Thermodynamics and activity coefficients at infinite dilution for organic compounds in the ionic liquid 1-hexyl-3-methylimidazolium chloride
Graphical abstract
Introduction
Ionic liquids (ILs) are of great interest to the chemical and related industries because of its unique physical and chemical properties, such as the high stability, non-flammable, negligible vapour pressure, adjustable acidity and alkalinity and flexible design. The replacement of volatile organic solvents by ILs has a promising application prospect in the fields of catalytic synthesis, electrochemical and chemical separation process.
In the application of ILs, it is primarily necessary to analyse the interaction between the ionic liquid and the solute molecules, and then choose the targeted ionic liquid. During this process, activity coefficient at infinite dilution is an important parameter in the ionic liquid separation process, not only reflects the dissolving ability of ionic liquids, but also describes the relationship between solute and IL [1]. It shows the essential information of the interaction between the solute molecules and the solvent molecules under the ideal condition that the solute molecules are surrounded by the solvent without considering the interactions between the solute molecules [2], [3]. In the meantime, the activity coefficient at infinite dilution can not only evaluate the separation properties of solvents, but also be used as an important basis to characterize the interaction between solute and solute molecules, which has important theoretical significance. It has important influences and wide application in chemical process and solution thermodynamics. In addition, it can be directly used in the design of units operation such as distillation, extraction and separation in petroleum and chemical fields. At the same time, ionic liquid has important applications in macromolecules, food, and petroleum processing.
A number of experimental techniques are available for direct measurement of, including steady-state gas–liquid chromatography (GLC), the dilutor technique (DT method) or inert gas stripping, differential ebulliometry, headspace, and dew point. Each of these techniques has some limitations. The GLC method is fast and accurate and requires only a small amount of the required agent. Due to the unique advantages of ILs, it is very suitable as a stationary phase for gas-liquid chromatography. The retention values of various organic and inorganic compounds are determined by gas-liquid chromatography, andof each compounds in the ionic liquid is calculated by specific formula. In this paper, the infinite dilution activity coefficients and the gas-liquid partition coefficients () of 33 organic solutes (alkanes, cycloalkanes, alkenes, aromatic hydrocarbons, acetonitrile, acetone, tetrahydrofuran, ethyl acetate, 1,4-dioxane, chloromethanes, alcohols) in ionic liquid 1-hexyl-3-methylimidazolium chloride ([HMIM][Cl]) have been measured at the temperatures from 313.15 K to 363.15 K by gas-liquid chromatography. By using the relationship between and the temperature, the values of the partial molar excess enthalpies at infinite dilution were obtained. The entropies and Gibbs energies of organic solutes at a reference temperature = 298.15 K were also determined from the values. These thermodynamic functions emphasize the interaction between solute and IL and are important information for the extraction of IL. The research results can provide basic data and theoretical basis for the further application of ILs as green solvents.
The selectivity and the capacity at infinite dilution directly calculated from offer an important means to evaluate the performance of ILs as solvents in various separation problems. The and at T = 323.15 K for IL [HMIM][Cl] have been also calculated for n-hexane ()/benzene (), cyclohexane ()/benzene (). The results were analysed in comparison to previously published literature data for [HMIM]-based or [Cl]-based ILs for the same separation problems.
Section snippets
Experimental Materials
The ionic liquid [HMIM][Cl] was supplied by Shanghai Chengjie Chemical Co., Ltd. and had a purity of mass fraction greater than 0.99. According to manufacturer’s specifications, with the following certified mass fraction of impurities: w (Cl−) <4 × 10−4, water <10−3. Before use, the IL was subjected to vacuum evapouration at T = (323–333) K over 24 h to remove any volatile chemicals and water from the ionic liquid. Karl Fischer titration indicates that the concentration of water in ionic liquid
Activity coefficients at infinite dilution ()
In (gas–liquid) chromatography, the activity coefficients at infinite dilution were obtained by the equation proposed by Cruickshank et al. [10] and Everett [11].where is the activity coefficient of solute at infinite dilution in the stationary phase (3), is the vapour pressure of the pure liquid solute, is the number of moles of the stationary phase component on the column, and is the standardized retention volume obtained by Eq.
Activity coefficients at infinite dilution ()
The results of for 33 solutes in the column with 45.35 percent mass ratio of the support material between the temperature range of (313.15 to 363.15) K are presented in Table 1. And the results for 10 selected solutes in the column with 35.49 mass % of the support material at 313.15 K and 323.15 K are listed in Table 4S in the Supplementary Materials. It has very similar results to that in the column of 45.35 mass %. The only disparity between both sets of values is the order of the
Conclusions
In this work, activity coefficients at infinite dilution for 33 solutes in the IL [HMIM][Cl] were measured by gas–liquid chromatography at the temperatures from (313.15–363.15) K. The gas–liquid partition coefficients for all solutes were discussed in [HMIM][Cl]. The thermodynamic functions, as the partial molar excess Gibbs energy with respective enthalpic and entropic at infinite dilution for the same solutes are discussed. The Hildebrand’s solubility parameters of [HMIM][Cl] were also
Acknowledgment
This work was supported by training program for graduate students' innovative activities and practical abilities (Zhang Miao)
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