Physicochemical properties of two 1-alkyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide ionic liquids and of binary mixtures of 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide with methanol or acetonitrile

doi:10.1016/j.jct.2013.12.009

The Journal of Chemical Thermodynamics

Volume 71, April 2014, Pages 171-181

https://doi.org/10.1016/j.jct.2013.12.009 Get rights and content

Highlights

•
Physicochemical characteristics of two 1-alkyl-1-methylpyrrolidinium ILs.
•
Similarities and differences between pyrrolidinium and imidazolium imides.
•
Volume and surface properties of [BMPyr][NTf₂] + methanol or + acetonitrile.
•
Some properties of mixtures with [BMPyr][NTf₂] and [BMIM][NTf₂] are very close.

Abstract

The physicochemical characteristics of two homologous room temperature ionic liquids (RTIL, IL) 1-methyl-1-propylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide [MPPyr][NTf₂] and 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide [BMPyr][NTf₂] are presented. Based on density, speed of sound, viscosity and surface tension measured in the temperature range of (288.15 to 323.15) K some related parameters, like isobaric coefficient of thermal expansion, isentropic compressibility coefficient, parameters of Vogel–Fulcher–Tamman (VFT) equation, surface entropy and enthalpy were calculated. These results were compared with those of some homologs from the 1-alkyl-3-methyl imidazolium bis[(trifluoromethyl)sulfonyl]imide [C_nMIm][NTf₂] family in order to systematize the knowledge about investigated substances. In the next part volumetric and surface properties, like excess molar volumes and Gibbs surface excess of methanol or acetonitrile in binary systems with [BMPyr][NTf₂] were determined at three temperatures (288.15, 298.15 and 308.15) K. The results of these studies indicate that the volumetric quantities of binary mixtures and the surface properties obtained with same solvents in two different classes of ionic liquids pyrrolidinium bis[(trifluoromethyl)sulfonyl]imides and imidazolium bis[(trifluoromethyl)sulfonyl]imides are very similar.

Introduction

Ionic liquids (ILs) or room temperature ionic liquids (RTILs) are still one of the most popular class of compounds and are studied intensively [1], [2], [3], [4], [5], [6]. However, the knowledge about ionic liquids and their mixtures is still incomplete. The complexity of structure of cations and anions gives more than a few possible forms of intermolecular interactions in a pure state and in mixtures. Thus, the collection of physicochemical properties of ILs and their binary solutions with some molecular solvents seems to be a reasonable start point to complete information on this topic. Furthermore, careful analysis of the results and comparison with other systems is necessary.

To our best knowledge, the first characterization of 1-alkyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imides as a new class of ionic liquids was present by MacFarlane et al. [7]. It seemed to be a good alternative to imidazolium imides due to higher stability and an interesting behavior of electrical conductivity in their crystalline state. One of the most significant feature was the fact that the first room temperature liquid representative for this family 1-propyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide, [MPPyr][NTf₂] had a melting temperature T = 285 K, whereas the most popular member of 1-alkyl-3-methylimidazoles family [EMIM][NTf₂] was found to have melting temperature at T = 257.1 K [8]. Recently, it was also shown that pyrrolidinium salts are less toxic than imidazolium ILs [9]. Other studies of additional properties of pyrrolidinium imides indicated similarities and differences between these two classes of compounds, or more often between [BMIm][NTf₂] and [BMPyr][NTf₂]. The pyrrolidinium ring is non-aromatic, in contrast to imidazolium, and has only one nitrogen atom; [BMIm][NTf₂] and [BMPyr][NTf₂] have the same alkyl groups, but with different structures, what should manifests in different physicochemical properties of these substances. Despite of this, it was shown from small angle X-ray scattering that pyrrolidinium-based ionic liquids, especially longer alkyl chains homologs can aggregate in analogous way to imidazolium-based ILs caused creating spatially heterogeneous domains [10]. Kolbeck et al. [11] found the surface tension of [BMPyr][NTf₂] at T = 298.15 K is higher than that of [BMIm][NTf₂]. This phenomenon was explained using a postulate presented by Deyko et al. [12]. They found from estimated enthalpies of vaporization (table 3 in Ref. [12]) that Coulomb forces for [BMIm][NTf₂] and [BMPyr][NTf₂] are very similar, (75 and 74) kJ ⋅ mol⁻¹, respectively. In contrast, the van der Waals interactions in pyrrolidinium IL are stronger than in imidazolium one, (79 and 59) kJ ⋅ mol⁻¹, respectively. This causes presumably the higher enthalpy of vaporization and higher surface tension of [BMPyr][NTf₂] compared to the imidazolium equivalent IL. Shimizu et al. [13] report some thermal properties of [BMIm][NTf₂] and [BMPyr][NTf₂]. It was supposed that the presence of methyl and butyl groups attached to the same charged nitrogen atom in a pyrrolidinium ring is probably responsible for weakening cation–anion interactions and for the lower melting point of [BMPyr][NTf₂]. According to Vranes et al. [14] [BMPyr][NTf₂] is more viscous than [BMIM][NTf₂] and has a lower electrical conductivity, which promotes the idea about stronger not weaker ion–ion interactions. In this work also the isobaric coefficient of thermal expansion of [BMPyr][NTf₂] was found as lower than of [BMIM][NTf₂]. On the other side, Kumełan et al. [15] found that the solubility of CO₂ in [BMPyr][NTf₂] is larger than in [BMIM][NTf₂]. This is caused probably by stronger interactions between the dissolved gas molecule and the anion of the former ionic liquid, accompanied by weaker cation–anion interactions. Moreover, presumably due to stronger interactions between the IL and alcohol, the UCST of the binary mixture of 1-butanol with [BMPyr][NTf₂] is higher than that with [BMIM][NTf₂] [16], which is in agreement with the theoretical predictions provided by Morrow and Maginn [17]. The situation of only [BMPyr][NTf₂] is not unambiguous, which becomes visible when a broader characterization of IL is considered.

