Elsevier

Fluid Phase Equilibria

Volume 372, 25 June 2014, Pages 21-25
Fluid Phase Equilibria

Miscibility behavior of trihexyl(tetradecyl)phosphonium tetrafluoroborate with cyclic hydrocarbons

https://doi.org/10.1016/j.fluid.2014.03.020Get rights and content

Abstract

Liquid–liquid miscibility temperatures as a function of composition have been determined experimentally for the binary systems formed by trihexyl(tetradecyl)phosphonium tetrafluoroborate with cyclohexane, cycloheptane, methylcyclohexane, 1,2-dimethylcyclohexane and 1,3-dimethylcyclohexane. All the measured systems present solubility curves characterized by the asymmetry with respect to the equimolar composition and represent the phase diagrams with the upper critical solution temperatures. The effect of the deuterium substitution in cyclohexane and methylcyclohexane has also been studied revealing small upward shift of the UCSTs which means slightly worse miscibility of ILs studied with selected deuterated cycloalkanes.

Introduction

Over the last decade, ionic liquids became one of the most popular objects of interest in the physical and organic chemistry as well as in the chemical technology. It is because of their unique properties – among others their versatile ability of solubility in different compounds. There are many data on miscibility of imidazolium based ionic liquids but there are few attempts only regarding the miscibility of phosphonium based ionic liquids. Sarcasan et al. [1] reported miscibility data on trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14]Cl) and bromide ([P6,6,6,14]Br) with alkanes while Domańska et al. [2], [3] presented extensive data on the phase behavior of tetrabutylphosphonium methanesulfonate and p-toluensulfonate + alcohols and aromatic hydrocarbons systems. Anderson et al. [4] reported the miscibility data on trihexyl(tetradecyl) phosphonium chloride with nonane and water. The solubility in water was also studied by Freire et al. [5]. Rebelo's group [6] described the miscibility behavior of selected tetralkylphosphonium ionic liquids with alkanes, alcohols and their fluorinated counterparts. They tested the ionic liquids with the following anions: acetate, bis(trifluoromethyl)sulfonyl imide, trifluoromethanesulfonate and dicyanamide. More recently, interesting work presented phase behavior of phosphonium based ionic liquids having triflate [OTf2] and bistriflamide [NTf2] anions with PEG of different average molecular mass were published by Calado et al. [7].

There are several reasons why one might consider phosphonium ionic liquids. At first, they are very stable, what is very important feature, in particular for processes operated at temperatures greater than 373 K. It should be however mentioned that high viscosity might be a disadvantageous factor in technological applications. They can be also attractive from another point of view. The steric bulk of the large alkyl side chain around the phosphonium cation interferes strongly with electrostatic interactions between anion and cation – which is generally localized on the phosphorous – allowing for greater impact of the properties of the anion itself, as compared to salts with more strongly interacting smaller cations (ammonium, imidazolium).

In the recent paper [8] we presented the phase diagrams characterizing the miscibility of [P6,6,6,14]Cl with the range of aliphatic hydrocarbons. As expected the phase diagrams obtained show upper critical solution temperature (UCST) with high asymmetry in respect to the equimolar composition. The solubility decreases with the increasing chain length of the alkanes. In the present study, we replaced chloride anion for tetrafluoroborate anion in the phosphonium ionic liquid. This replacement had a huge impact on its phase behavior. It appeared that contrary to [P6,6,6,14]Cl, this new ionic liquid is completely immiscible with aliphatic hydrocarbons. On the other hand, it shows interesting phase behavior for the mixtures of [P6,6,6,14]BF4 with cyclic hydrocarbons. In this paper we present the miscibility study of [P6,6,6,14]BF4 with cyclohexane, methylcyclohexane, dimethylcyclohexanes and cycloheptane along with the study of the isotope effect on miscibility of deuterated cyclohexane and methylcyclohexane.

Section snippets

Materials and methods

The list of the cyclic hydrocarbons used in this study is as follows: cyclohexane, cyclohexane-d12, methylcyclohexane, methylcyclohexane-d14, cycloheptane, 1,2-dimethylcyclohexane and 1,3-dimethylcyclohexane. Their suppliers and the related stated purities have been presented in Table 1. They were additionally dried over molecular sieves 5 Å. Deuterated cyclohexane-d12 and methylcyclohexane-d14 were stored over molecular sieves previously treated with D2O and carefully dried.

Results and discussion

The experimental values of the liquid–liquid phase separation temperatures as a function of composition (mole fractions, xIL) for the binary mixtures of [P6,6,6,14]BF4 with cyclohexane, methylcyclohexane, dimethylcyclohexanes, cycloheptane were used to construct the appropriate phase diagrams shown in Fig. 1. The detailed experimental numerical cloud points for the above-mentioned systems in the form of T  x data are given in Table 2. Because of the wide temperature range, the data were fitted

Conclusions

In this paper, we have presented the results of the miscibility studies of trihexyl(tetradecyl)phosphonium tetrafluoroborate with cyclic alkanes: cyclohexane, cycloheptane, methylcyclohexane, 1,2-dimethylcyclohexane and 1,3-dimethylcyclohexane. In additions to these systems, the effect of deuteration in cyclohexane and methylcyclohexane on the miscibility has been also reported. All systems studied are characterized by the phase diagrams with the upper critical solution temperature. The

Acknowledgment

This work was partially supported by the Ministry of Science and Higher Education of Poland under the Grant no. N N204 030336.

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