phase, where from the latter an aqueous two-phase system is generated by addition of salts or polymers. This is a multicomponent separation process that is being tested for recovery of metals, extraction of natural products, wastewater treatment, etc [5].

However, the coexistence of three liquid phases is important not only in separation processes but also in chemical reactions. In some phase transfer catalytic reactions, the reaction is sharply accelerated when a third liquid phase is generated. This third liquid contains a high concentration of catalyst (typically having surfactant-like properties) as well as significant amounts of water and the organic phase [6]. Also balanced catalytic surfactants have been proposed [7], [8] to form three-liquid-phase microemulsion systems to carry out catalytic oxidation reactions.

With the revival of ionic liquids in the last decade, traditional processes were tested again analysing the viability of these compounds to improve them, and also several new alternatives have appeared. Thus, ionic liquids have been incorporated as phase-forming components to produce aqueous two-phase systems and three-phase systems [9], [10]. Multiphasic systems have been widely proposed as reaction media for catalytic reactions with the catalyst dissolved in the ionic liquid. For instance, Heck coupling of aryl halides or benzoic anhydride with alkenes has been proposed in a three-phase system: ionic liquid with the catalyst, water (that removes salt by-products) and organic phase as extractant [11].

Surfactant ionic liquids have also been proposed as suitable chemicals to be used in surfactant or micellar flooding due to its capacity to reduce oil/water interfacial tension and solubilise oil [12], [13], [14], [15], [16], [17], [18], [19], [20]. We have undertaken a study [12], [13], [21], [22], [23] about the equilibria established among surfactant ionic liquids, water and n-dodecane (as oil phase). We are looking for Winsor type III systems and an important reduction of interfacial tension. When systems are Treybal type III (ionic liquid immiscible with water and oil) [12], [13], [21], [22], the formation of a three-phase system is highly probable. But even with surfactant ionic liquids miscible with oil and/or water, a Winsor Type III system can be found [23]. For all these systems we have found the formation of a triphasic system, without the need of co-surfactant, stable with temperature and in presence of salt.

In this paper, phase diagram for the system water + surfactant + n-dodecane is determined at two temperatures (298.15 K and 348.15 K) and atmospheric pressure, using the room temperature ionic liquid trihexyltetradecylphosphonium dicyanamide [P6 6 6 14][DCA] as surfactant. The goal is to analyse the influence of the anion on phase diagram and in the interfacial tension reduction of the oil/water and oil/brine systems. Besides the interfacial tension, other physical properties useful in process design (density and viscosity) are also determined. The knowledge of the range of concentrations where the three-phase system is found is of interest not only for the mentioned application but also for any other that implies the coexistence of the three-liquid phases. Moreover, the interfacial tensions in the system control phenomena of dispersion and coalescence of immiscible phases.

Section snippets

Chemicals

n-Dodecane (≥99 wt%), purchased from Sigma-Aldrich, was washed six times with bi-distilled water and passed many times through a column with alumina (activated, basic Brockman I, Sigma–Aldrich). The ionic liquid trihexyltetradecylphosphonium dicyanamide, [P6 6 6 14][DCA], (CAS no. 701921-71-3), was supplied by IOLITEC with purity >95 wt%. The structure is shown in Fig. 1. It was purified by washing with double-distilled water (4 times), passing it through a column filled with alumina and active

Results and discussion

The liquid–liquid–liquid equilibrium data for the ternary system water + [P6 6 6 14][DCA] + n-dodecane were determined at 298.15 and 348.15 K and atmospheric pressure. The results are presented in Table 3, Table 4. Fig. 2, Fig. 3 show the phase equilibria diagrams. It can be seen from these figures that the ternary systems are Winsor type III, with a triphasic region surrounded by two visible biphasic regions and one negligible biphasic region (at the bottom axis). This is common for traditional

Conclusions

In this work, the phase diagram for the ternary system water + [P6 6 6 14][DCA] + n-dodecane was determined at 298.15 K or 348.15 K and atmospheric pressure. Phase diagram shares many common features with other water + [P6 6 6 14][X] + n-dodecane systems, being X: Cl or NTf2. All these systems can be classified as Winsor Type III, and the triphasic systems are formed without the need of any co-surfactant. In all these systems, a small amount of surfactant is required to reach the three-phase region, and this

Acknowledgments

The authors acknowledge the Ministry of Economy and Competitiveness (Spain) for financial support through project CTQ2012-33359. I. Rodríguez-Escontrela and O. Rodriguez also acknowledge financial support from Ministry of Economy and Competitiveness (Spain) (FPI grant ref. BES-2013-062735, Ramón y Cajal fellow RYC-2012-10378).

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