Solubilities of some organic solutes in 1-ethyl-3-methylimidazolium acetate. Chromatographic measurements and predictions from COSMO-RS
Highlights
► New solubility data are reported. ► Two experimental procedures for measuring solubilities are used. ► The experimental results from both procedures are compared. ► Experiments are compared with the theoretical calculations.
Introduction
Applications of room-temperature ionic liquids (ILs) have gained much interest during the past decade. Their low melting point, thermal stability, negligible vapor pressure and solvent power for polar and non-polar organic and inorganic solutes make them suitable for separation processes as substitutes for conventional volatile solvents. Ionic liquids are useful as media for chemical reactions, for batteries and electrochemistry [1], [2], [3]. Ionic liquid 1-ethyl-3-methylimidazolium acetate [EMIM][Ac] provides a non-toxic and non-corrosive medium for dissolution of cellulose from lignocellulosic biomass in a pretreatment process for a biorefinery. Unlike the chloride anion, the acetate anion is tolerated by enzymes used for hydrolysis of cellulose to yield glucose. [EMIM][Ac] is also useful as an absorbent for carbon dioxide from flue-gas streams [3].
An ionic liquid typically consists of an organic cation and an inorganic anion. Numerous possible combinations of various ions result in different properties such as melting point, viscosity, and solvent power for various solutes. Because of their desirable properties, cations based on alkylimidazolium and pyridinium have become popular. The search for task-specific ionic liquids has generated a substantial amount of research in recent years [1], [2].
Because [EMIM][Ac] is a promising solvent for a variety of applications, this work reports experimental measurements for infinite-dilution activity coefficients and Henry’s constants for some typical organic solutes in [EMIM][Ac].
Gas–liquid chromatography provides a useful method for measuring activity coefficients of volatile solutes in a non-volatile solvent such as a polymer or an ionic liquid. In this work we used two chromatographs: one uses a packed column (PC) where the packing is coated with ionic liquid, while the other uses an open-tubular capillary column (OTWC) where the inside wall of a capillary tube is coated with ionic liquid. Our experimental work is similar to that of previous studies [4], [5], [6], [7], [8] using packed-column gas–liquid chromatography to determine infinite-dilution activity coefficients of organic solutes in several ionic liquids. However, the previous studies did not include [EMIM][Ac].
This work presents a comparison of results obtained by a conventional packed column with those obtained by an OTWC column. The latter has an advantage relative to the former as discussed later. We report new experimental data for infinite-dilution activity coefficients for 16 organic solutes: n-hexane, 1-hexene, cyclohexane, heptane, benzene, toluene, ethyl acetate, tetrahydrofuran, 1,4-dioxane, acetonitrile, nitromethane, 1,2-methoxyethane, 1-propanol, 2-propanol, t-butanol and t-amyl alcohol in [EMIM][Ac] from T = 313 K to T = 413 K. We compare our experimental results with those predicted by COnductor-like Screening MOdel for Real Solvents (COSMO-RS) [9].
Section snippets
Materials
Analytic-grade organic solutes, purity ⩾0.999 (mole fraction), were purchased from Sigma–Aldrich. No significant peaks for impurities were observed during measurements on the GC.
[EMIM][Ac] was purchased from Io-li-tech with certified purity >0.95 (mass fraction) and having water content of about 4 wt%. Before use, the IL was dried in an oven at elevated temperature for about 12 h. Nuclear magnetic resonance studies of the dried IL was conducted to make sure that the columns received coatings of
Results and discussion
TABLE 2, TABLE 3 present infinite-dilution activity coefficients and Henry’s constants Hi for 9 solutes in [EMIM][Ac] obtained from packed column measurements. Results from the column with 25% IL loading agree well with those from the column with 35% IL loading. Infinite-dilution activity coefficients decrease with rising temperature for n-hexane, 1-hexene and cyclohexane but increase for all other solutes studied in this work. However, Henry’s constants for all solutes studied here
Conclusions
New experimental data are presented for infinite-dilution activity coefficients and Henry’s constants for 16 solutes in [EMIM][Ac] from 313 K to 413 K using two packed columns with different loadings and one OTWC column. The OTWC column gives results very close to those obtained from the packed columns. However, because the OTWC column has a smaller volume, retention times in the OTWC are smaller than those in a packed column. Thus, an OTWC column is more convenient than a packed column for
Acknowledgements
The authors are grateful to the Energy Biosciences Institute and to the Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory for financial support, and to Prof. Alexis Bell and coworkers for general assistance. We much appreciate the advice of Dr. Sasisanker Padmanabhan and the helpful critical comments of Prof. Clayton Radke. We thank Prof. Michael Manga for providing the density meter. We thank the German Academic Exchange Service (DAAD) for providing a foreign
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Exchange student from Technische Universität Hamburg-Harburg, 21071 Hamburg, Germany.