Excess molar properties for binary systems of alkylimidazolium-based ionic liquids + nitromethane. Experimental results and ERAS-model calculations

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Abstract

Density, isobaric molar heat capacity, and excess molar enthalpy were experimentally determined at atmospheric pressure for a set of binary systems ionic liquid + nitromethane. The studied ionic liquids were: 1-butyl-3-methylimidazolium tetrafluoroborate, 1-hexyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylpyridinium tetrafluoroborate, 1-ethyl-3-methylimidazolium ethylsulfate, 1-butyl-3-methylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, and 1-butyl-3-methylimidazolium trifluoromethanesulfonate. Density and heat capacity were obtained within the temperature range (293.15 to 318.15) K whereas excess molar enthalpy was measured at 303.15 K; excess molar volume and excess molar isobaric heat capacity were calculated from experimental data. The ERAS-model was applied in order to study the microscopic mechanisms involved in the mixing process. Although the studied compounds are not self-associated, ERAS-model describe adequately the experimental results if cross-association between both compounds is considered.

Introduction

Study of room temperature ionic liquids (RTILs) as solvents has received an increasing attention in the last years from fundamental and applied points of view [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. As for thermodynamics of RTILs in solution with other compounds, there are many experimental and theoretical studies which deal on the behavior of thermodynamic properties of mixtures RTIL + common organic solvent [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]; however, due to the enormous number of possible binary systems, a lot of work remains to be done. Polar compounds present good solubility with ionic liquids; therefore, there are several works that study the thermodynamic properties of RTILs + polar compound, being the RTIL + CO2 system extensively studied due to its importance for supercritical extraction [16], [17], [18]. On the other hand, much effort in this field has been devoted to the study of RTIL + water and RTIL + alcohol binary systems, mainly motivated by their solubility with RTILs as well as their low environmental impact [19], [20], [21], [22], [23], [24], [25], [26], [27]. The thermodynamic properties of RTILs with other polar compounds have been studied in some extent [29], [30], [31], [32], [33], especially volumetric properties, but there are few works that study their thermodynamic behavior considering thermal properties as enthalpy or heat capacity.

The present work analyzes the excess molar enthalpy, excess molar volume, and excess molar isobaric heat capacity of RTIL + nitromethane systems by means of the ERAS-model. With this purpose, excess enthalpy, density, and heat capacity were experimentally determined for a set of RTIL + nitromethane binary systems. The RTILs were selected in order to be able to study the effect of changing the chemical structure of the cation and/or the anion on thermodynamic properties. To observe the influence of varying the chemical nature of only the cation or the anion, most of the selected RTILs present a common ion. Considering the bibliographic data available, the selected RTILs were: 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4]; 1-hexyl-3-methylimidazolium tetrafluoroborate, [hmim][BF4]; 1-butyl-3-methylpyridinium tetrafluoroborate, [bmpyr][BF4]; 1-ethyl-3-methylimidazolium ethylsulfate, [emim][EtSO4]; 1-butyl-3-methylimidazolium methylsulfate, [bmim][MetSO4]; 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, [emim][triflate]; and 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [bmim][triflate]. Density and isobaric molar heat capacities were measured in a temperature interval from (293.15 to 318.15) K at atmospheric pressure for all systems except for [bmpyr][BF4], since these data are available in literature [29]; from these, excess molar volume and excess molar isobaric heat capacity were calculated. Excess molar enthalpy was measured at 303.15 K and atmospheric pressure for all systems except for those containing [bmim][BF4] and [bmim][MetSO4] since there are available data in literature [31]. The experimental results are analyzed in the frame of the Extended Real Associated Solution theory, ERAS [34], [35], [36], with the hypothesis that RTILs and nitromethane cannot self-associate, but they can cross-associate. The obtained ERAS parameters are analyzed in order to determine the main microscopic processes that explain the values of the studied excess thermodynamic properties.

Section snippets

Chemicals

Nitromethane was obtained from Fluka with purity greater than 0.985 in mass fraction and was dried in Fluka 0.3 nm molecular sieves before use. All ionic liquids were purchased from Solvent Innovation and they were dried in vacuum at 333.15 K for 48 h prior to use in order to eliminate water and other volatile compounds. It was not observed decomposition of the RTILs at the experimental conditions. The water mass fraction of the dried RTILs was determined through Karl Fisher titration, and it was

Results

In table 1 the densities and isobaric molar heat capacities of pure compounds at 298.15 K are presented and compared to the literature data. As a rule, good agreement was achieved; only the heat capacity of [emim][EtSO4] differs significantly from that reported in literature. Taking into account the similar chemical structure of this RTIL and [bmim][MetSO4], the heat capacity of both compounds must be similar, and therefore, the value reported in literature seems to be wrong. TABLE 2, TABLE 3,

ERAS-model

The Extended Real Associated Solution Theory, ERAS, [34], [35], [36] was applied to the studied RTIL + nitromethane systems. Although this is a model devoted to describe associated compound properties and neither RTILs nor nitromethane self-associate, previous works have shown that it is possible cross-association between an RTIL and polar compound [18], [47], [48], [49], fact which have motivated the use of this model, assuming only cross-association between RTIL and nitromethane. The ERAS

Conclusions

Excess molar volume, heat capacity, and enthalpy were measured for a set of binary systems of RTIL + nitromethane, showing all the systems a quite similar behavior, which does not highly deviate from that of the ideal mixture. In order to get information about the microscopic processes involved in the mixing process, the ERAS-model was applied to experimental data under the assumption that both compounds do not self-associate but they cross-associate, obtaining good results for the studied excess

Acknowledgements

The authors are grateful to the Dirección Xeral de I + D da Xunta de Galicia (Project # PGIDIT-03-DPI-38301-PR) and to the Ministerio de Educación y Ciencia, Secretaría de Estado de Política Científica y Tecnológica del Gobierno Español (Projects # BFM2003-09295 and # MAT2006-12984) for financial support. G. García-Miaja is supported by the “Programme Alban, the European Union Programme of High Level Scholarships for Latin America”, Scholarship No. E05D056044MX.

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