Liquid–liquid phase equilibria of ionic liquid solutions in the critical region: 1-Methyl-3-octylimidazolium tetrafluoroborate with 1-pentanol or 1-hexanol

doi:10.1016/j.fluid.2014.07.036

Fluid Phase Equilibria

Volume 380, 25 October 2014, Pages 58-66

https://doi.org/10.1016/j.fluid.2014.07.036 Get rights and content

Highlights

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Liquid–liquid equilibria of binary RTIL/alcohol solutions were measured.
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Asymmetry of the coexistence curves was analyzed.
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Critical behaviors of RTIL/alcohol solutions were investigated.
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Dependences of critical parameters on the solvent's permittivity were studied.

Abstract

The liquid–liquid coexistence curves for the binary solutions of the room temperature ionic liquid (RTIL) 1-methyl-3-octylimidazolium tetrafluoroborate ([C₈mim][BF₄]) in 1-pentanol and 1-hexanol were measured in the critical region. The critical exponent β and the critical amplitude B were deduced and the former was found to be consistent with the 3D-Ising value. The complete scaling theory was applied to describe the asymmetry of the diameters of the coexistence curves. Moreover, the dependences of the critical temperature, the critical mole fraction, the reduced critical temperature and the reduced critical density on the permittivity of alcohol for a series of binary solutions of RTIL in alcohols were discussed.

Introduction

The liquid–liquid phase transitions of room temperature ionic liquids (RTIL) in organic solvents have drawn much attention both in applied and theoretical fields [1], [2], [3], [4], [5]. The mixtures of RTILs/organic solvents have been used as the media of chemical reactions and their liquid–liquid phase transitions are favorable to the separation of products, catalyst and solvent by changing temperature or composition [6], thus it is important to acquire more knowledge of the liquid–liquid phase equilibriums of the RTIL solutions [3], [4], [5]. It was proposed that the liquid–liquid phase transitions of RTIL solutions near the critical points are driven by the coulombic interaction or the solvophobic mechanism, dependent on the permittivity of the organic solvents, which decides the critical behavior of the liquid–liquid equilibrium and possibly a crossover between solvophobic and coulombic characters [3], [5], [7]. This characteristic criticality of ionic solutions motivates us to carry out more precise measurements of the coexistence curves of ionic solutions in the critical region.

In the region close to the critical point, the difference Δρ of the values of the general density variable between the two coexisting phases has the expression $Δ ρ = |\frac{ρ_{L} - ρ_{U}}{2}| = B τ^{β}$ where ρ is the general density variable; the subscript U or L relates to the upper or lower phase; $τ = |T_{c} - T| / T_{c}$ is the reduced temperature with T_c being the critical temperature; β is the critical exponent with the theoretical value of 0.326 [8] and B is a system-dependent critical amplitude.

Based on the complete scaling formulation proposed by Fisher and co-workers [9], [10], [11] for one-component fluids, Anisimov and co-workers [12], [13], [14] pointed that the scaling fields for incompressible or weakly compressible binary liquid mixtures may be expressed by the linear combinations of all physical fields: the chemical potential of the solvent μ₁, the difference of the chemical potentials between the solvent and the solute Δμ, the temperature T and the pressure P. The complete scaling theory for binary solutions has been tested by some experimental liquid–liquid equilibrium data of binary molecular liquid mixtures [15], [16], including a few ionic solutions [17], [18], [19], [20], [21], [22], however more precise liquid–liquid phase equilibrium data of RTIL solutions in the critical region are still highly required for deeper insight into this issue.

In this paper, we report the liquid–liquid coexistence curves of {x 1-methyl-3-octylimidazolium tetrafluoroborate ([C₈mim][BF₄]) + (1 − x) 1-pentanol} and {x[C₈mim][BF₄] + (1 − x) 1-hexanol}. The experimental results are used to determine the critical exponent β and the critical amplitude B, to discuss the asymmetric behavior of the diameters of the coexistence curves through the complete scaling theory, to examine the importance of the contribution of the heat capacity in the complete scaling formulation, and to investigate the dependences of the critical parameters on the permittivity of the alcohol solvents.

Section snippets

Chemicals

The purities, the suppliers, and the purified methods of the chemicals [C₈mim][BF₄], 1-pentanol and 1-hexanol are listed in Table 1. The mass fractions of water remaining in the samples after drying were checked by the coulometric Karl–Fischer titration, and are also listed in Table 1.

Apparatus and procedure

The critical mole fraction x_c of [C₈mim][BF₄], which was defined as the second component and has larger molecular volume than the solvent alcohols in the binary solutions, was determined by the “equal volume”

Results and discussion

The critical mole fractions and the critical temperatures were determined to be x_c = (0.109 ± 0.001), T_c = (290.7 ± 0.1) K for {x[C₈mim][BF₄] + (1 − x) 1-pentanol} and x_c = (0.121 ± 0.001), T_c = (300.1 ± 0.1) K for {x[C₈mim][BF₄] + (1 − x) 1-hexanol}, respectively.

The refractive indexes n measured for each coexisting phase at various temperatures are listed in columns 2 and 3 for {x[C₈mim][BF₄] + (1 − x) 1-pentanol} in Table 2 and for {x [C₈mim][BF₄] + (1 − x) 1-hexanol} in Table 3. Figs 1a and 2a show the corresponding plots

Conclusion

We measured the coexistence curves of systems {x[C₈mim][BF₄] + (1 − x) 1-pentanol} and {x[C₈mim][BF₄] + (1 − x) 1-hexanol}, from which the values of the critical exponent β were determined and found to be consistent with the 3D-Ising value. The experimental data were used to study the asymmetry of the diameters of the coexistence curves in terms of the complete scaling theory proposed by Anisimov under incompressible condition and neglect of the excess volume. The results indicated that the heat

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Projects 20973061, 21173080, 21373085 and 21303055).

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Liquid–liquid phase equilibria of ionic liquid solutions in the critical region: 1-Methyl-3-octylimidazolium tetrafluoroborate with 1-pentanol or 1-hexanol

Highlights

Abstract

Introduction

Section snippets

Chemicals

Apparatus and procedure

Results and discussion

Conclusion

Acknowledgements

Chem. Phys. Lett.

Fluid Phase Equilib.

Fluid Phase Equilib.

J. Chem. Thermodyn.

J. Mol. Liq.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

Science

Nat. Mater.

Phys. Chem. Chem. Phys.

Phys. Chem. Chem. Phys.

J. Phys.: Condens. Matter

Angew. Chem. Int. Ed. Engl.

J. Chem. Phys.

J. Stat. Phys.

Phys. Rev. Lett.

J. Chem. Phys.