Osmotic coefficients and apparent molar volumes of 1-hexyl-3-methylimidazolium trifluoromethanesulfonate ionic liquid in alcohols
Introduction
Ionic liquids (ILs) are salts with a melting point lower than 100 °C. As it is known, these solvents have interesting properties, which make them attractive for a number of applications. Some of these properties are their negligible vapor pressure, their non-flammability, their chemical stability and their excellent solubility for organic and inorganic compounds. Due to their relative novelty, there is still a lack of experimental data, especially concerning physical properties and phase behavior, which becomes vital for the application of ILs in industrial processes or for developing thermodynamic models.
Studies of thermodynamic properties of mixtures containing ILs mixed with other compounds are crucial because they provide useful information to understand the behavior of these liquid salts in common solvents. As example, density and derived properties are usually required to develop specific thermodynamic models and they are also of great relevance in assessing intermolecular interactions between the ionic liquid and the solvent. The calculation of osmotic and activity coefficients allows to analyze the nonideality of mixtures, studying their thermodynamics, and to test the usually used thermodynamic tools. To date, most of the works are based on the study of physical properties of ILs in water, while the studies of non-aqueous mixtures containing ILs are still not abundant [1], [2], [3], [4], [5].
This paper is an extension of our studies on volumetric and osmotic properties of ILs in alcohols [6], [7]. In this work, densities for the binary mixtures {1-propanol (1), or 2-propanol (1), or 1-butanol (1), or 2-butanol (1), or 1-pentanol (1) + 1-hexyl-3-methylimidazolium trifluoromethanesulfonate (2)} are reported at T = 323.15 K and atmospheric pressure. From these data, the corresponding apparent molar volumes were obtained and fitted to a Redlich–Meyer type equation [8]. On the other hand, a vapor pressure osmometry technique was used to obtain osmotic and activity coefficients and vapor pressures of the above mentioned binary mixtures. The Extended Pitzer model modified by Archer [9], [10] was used for modeling the experimental data. Finally, the mean molal activity coefficients and the excess Gibbs free energy for the studied binary mixtures were calculated from the obtained parameters.
The ionic liquid studied in this work was selected in order to compare the experimental results obtained here with those previously published for the binary mixtures of 1-butyl-3-methylimidazolium trifluoromethanesulfonate in alcohols [7]. The comparison of these results allows evaluating the effect of the alkyl chain length of the cation on the properties reported in this work.
To our knowledge, some papers containing physical property data for the pure ionic liquid were previously published [11], [12], [13]; nevertheless, the experimental data reported in this work were not previously determined.
Section snippets
Chemicals
The ionic liquid 1-hexyl-3-methylimidazolium trifluoromethanesulfonate, [HMim][TfO], was supplied by Iolitec GmbH (Germany) with a purity higher than 0.990 by mass. Prior to its use, this compound was dried under vacuum (p = 2 · 10−1 Pa; T = 323.15 K). The water content, ww, for the dried ionic liquid was 520 ppm.
Alcohols were purchased in specialized companies. They were ultrasonically degassed and dried over molecular sieves type 4 · 10−10 m, supplied by Aldrich, and kept in bottles under inert
Apparent molar volume
In this work, densities, ρ, for the binary systems of [HMim][TfO] in alcohols (1-propanol (1), or 2-propanol (1), or 1-butanol (1), or 2-butanol (1), or 1-pentanol (1)) were measured at T = 323.15 K and atmospheric pressure. The experimental data, which are reported in table 2, show that this physical property increases with molality, following a similar behavior to that presented by alcoholic solutions of “classical” electrolytes [17], [18]. Moreover, density data decrease in the order: 1-pentanol
Conclusions
In this study, densities and apparent molar volumes for mixtures containing [HMim][TfO] ionic liquid and alcohols (1-propanol, or 2-propanol, or 1-butanol, or 2-butanol, 1-pentanol) are reported at T = 323.15 K. A Redlich–Meyer type equation was applied to fit the apparent molar volumes. Then, the corresponding apparent molar volumes at infinite dilution were also calculated.
The obtained results show that apparent molar volume for [HMim][TfO] in primary and secondary alcohols increases with
Acknowledgements
This work is partially supported by project PEst-C/EQB/LA0020/2011, financed by FEDER through COMPETE – Programa Operacional Factores de Competitividade and Fundação para a Ciência e a Tecnologia – FCT (Portugal). Emilio J. González and Noelia Calvar are thankful to FCT for awarding their postdoctoral grants (SFRH/BPD/70776/2010 and SFRH/BPD/37775/2007, respectively).
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