Elsevier

Chemical Physics Letters

Volume 628, 16 May 2015, Pages 108-112
Chemical Physics Letters

Density fluctuations in aqueous solution of ionic liquid with lower critical solution temperature: Mixture of tetrabutylphosphonium trifluoroacetate and water

https://doi.org/10.1016/j.cplett.2015.03.026Get rights and content

Highlights

  • Aqueous solution of ionic liquid [P4444]CF3COO shows LCST-type phase separation.

  • The solution exhibits unique mixing states in meso scale.

  • A quantitative expression of mixing state is density fluctuations.

  • We experimentally obtain the density fluctuations.

  • The density fluctuations drastically increase as approach to the critical point.

Abstract

Aqueous solutions of tetrabutylphosphonium trifluoroacetate ([P4444]CF3COO) exhibit a LCST-type phase transition with the critical point near 0.025 in mole fraction of [P4444]CF3COO at T = 302 K. The phase behavior of [P4444]CF3COO–water mixtures was investigated by evaluating their density fluctuations, which provide quantitative descriptions of the mixing states of the solutions. The concentration dependence of the density fluctuations was investigated at 293 and 301 K for the mixtures without distinguishing the components and for the individual components ([P4444]CF3COO and water). A drastic change in the mixing state was observed for the solution when the critical point was approached.

Introduction

Ionic liquids (ILs) are salts with unusually low melting points below 373 K. They also have many unique properties such as non-volatility, non-flammability, high thermal stability, high ionic conductivity and amphiphilicity [1]. Because of these characteristic properties, ILs have attracted considerable attention in various fundamental and application fields. In addition to the reports on simple ILs, extensive studies on mixtures of ILs and other materials have appeared in the literature recently. In particular, mixtures with water, which is regarded as a counter material to ILs, have been investigated as novel reaction media and functional materials [2], [3], [4]. One of the examples, in which the functions of aqueous IL solutions are designed and utilized, is a positive use of a phenomenon of phase separation such as a separation technique of solved materials into each separated phase [5].

With regard to aqueous solutions of ILs that exhibit thermo-responsive phase transitions, those with upper critical solution temperature (UCST)-type transitions are popular and some studies on them have been reported [6], [7], [8]. For example, it has been reported that binary mixtures of imidazolium-based ILs ([Cnmim]BF4 (n = 4, 6, 8)) and water show UCST-type phase transitions, and the transition depends strongly on the length of the alkyl chain in the imidazolium cation [6], [7], [9], [10]. To achieve the further functional aqueous system, Ohno et al. performed systematic studies on the mixtures of phosphonium-based ILs and water and observed that some of the mixtures exhibited lower critical solution temperature (LCST) depending on the counter anion [11]. This result was of interest because the LCST-type phase transition is rarely observed in IL–water mixtures. A mixture with an LCST remains homogeneous upon cooling, but the phase separates upon heating. Such ILs were prepared by appropriately adjusting the total hydrophilicity of the cation and anion [11], [12], and the critical temperature was also controllable by varying the hydrophilicity of the component ions [13]. However, as far as we know, only one study of the mechanism of LCST-type transitions in IL–water mixtures have been reported by Gao's and Li's groups [14]. It is necessary to analyze IL–water mixtures from multiple perspectives to gain a fundamental understanding of this behavior and for the development of practical application.

Density and concentration fluctuations express the spatial inhomogeneity of the particle distribution in a sample with a disordered structure, such as a solution, and can therefore be used for the quantitative determination of the mixing state. More specifically, the concentration fluctuation, N¯(Δx2), and the density fluctuation, (〈ΔN2), are defined as the mean squares of the differences from the average quantities for the concentration and particle distribution, respectively. For a binary system, these two fluctuations are directly related to the X-ray intensity at s = 0 (I(0)) [15], [16], [17], where s is the absolute magnitude of the scattering vector defined by 4π sin θ/λ, and θ and λ are half of the scattering angle and the X-ray wavelength, respectively. Nishikawa and Morita determined the inhomogeneities in aqueous solutions of various molecular liquids not only under ambient conditions [18], [19], [20], [21], [22] but also in supercritical states [23], [24] using the concentration and density fluctuations. Furthermore, the fluctuations in a [C4mim]BF4–water mixture, which exhibit a UCST at 277 K, were determined via small-angle neutron scattering analysis at 298 K by Almásy et al. [25].

In the present study, the mixing state of a binary tetrabutylphosphonium trifluoroacetate ([P4444]CF3COO)–water mixture was investigated by evaluating the density fluctuations in the solution. The mixture exhibits an LCST-type phase transition with the critical point near xIL = 0.025 (xIL: mole fraction of [P4444]CF3COO) at T = 302 K [11]. Using small-angle X-ray scattering (SAXS) analysis and density measurements, we investigated three density fluctuations; the density fluctuations without distinguishing the components and the individual fluctuations in [P4444]CF3COO and water.

Section snippets

Experimental

[P4444]CF3COO was prepared by directly neutralized aqueous solutions of the tetrabutylphosphonium hydroxide (Hokko Chem Co.) and trifluoroacetate. After evaporation, the product was added to a dichloromethane/water biphasic system, and the resulting mixture was washed several times with distilled water. The prepared [P4444]CF3COO was dried in vacuum for 24 h at 333 K prior to use. The structure and purity of the salt were confirmed by 1H NMR and elemental analyses.

SAXS experiments were performed

Results and discussion

Figure 1 shows the phase diagram for the [P4444]CF3COO–water mixture [11] and the analysis points over the range xIL = 0.016–0.079 at T = 293 and 301 K. Hereafter, the green and red symbols used in the figures for the mixture refer to the data at 293 K and 301 K, respectively. In this concentration range, judging from the electrical conductivity, it can be concluded that the cation and anion form a pair in water. Therefore, the ion pair was regarded as a particle, and the solution was treated as a

Acknowledgments

This work was partially supported by JSPS KAKENHI (Grant Number: 25248003). The SAXS experiment in this work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 14G689).

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    Present address: Department of Chemical & Biological Engineering, University of Colorado, Boulder, CO 80309, USA.

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