Infinite dilution activity coefficients of volatile organic compounds in two ionic liquids composed of the tris(pentafluoroethyl)trifluorophosphate ([FAP]) anion and a functionalized cation
Introduction
During more than a decade, ionic liquids (ILs) have been promoting burgeoning research due to their special chemical and physicochemical properties offering prospects for great technological innovations. Since properties of these media can be controlled by almost countless structure variations of composing ions, still new ILs are synthetized and explored with the aim to improve their properties and tailor them for specific applications. In order to replace rather unstable hexafluorophosphate containing ILs, Merck developed and made commercially available a group of ILs based on tris(pentafluoroethyl)trifluorophosphate ([FAP]) anion [1]. The [FAP] based ILs exhibit excellent hydrolytic, thermal, and electrochemical stability as well as a lower water uptake and have been shown to provide advantages in various applications in electrochemistry [2], tribology [3], analytical chemistry [4], [5] and gas absorption [6], [7]. Studies on solvation properties of ILs containing [FAP] anion [8], [9] have indicated their great potential for use in solvent-aided separations, especially for ILs in which [FAP]− is paired with a functionalized counter-cation. Measurements of infinite dilution activity coefficients for a number of test solutes in [FAP] based ILs have been reported confirming this potential [10], [11], [12], [13], [14]. Notable in this respect are systematic measurements recently carried out by Marciniak and Wlazlo on 1-(3-hydroxypropyl)pyridinium [HO-PPY] [12], 4-(2-methoxyethyl)-4-methylmorfolinium [MO-EMMOR] [13], and 1-(2-methoxyethyl)-1-methylpiperidinium [MO-EMPIP] [14] [FAP]s.
In this work, we examine interactions of selected VOCs with two [FAP] based ionic liquids containing functionalized counter-cations, namely 1-(2-hydroxyethyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HO-EMIM][FAP]) and 1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([MO-EMPYR][FAP]).The methodology of the present investigation is closely parallel to that we used in our previous studies devoted to selected [EMIM] or [BMPYR] based ILs [15], [16], [17], [18], [19]. Here we thus report infinite dilution activity coefficients and gas–liquid partition coefficients measured by gas–liquid chromatography (GLC) for the same representative set of 30 selected hydrocarbons, alcohols, ketones, ethers, esters, haloalkanes, and nitrogen- or sulfur-containing compounds in [HO-EMIM][FAP] and [MO-EMPYR][FAP] as a function of temperature. The obtained thermodynamic properties are analyzed to disclose intermolecular interactions governing the observed behavior and identify the potential of these ILs for use as entrainers in solvent-aided separations. Through comparisons with available literature data for unfunctionalized analogs of similar structure also effects brought by the functionalization are disclosed.
Concurrently, infinite dilution activity coefficients for some of the examined solutes in [MO-EMPYR][FAP] have been measured by Marciniak and Wlazlo and presented just now [20]. A detailed comparison of our results to their values is also presented.
Section snippets
Theory
In gas–liquid chromatography (GLC), the infinite dilution activity coefficient and the gas–liquid partition coefficient for a solute (1) partitioning between a carrier gas (2) and a non-volatile liquid solvent (3) are calculated from the solute retention according to the following equations [21]where T is temperature of the column, m3, M3 and mass, molar mass and density of the solvent,
Materials
The ionic liquids 1-(2-hydroxyethyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([OH-EMIM][FAP], M = 572.17 g · mol−1) and 1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([MO-EMPYR][FAP], M = 589.24 g · mol−1) were obtained in the high purity grade from Merck. Their purity of these ILs according to the producer’s specification was >0.99, the certified water content being ⩽100 · 10−6 (mass basis). The samples were handled with special precautions to avoid
Results
Infinite dilution activity coefficients and gas–liquid partition coefficients in [HO-EMIM][FAP] and [MO-EMPYR][FAP] were determined for a set of 30 selected solutes at (318.15, 323.15, 333.15, 343.15, and 353.15) K. The retention measurements were carried out on columns loaded with different amounts of the IL, namely 2.833 g (, L = 1.2 m) and 1.864 g (, L = 0.65 m) for [OH-EMIM][FAP], and 2.893 g (, L = 1.2 m) and 2.107 g (, L = 0.65 m) for [MO-EMPYR][FAP]. The values of and were
Comparison with existing data
Infinite dilution activity coefficients for some volatile organic compounds in [MO-EMPYR][FAP] have been reported by Marciniak and Wlazlo [20] during the final stages of preparation of this paper. Table 6 compares the present measurements with their data at the lowest and the highest temperature of our measurements. As seen, agreement between the two determinations is not entirely satisfactory. Considering that for both these determinations the declared relative standard uncertainty is 0.03 and
Conclusions
In this work, we have examined interactions of two ionic liquids containing [FAP] anion and a functionalized counter-cation, namely [HO-EMIM][FAP] and [MO-EMPYR][FAP], with various types of organic solutes and assessed the potential of these ILs to be used as solvents in separation processes. Through methodical GLC retention measurements, infinite dilution activity coefficients and gas–liquid partition coefficients of 30 hydrocarbons, alcohols, ketones, ethers, esters, haloalkanes, and
Acknowledgment
This work was funded from Ministry of Education of the Czech Republic through financial support for specific university research (MSMT No. 21/2012). The stay of E.F.O. at ICT, Prague was supported by EU Erasmus Scholarship and she thanks her teachers Dr. S. Pinho and Dr. O. Ferreira for preparing the way.
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Visiting Erasmus M.Sc. student from Escola Superior de Tecnologia e de Gestão, Instituto Politécnico de Bragança, Portugal.