Gas-liquid chromatography measurements of activity coefficients at infinite dilution of hydrocarbons and alkanols in 1-alkyl-3-methylimidazolium bis(oxalato)borate
Introduction
This work is a continuation of our investigations on the determination of activity coefficients at infinite dilution for many organic solutes in ionic liquids using gas-liquid chromatography. Our previous work includes measurements of for organic solutes in the ionic liquids (ILs): 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate [EMIM][FAP] [1] and 1-ethyl-3-methylimidazolium tetracyanoborate [EMIM][TCB] [2]. Due to their unique properties such as no flammability, wide liquid range, stability at high temperatures, and negligible vapor pressure, so not surprisingly, one of the most significant applications for ionic liquids is using them as solvents for clean liquid–liquid extraction replacement for conventional volatile, flammable and toxic organic solvents [3], [4]. Activity coefficients at infinite dilution of a solute i is a suitable and widely accepted approach for quantify the volatility of the solute as well as to provide information on the intermolecular energy between solvent and solute [5], [6]. Since ILs have a negligible vapor pressure, the gas-liquid chromatography (GLC) using the ionic liquid as stationary phase has been proven to be a good method for measuring activity coefficients at infinite dilution .
Until now, there are so many publications about the ILs based on some common anions, such as bis(trifluoromethanesulfonyl)imide[NTf2−] [7], [8], [9], [10], [11], [12], [13], tetrafluoroborate [BF4−] [14], [15], [16], [17], hexafluorophosphate [PF6−] [18] and trifluoromethanesulfonate [CF3SO3−] [19], [20], [21]. In order to expand our knowledge about the nature of ILs, influence of the anion structure on the thermodynamic properties of the disubstituted imidazolium based ionic liquid with bis(oxalato)borate [BOB−] anion was studied in this work. For this study, activity coefficients at infinite dilution, values for 23 solutes: alkanes, cycloalkanes, 1-alkenes, 1-alkynes, benzene, alkylbenzenes, and alcohols in the ionic liquids 1-butyl-3-methylimidazolium bis(oxalato)borate [BMIM][BOB] and 1-hexyl-3-methyl-imidazolium bis(oxalato)borate [HMIM][BOB] have been determined over the temperatures range from (308 to 348) K. The partial molar excess enthalpies at infinite dilution, , of these solutes in the ionic liquids were also derived from the temperature dependence of the values. This work also intends to provide information about the slight modification in the length of these substituents and its influence on the value of and selectivity of ionic liquids based on the bis(oxalato)borate anion.
Section snippets
Materials
1-Alkyl-3-methylimidazolium bis(oxalato)borate (R = butyl, hexyl) were synthesized from 1-alkyl-3-methylimidazolium bromide (R = butyl, hexyl) with lithium bis(oxalato)borate (LiBOB) according to the procedures [22]. A silver nitrate test was performed on the RTIL and indicated that a less than 50 ppm of bromide ion impurity existed in the RTIL. The ionic liquids were purified and dried under high vacuum at 343 K for 24 h to remove organic solvents and water, and its structure was confirmed by 1H NMR,
Theoretical basis
The equation developed by Everett [24] and Cruickshank et al. [25] was used in this work to calculate the of solutes in the ionic liquid:VN is the standardized retention volume of the solute, Po is the outlet pressure, n3 is the number of moles of the ionic liquid on the column packing, T is the column temperature, is the saturated vapor pressure of the solute at temperature T, B11 is the second virial coefficient of the pure solute,
Results and discussion
Table 1, Table 2 list the values of of different solutes (alkanes, cycloalkanes, 1-alkenes, 1-alkynes, benzene, alkylbenzenes, and alcohols) in the ionic liquids [BMIM][BOB] and [HMIM][BOB] in the temperature range from 308 to 348 K and partial molar excess enthalpies at infinite dilution , determined from the Gibbs–Helmholtz equation:
Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8 show the natural logarithm of the activity coefficients in the ionic
Conclusions
Activity coefficients at infinite dilution for various solutes (alkanes, cycloalkanes, 1-alkenes, 1-alkynes, benzene, alkylbenzenes, and alcohols) in the ionic liquids 1-butyl-3-methylimidazolium bis(oxalato)borate [BMIM][BOB] and 1-hexyl-3-methyl-imidazolium bis(oxalato)borate [HMIM][BOB] were measured in the temperature range from 308 K to 348 K using GLC method. These results show the influence of the cation's and anion's alkyl chain length on the and values. For the separation of
Acknowledgments
Funding for this research was provided by National Natural Science Foundation of China (NSFC No. 20901076), Knowledge Innovation Program of the Chinese Academy of Sciences, DICP (Grant No. K2009D03), and State Key Laboratory of Fine Chemicals (KF0811).
References (35)
- et al.
J. Chem. Thermodyn.
(2010) - et al.
Fluid Phase Equilib.
(1991) - et al.
J. Chem. Thermodyn.
(1976) - et al.
J. Chem. Thermodyn.
(2010) - et al.
Fluid Phase Equilib.
(2009) - et al.
J. Chem. Thermodyn.
(2009) - et al.
J. Chem. Thermodyn.
(2008) - et al.
J. Chem. Eng. Data
(2010) - et al.
Chem. Commun.
(1998) - et al.
Chem. Commun
(2001)