In our present work, we have carried out comparative studies of some experimental and calculated properties like density, speed of sound, isobaric coefficient of thermal expansion, isentropic compressibility coefficient, parameters of Vogel–Fulcher–Tamman (VFT) equation, surface tension, surface entropy and enthalpy of two representative ILs from 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide family with propyl- an butyl-chain, [MPPyr][NTf₂] and [BMPyr][NTf₂] at few temperatures.

We also investigated volumetric and surface properties of binary mixtures of [BMPyr][NTf₂] with the protic and aprotic molecular solvent: methanol and acetonitrile. In the literature some data of binary mixtures of [BMIM][NTf₂] with methanol and with acetonitrile are reported. They are viscosities and conductivities of the solutions; these later are significantly higher for acetonitrile mixtures at low concentrations of IL (x_IL = 0.2–0.3) [18]. What is more, for [BMIM][NTf₂] + acetonitrile system at these concentrations a minimum on V^E(x) occurs, and a significant change of σ(x) curves is observed [19], [20]. We intend to check a behavior of these properties for [BMPyr][NTf₂] + methanol, or + acetonitrile system. Additionally, our results for [BMPyr][NTf₂] + methanol are compared with the proper quantities for binary mixtures of [BMPyr][NTf₂] with other homologous alcohols [21] in order to check the systematic change with the elongation of alkyl chains.

Section snippets

Chemicals

The basic specifications of chemicals used in an experimental work are given in table 1. The structures of ionic liquids 1-methyl-1-propylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide [MPPyr][NTf₂] and 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide [BMPyr][NTf₂] (Ionic Liquids Technologies GmbH, Io-li-tec, Germany) are presented in figure 1. Prior to the experiments the ILs were treated under vacuum for around 48 h (optionally with heating at T = 333 K) or even longer until

Pure ionic liquids

Data for densities, surface tensions, speeds of sound and kinematic viscosities of [MPPyr][NTf₂] and [BMPyr][NTf₂] (from 288.15 to 323.15 K) are listed in table 2. The temperature dependence of these quantities was described using the polynomial: $q = \sum_{i = 0}^{n = 2} a_{i} \cdot {(T / K)}^{i},$ where q = ρ/(g ⋅ cm⁻³), σ/(mN ⋅ m⁻¹) or c/(m ⋅ s⁻¹).

Coefficients of equation (1) are collected in table 3. The root-mean-square deviation was calculated according to equation: $δ (q) = {[\sum_{i = 1}^{N} {(q_{i, \exp} - q_{i, cal})}^{2}/ N]}^{1 / 2},$ where N – is the number of data

Conclusions

The results of analysis of the studied physicochemical properties of two pure 1-alkyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide ionic liquids [MPPyr][NTf₂] and [BMPyr][NTf₂] reveal different characteristics than of 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ionic liquids. They have lower densities than [C_nMIm][NTf₂] ILs and lower isobaric coefficient of thermal expansion with positive slope of its temperature dependence. They are characterized by higher

Acknowledgements

The Authors are grateful to Dagmar Klasen and Anika Rose for supporting the experimental work.

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Physicochemical properties of two 1-alkyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide ionic liquids and of binary mixtures of 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide with methanol or acetonitrile

Highlights

Abstract

Introduction

Section snippets

Chemicals

Pure ionic liquids

Conclusions

Acknowledgements

J. Chem. Thermodyn.

J. Chem. Thermodyn.

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J. Chem. Thermodyn.

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Thermochim. Acta

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J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

J. Chem. Thermodyn.

Science

J. Phys. Org. Chem.

Pure Appl. Chem.

Ionic Liquids in Synthesis

Green Chem.

J. Phys. Chem. B

J. Phys.: Condens. Matter

Ecotoxicology

J. Phys. Chem. Lett.

J. Phys. Chem. B

Phys. Chem. Chem. Phys.

J. Phys. Chem. B

J. Chem. Eng. Data

J. Chem. Eng. Data

J. Chem. Eng. Data

J. Phys. Chem. B

J. Chem. Eng. Data

J. Solution Chem.

J. Chem. Eng. Data