Heat capacities of the mixtures of ionic liquids with methanol at temperatures from 283.15 K to 323.15 K

doi:10.1016/j.jct.2007.07.001

The Journal of Chemical Thermodynamics

Volume 40, Issue 2, February 2008, Pages 203-207

https://doi.org/10.1016/j.jct.2007.07.001 Get rights and content

Abstract

The molar isobaric heat capacities of (methanol + 1-hexyl-3-methylimidazolium tetrafluoroborate) and (methanol + 1-methyl-3-octylimidazolium tetrafluoroborate) mixtures have been determined over the temperature range from 283.15 K to 323.15 K within the whole composition range. The excess molar heat capacities of investigated mixtures have been fitted to the Redlich–Kister equation at several selected temperatures. Positive deviations from the additivity of molar heat capacities have been observed in both examined systems. The results obtained have been discussed in terms of molecular interactions in binary mixtures.

Introduction

Huge amounts of volatile compounds, often toxic substances, have been used both in laboratory practice and in the industry; unfortunately, they penetrate into the atmosphere to a lesser or greater extent. Thus, it seems justifiable to search intensively for solvents that could replace those used so far, and would be environmentally friendly at the same time. Room-temperature ionic liquids, salts composed of an organic cation and inorganic or organic anion, seem to be such solvents [1]. The major attention devoted to ionic liquids results from the fact that their physical and chemical properties change over a very wide range depending on the type of cation and anion. It is quite possible to obtain an ionic liquid with desired, specific properties such as melting temperature, viscosity, density, or miscibility with water and organic solvents. Therefore, ionic liquids are often defined as “designer solvents” as it is possible to design such an ionic liquid that would be appropriate for the given process in question [2]. Particular physico-chemical properties of ionic liquids allow one to classify them in the category of so-called “green solvents” as the ionic liquids are practically non-volatile (their vapour pressures are non-measurably low), moreover, they are inflammable and mostly not explosive. These compounds can then replace the conventional solvents used in the industry, especially volatile organic compounds that are the main source of environmental pollution. Therefore, considering the possibilities of replacing conventional solvents with ionic liquids, it is necessary to study systematically the physical and chemical properties of ionic liquids, including their mixtures with other solvents. Compounds consisting of 1,3-dialkylimidazolium cations and various polyatomic anions (e.g. ${BF}_{4}^{-}, {PF}_{6}^{-}, and [{CF}_{3} ({SO}_{2})_{2} N]^{-}$ ) have been the most often investigated and described group of low-temperature ionic liquids with a very wide potential application in new technologies. The mixtures of 1-alkyl-3-methylimidazolium tetrafluoroborates with alcohols have been intensively studied for several years [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Much attention in these studies has been focused on the phase equilibria occurring in these mixtures [3], [4], [5], [6], [7], [8], which is of paramount importance in using ionic liquids in separation processes. Researchers have been also interested in various properties of the mixtures of 1-alky-3-methylimidazolium tetrafluoroborates with alcohols, such as density [7], [8], [9], [10], viscosity [7], [9], refractive index [9], activity [11], [12] and osmotic [12] coefficients, speed of sound [9], [10], etc. The aim of this work is to determine the heat capacities of (1-hexyl-3-methylimidazolium tetrafluoroborate + methanol) and (1-methyl-3-octylimidazolium tetrafluoroborate + methanol) mixtures. The assessment of the effect of conventional solvents on the physical and chemical properties of ionic liquids is indispensable to determine their potential use in various chemical processes. Such examinations can also provide valuable information about intermolecular interactions in the liquid phase.

Section snippets

Experimental

Ionic liquids (ILs), 1-hexyl-3-methylimidazolium tetrafluoroborate (HMImBF₄) and 1-methyl-3-octylimidazolium tetrafluoroborate (OMImBF₄), supplied from Fluka with minimum mass fraction purity of 0.97, were dried under vacuum and then stored in desiccators.

The chemical structure of used ILs is shown in figure 1.

Methanol (MeOH), from POCH (Polish Chemicals), was purified by distillation and out gassed under reduced pressure before use.

A differential scanning calorimeter Micro DSC III,

Results

The molar heat capacities at constant pressure of the pure solvents and their mixtures, C_p, were determined over the temperature range from 283.15 K to 323.15 K. For clarity, only values obtained every 5 K are presented in table 2.

The excess molar heat capacities, $C_{p}^{E}$ , of the {methanol (1) + HMIMBF₄ (2)} and {methanol (1) + OMIMBF₄ (2)} mixtures were calculated from experimental data from the following relationship: $C_{p}^{E} = C_{p} - (x_{1} C_{p 1}^{\circ} + x_{2} C_{p 2}^{\circ}),$ where C_p, $C_{p}^{\circ}$ , and x denote molar heat capacity of the mixture,

Discussion

Figure 2 shows the dependences of excess heat capacities of the systems under investigation on the molar fraction of ionic liquids.

As is seen, the molar heat capacities of the examined systems, both (HMIMBF₄ + methanol) (figure 2a) and (OMIMBF₄ + methanol) (figure 2b), show clear positive deviations from additivity. However, one can observe very small negative values of $C_{p}^{E}$ for the {HMIMBF₄ + methanol} system at x₂ = 0.88 (table 2). In addition, curves $C_{p}^{E} = f (x_{2})$ show asymmetry with a maximum at about x₂

Conclusions

Using the molar isobaric heat capacities of (methanol + 1-hexyl-3-methylimidazolium tetrafluoroborate) and (methanol + 1-methyl-3-octylimidazolium tetrafluoroborate) mixtures, the excess molar heat capacities of these mixtures were determined and satisfactorily fitted at several selected temperatures to the Redlich–Kister equation. In both systems under investigation, (HMIMBF₄ + MeOH) and (OMIMBF₄ + MeOH), positive deviations from the additivity of molar heat capacities were observed, which suggests

Acknowledgements

I thank Prof. H. Piekarski and Dr. P. Goralski for their helpful suggestions on the manuscript.

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Heat capacities of the mixtures of ionic liquids with methanol at temperatures from 283.15 K to 323.15 K

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

Acknowledgements

Fluid Phase Equilibr.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

Fluid Phase Equilibr.

Pure Appl. Chem.

J. Phys. Chem. B

Phys. Chem. Chem. Phys.

Phys. Chem. Chem. Phys.

Phys. Chem. Chem. Phys.

J. Solution Chem.

J. Solution Chem.