Density, electrical conductivity, viscosity and excess properties of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide + propylene carbonate binary mixtures

doi:10.1016/j.jct.2013.08.034

The Journal of Chemical Thermodynamics

Volume 68, January 2014, Pages 98-108

https://doi.org/10.1016/j.jct.2013.08.034 Get rights and content

Highlights

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Densities of [bmim][NTf₂] mixtures with propylene carbonate were measured.
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Excess properties were calculated.
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Formation of hydrogen bonds between IL and PC was discussed.
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Electrical conductivity and viscosity were also measured.
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Influence of temperature and composition on mixture properties were studied.

Abstract

Densities of binary liquid mixtures of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim][NTf₂], with propylene carbonate (PC) were measured at temperatures from (293.15 to 323.15) K and at atmospheric pressure over the whole composition range. The electrical conductivity was measured in the range from (293.15 to 328.15) K. Also, viscosity of [bmim][NTf₂] + PC binary mixtures was measured from (298.15 to 333.15) K. Excess molar volumes, V^E, have been obtained from the experimental densities and were fitted to Redlich–Kister polynomial equation. Other volumetric properties, such as isobaric thermal expansion coefficients, partial molar volumes, apparent molar volumes and partial molar volumes at infinite dilution have been also calculated, in order to obtain information about interactions between PC and selected ionic liquid. Results are discussed in order to understand the hydrogen bonds formation between components of the mixture.

Introduction

Due to the noteworthy physical and chemical properties, ionic liquids (ILs) as a class of chemical compounds formed principally of ions, gained great interest of the scientific community [1], [2], [3], [4]. Increasing attention has been paid to their application as green solvents, since many of them are non-volatile, biodegradable and recyclable materials [5]. Their benefits are reflected in the retention and stability of the liquid state in the high temperature range, because of the low melting point and high temperature of degradation [6], [7], [8], [9], [10], [11]. Also, ionic liquids have suitable solvation properties, air resistivity, high electrical (ionic) conductivity, low vapor pressure and selectivity in different organic reactions [12], [13], [14], [15], [16]. Thus, these noteworthy physicochemical properties of ILs open the possibility of a wide range of their commercial applications [17], [18], [19], [20], [21], electrochemical extractions [22], [23] and battery production [24], [25], [26]. Recently, ILs have been used as electrolytes in solar and fuel cells [27].

However, a limiting factor in the application of ionic liquids is their high viscosity [28], which reduces transport capabilities and slow down the chemical processes that occur in these solvents. The high viscosity of ILs can be overcome by mixing with some molecular solvents [29], [30], [31], [32], [33], such is propylene carbonate (PC). Binary mixtures of ILs with molecular liquids are widely used as the electrolytes in lithium-ion batteries [32], [33] or as a key component in smaller portable electronic devices [34], [35], [36], [37] incorporated in electric vehicles and hybrid electric vehicles [38], [39], [40], [41], [42], [43], [44], [45], [46], [47]. Flammable electrolytes in lithium-ion batteries were used so far [48], [49], [50], [51], [52], [53], [54]. Propylene carbonate can be used as a component in these batteries, since the polarity of PC provides excellent solvation of lithium ions and thus the increasing electrical conductivity. Propylene carbonate has a low melting point (−49.7 °C), high flash point (132 °C) and high relative permittivity, which favors dissociation of electrolytes [55], [56]. The limiting factors are the narrow temperature range where PC can be used [57], [58] and the formation of solid electrolyte interphase. Thus, further physicochemical characterization of PC binary mixtures with different ILs is required [59], [60].

In this study, the thermodynamic properties of selected ionic liquid namely 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim][NTf₂] mixed with propylene carbonate have been investigated. Volumetric, thermal and transport properties are presented as a function of temperature and composition.

Section snippets

Materials

Ionic liquid [bmim][NTf₂] (Merck, mass fraction purity ⩾ 0.99) was used after determination of the water content. The water content in commercial IL was determined using a 831 Karl Fischer coulometer showing (0.05306 ± 0.0002)% mass fraction of water. After drying, the water content was observed to be less than 10⁻⁴% mass fraction. Propylene carbonate (Baker, mass fraction purity ⩾ 0.99) was used without prior treatment. Chemicals were stored in a desiccator above P₂O₅. The summary of the provenance

Densities

Densities of pure components and ([bmim][NTf₂] + PC) mixtures were measured as a function of temperature in the range from (293.15 to 323.15) K and the results are shown in TABLE 2, TABLE 3 and also in figure S1 in the supporting information. It may be noted that the density decreases with the increasing temperature and increases with the mole fraction of ionic liquid in the binary mixture. Density data obtained were fitted as a function of temperature using a linear fit. These parameters are

Conclusions

Volumetric properties of binary liquid mixtures of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with propylene carbonate were determined at various temperatures at atmospheric pressure in the whole composition range. Based on the experimental density values, the excess molar volumes, apparent and partial molar volumes and excess molar volumes at infinite dilution were calculated. Also, the coefficient of thermal expansion and the excess of thermal expansion coefficient were

Acknowledgments

This work was financially supported by the Ministry of Education and Science of Serbia under project contract ON172012 and The Provincial Secretariat for Science and Technological Development of APV.

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