Heat capacities of the mixtures of ionic liquids with acetonitrile
Introduction
Since the 90s of the previous century the chemical industry has been increasingly interested in ionic liquids, mainly because they can replace volatile organic solvents that are used in huge amounts and constitute the main source of environment pollution. With respect to chemical structure, ionic liquids are salts composed of an organic cation (as a rule, a large and asymmetric one) and an inorganic or organic anion, being liquid at a temperature below T = 373 K (often already at room temperature). Ionic liquids are characterized by a high thermal as well as chemical stability. Moreover, they are practically non-volatile, non-flammable, and apart from few exceptions, non-explosive, owing to which they meet the most significant requirements imposed on them by “green chemistry” [1], [2], [3], [4]. Ionic liquids are often called “designed solvents” [5], as by appropriate choice of anion and cation, it is possible to design an ionic liquid with specified physical and chemical properties such as melting point, viscosity, density, and miscibility with water or organic solvents [3], [6]. This extends the range of their use and makes them even more competitive in relation to conventional solvents being currently used. The use of ionic liquids as solvents makes it possible to carry out reactions that were so far difficult or impossible to perform. Reactions carried out in the medium of an ionic liquid proceed as a rule at lower temperature with a less consumption of catalyst and result in products with appropriate yield and selectivity [1], [2], [7], [8], [9], [10].
At present the most popular ionic liquids consist of 1,3-dialkylimidazolium cations and various anions. This is due to the fact that these salts show an exceptional chemical and thermal stability (their decomposition takes place often over T = 573 K), they are liquid within a very wide range of temperature and their synthesis is relatively simple and many are commercially available. In order to utilize the salts containing 1,3-dialkylimidazolium cation on a larger scale in the chemical industry, it is required to know precisely their physical and chemical properties.
The amount of thermo-chemical information in the scientific literature concerning even pure ionic liquids is insufficient. There are still few experimental values concerning the mixtures of ionic liquids and popular organic solvents. Thermo-chemical studies on pure ionic liquids (in particular 1-alkyl-3-methylimodazolium tetrafluoroborates) and their mixtures with water and organic solvents have been carried out by us for several years. They include among others the measurements of heats of solution of ionic liquids in conventional solvents [11], the thermal stability of ionic liquids and measurements of molar heat capacities of both pure ionic liquids [11], [12], and their mixtures with molecular solvents (classical organic solvents) [12]. The present study is a continuation of our previous investigations.
The available data concerning the mixtures of 1-alkyl-3-methylimodazolium tetrafluoroborates and a popular aprotic organic solvent such as acetonitrile relate mainly to their properties such as density [13], [14], [15], [16], viscosity [14], activity [17], [18], and osmotic coefficients [19], speed of sound [15], [16], but there is a lack of data concerning an important thermodynamic value such as molar heat capacity. The knowledge of heat capacity as a function of temperature is necessary to describe the system; it is also extremely useful in physical chemistry, technology and chemical engineering. Therefore the aim of the present study is to determine the heat capacities of the mixtures of two popular ionic liquids: 1-metyl-3-octylimidazolium tetrafluoroborate and 1-hexyl-3-methylimidazolium tetrafluoroborate with acetonitrile. The examination of the effect of conventional solvents on the physical and chemical properties of ionic liquids can provide valuable information about intermolecular interactions in the liquid phase and they are also necessary to determine their potential use in various chemical processes.
Section snippets
Experimental
Ionic liquids (ILs): 1-hexyl-3-methylimidazolium tetrafluoroborate (HMImBF4) and 1-methyl-3-octylimidazolium tetrafluoroborate (OMImBF4), both from Fluka (minimum 0.97 mass fraction purity), were dried under vacuum and then stored in desiccators. The mass fraction of water determined by the Karl Fisher method was less than 2 · 10−3.
Acetonitrile (MeCN), from Aldrich (anhydrous, minimum 0.998 mass fraction purity), was outgased under reduced pressure before use.
The specific isobaric heat capacities
Results
The specific isobaric heat capacities of binary mixtures of {MeCN (1) + 1-hexyl-3-methylimidazolium tetrafluoroborate (2)} and {MeCN (1) + 1-methyl-3-octylimidazolium tetrafluoroborate (2)} were measured over the whole concentration range from pure acetonitrile to pure ILs at atmospheric pressure and at temperatures from (283.15 to 323.15) K. The results were recorded at each 0.02 K, thus giving ca. 2000 data points over the temperature range studied. For clarity, only the values of the molar heat
Discussion
FIGURE 1, FIGURE 2 show the dependences of the excess molar heat capacities of the system under investigation on the molar fraction of ionic liquids for five temperatures of the range tested (283.15 to 323.15).
As seen, the molar heat capacities of all the examined mixtures: {MeCN (1) + HMIMBF4 (2)} (figure 1) as well as {MeCN (1) + OMIMBF4 (2)} (figure 2) show negative deviations from additivity. The curves for show also asymmetry with a minimum lying within the range 0.2 ⩽ x2 ⩽ 0.3.
Conclusions
The experimental values of specific heat capacities of {1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4) + acetonitrile (MeCN)} as well as {1-octylimidazolium tetrafluoroborate (OMIMBF4) and MeCN} were used to determine the excess molar heat capacities, which were satisfactorily fitted for several selected temperatures to the Redlich–Kister equation. In both systems under investigation, (HMIMBF4 + MeCN) and (OMIMBF4 + MeCN), negative deviations from the additivity of the molar heat capacities
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