Excess molar volumes and isentropic compressibility of binary systems {trioctylmethylammonium bis(trifluoromethysulfonyl)imide + methanol or ethanol or 1-propanol} at different temperatures

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Abstract

This paper reports measurements of densities for the binary systems of an ionic liquid and an alkanol at T = (298.15, 303.15, and 313.15) K. The IL is trioctylmethylammonium bis(trifluoromethylsulfonyl)imide [OMA]+[Tf2N] and the alkanols are methanol, or ethanol, or 1-propanol. The speed of sound at T = 298.15 K for the same binary systems was also measured. The excess molar volumes and the isentropic compressibilities for the above systems were then calculated from the experimental densities and the speed of sound, respectively. Redlich–Kister smoothing polynomial equation was used to fit the excess molar volume and the deviation in isentropic compressibility data. The partial molar volumes were determined from the Redlich–Kister coefficients. For all the systems studied, the excess molar volumes have both negative and positive values, while the deviations in isentropic compressibility are negative over the entire composition range.

Introduction

Ionic liquids (ILs) are organic salts, which results from a combination of organic cations and various anions. As numerous combinations of cations and anions are possible, they are considered as “designer solvents” thereby optimizing the ionic liquid physical properties for a specific application [1]. ILs have widely tuneable properties with regard to polarity, hydrophobicity, and solvent miscibility through the appropriate selection of the anion and cation. The ILs that are moisture and air stable (e.g. with [Tf2N] anion) at room temperature have potential uses for new chemical processes and technologies [2], [3].

Recently, there have been numerous publications about ILs properties or their application in catalytic reactions, separation processes, membrane technology, chemical analysis, batteries, solar cells, or as lubricants. The information about physical and thermodynamic properties of ionic liquids is limited because, the number of ILs are large. These new solvents are highly hydroscopic, and small quantities of water or other compounds in ionic liquids cause considerable changes in the physical properties. For this reason, to ensure the purity of ionic liquids, the samples should be prepared and stored in an inert atmosphere.

To design any process involving ILs on an industrial scale, it is necessary to know their thermodynamic or physico-chemical properties such as density, speed of sound, activity coefficients at infinite dilution. Since it is impossible to measure every combination of IL systems, it is necessary to make measurements on selective systems to provide results that can be used to develop correlations and predictive methods [1], [4], [5].

Measurements of the speed of sound, u, in liquids is a powerful source of information (e.g. to detect small changes in gas composition or the effects of small concentrations changes) about the thermo physical properties of chemical substances and their mixtures [6]. The excess molar volume VmE, data can be used to predict (vapour + liquid) equilibria using appropriate EOS models [7].

In this work the VmE were determined for the binary system (ionic liquid + an alkanol) over the entire composition range at T = (298.15, 303.15, and 313.15) K and the speed of sound u at T = 298.15 K and 1 MHz. The IL used was trioctylmethylammonium bis(trifluoromethylsulfonyl)imide [OMA]+[Tf2N] and the alkanols were methanol, or ethanol, or 1-propanol. To our best knowledge no thermodynamic data is available in the literature for the pure IL or its solution with solvents. The Redlich–Kister smoothing equation was used to fit the experimental data.

The results are discussed in terms of intermolecular forces. The partial molar volumes were calculated from the Redlich–Kister correlation coefficients.

This work is a continuation of our research group’s work on thermodynamic properties of ILs [8], [9], [10], [11], [12], [13].

The structure of the ionic liquid used in this work is presented in figure 1.

Section snippets

Materials

The chemicals, suppliers, purity, the literature, and experimental densities (ρ) are given in table 1. The densities of pure liquids and the mixtures were determined at T = (298.15, 303.15, and 313.15) K and at atmospheric pressure. The molar mass of the IL is 648.34 g · mol−1.

The ionic liquid {trioctylmethylammonium bis(trifluoromethyl-sulfonyl)imide [OMA]+[Tf2N], ethanol, and 1-propanol were used without any further purification. Methanol was first dried with potassium carbonate and then distilled

Results and discussion

The excess molar volumes of the studied systems were calculated from the experimental density values, using the following equation:VmE=x1M1+x2M2ρ-x1M1ρ1-x2M2ρ2,where x1 and x2 are mole fractions, M1 and M2 are molecular masses, ρ1 and ρ2 are densities of the pure components 1 and 2, respectively, where 1 refers to IL and 2 to the alkanol, and ρ is density of the mixtures.

The Redlich–Kister equation was used to fit the experimental data, using a commercial software programme (MathCAD) to obtain

Conclusion

In this paper the densities were measured at T = (298.15, 303.15, and 313) K, over the entire composition range for the three binary systems (ionic liquid + methanol, or ethanol, or 1-propanol). The excess molar volumes were calculated from the density measurements and the Redlich–Kister smoothing equation was used to fit the experimental data. The speed of sound for the three binary systems was measured at T = 298.15 K. The results were interpreted in terms of the alcohol chain length and, it was

Acknowledgement

The authors acknowledge the National Research Foundation, South Africa for funding.

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