Determination of activity coefficients at infinite dilution of organic solutes in the ionic liquid, trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl) trifluorophosphate, by gas–liquid chromatography
Introduction
The investigation of the use of ionic liquids as stationary phases in gas–liquid chromatography extends as far back as the early research studies of Poole et al. [1], where the use of ionic liquids as standard reference polar phases was shown to be favourable, and more recently in the work of Armstrong et al. [2], where the use of ionic liquids in separation science was advocated. In recent years the widespread potential applications of ionic liquids in the fields of transition metal and enzymatic catalysis, solvent extraction, organic synthesis, thermal energy storage, electrochemical devices, biochemical engineering etc. have been well-documented by many researchers in review articles [3], [4], [5]. The widely acknowledged potential use of ionic liquids in the above-mentioned applications stems from the favourable physico-chemical properties of this distinctive class of solvents.
Ionic liquids are constituted principally by ions, in an analogous fashion to conventional inorganic ionic substances; however, the incorporation of a bulky and asymmetric organic cation inhibits an ordered crystalline structure hence allowing for a large liquidus range. Ionic liquids also have negligible vapour pressures, favourable thermal stability, variable miscibility characteristics in both polar and non-polar organic and aqueous media, are non-corrosive, non-flammable and can be “tailor-made” for specific applications yielding an almost infinite number, around 1018 [6], of ionic liquids. Of great significance is their “greenness”, which favours their incorporation into industrial processes such as solvent-enhanced separations in the form of liquid–liquid extraction and extractive distillation, where many concerns over the detrimental impact of conventional organic solvents, which are volatile, flammable and not easily recyclable, have surfaced.
The screening of solvents for potential application in solvent-enhanced separation processes of organic liquid mixtures [8] can be achieved through the examination of the activity coefficients at infinite dilution,, of organic solutes. The use of the steady-state gas–liquid chromatographic technique for the determination of values has been employed by many researchers and has proved quite reliable in this respect [7]. Ionic liquids are particularly amenable for the determination of by this method because of their negligible vapour pressures, making ionic liquids an ideal stationary phase. In this research investigation, values have been determined at T = 308.15, 318.15 and 328.15 K for normal alkanes (pentane, hexane, heptane and octane), alkenes (1-hexene, 1-heptene and 1-octene), alkynes (1-hexyne, 1-heptyne and 1-octyne), cycloalkanes (cyclopentane, cyclohexane and cycloheptane), alcohols (methanol, ethanol and propanol) and benzene in the high molecular weight ionic liquid, trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl) trifluorophosphate. This represents the first set of activity coefficient data that has been measured for the phosphonium-based ionic liquid, whose structure is shown below:Values for the partial molar excess enthalpies at infinite dilution, , were also determined for the solutes from the temperature dependency of the . For the assessment of the feasibility of the use of the ionic liquid solvent in extractive distillation, the experimentalvalues are used to compute the values for the selectivity, of the ionic liquid solvent [8] for the separation of different mixture types, i.e. alkane–aromatic, cycloalkane–aromatic and alkane–alkene.
This work is part of an ongoing scientific foray [9], [10], [11], [12], [13] by this research group into the search for viable ionic liquid alternatives for molecular solvents presently employed in extractive distillation and solvent extraction (e.g. sulfolane and N-methylpyrrolidone).
Section snippets
Chemicals
The ionic liquid, trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl) trifluorophosphate, was purchased from Merck with a water content of less than 100 ppm. Prior to use, the ionic liquid was purified by subjecting the liquid to a low-pressure vacuum (10−4 Torr) for 6 h, to remove any traces of volatile contaminants including water. The organic solutes were used without any further purification due to the nature of the chromatographic process, where small amounts of impurities are separated
Theory
The equation proposed by Everett [16] and Cruickshank [17] shown below, was used for the determination of values for the solute (1) eluting in a carrier gas (2) through a non-volatile liquid solvent (3).In Eq. (1), VN is the net retention volume of the solute, po is the column outlet pressure (equal to atmospheric pressure), the term is the mean column pressure, n3 is the number of moles of the ionic liquid in the stationary phase,
Results
The values are presented in Table 3 for the trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl) trifluorophosphate ionic liquid stationary phase at T = 308.15, 318.15 and 328.15 K for the three different column loadings or moles of solvent in the column, i.e. n3 = 1.244, 0.9948 and 0.9954 mmol. The averaged values for over the three column loadings are presented in Table 4 to allow facilitate an analysis of the results. As in previous work [9], [10], [11], [15], the estimated error in the
Discussion
The values of the activity coefficients for all the solutes, apart from the alcohols at the lower temperatures, are below unity, indicating a strong affinity of the solutes for the ionic liquid stationary phase. This affinity is principally due to the long alkyl chains (hexyl and tetradecyl) present in the phosphonium-based cation, as well the interaction between the positive charge of the cation and especially the pi-electrons of the unsaturated hydrocarbons (alkenes, alkynes, benzene). In
Acknowledgement
The researchers wish to express their gratitude to the National Research Foundation of South Africa for financial support.
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