Elsevier

Fluid Phase Equilibria

Volume 378, 25 September 2014, Pages 34-43
Fluid Phase Equilibria

Experimental and theoretical study of interaction between organic compounds and 1-(4-sulfobutyl)-3-methylimidazolium based ionic liquids

https://doi.org/10.1016/j.fluid.2014.06.021Get rights and content

Abstract

Activity coefficients at infinite dilution (γ1,2) for diverse probe solutes in three homologous 1-(4-sulfobutyl)-3-methylimidazolium based ionic liquids were measured by inverse gas chromatography at temperatures from 323 to 343 K. Accordingly, this family of ionic liquid can be ranked according to their interaction with organic compounds as follows: [HSO4]  [OTf]  [N(Tf)2]. The retention data were further converted to gas-to-IL and analyzed using the Abraham solvation parameter model. The LSER treatment indicates that the most dominant interaction constants for this family of ILs are strong dipolarity, hydrogen bond basicity and acidity. Then, the ChelpG atomic charges on the cation and the anions obtained by ab initio calculations are used to explain the interactions between methanol, thiophene and benzene and the anions or the cation.

Introduction

New strategies are necessary for efficient and clean separation processes in the biotechnology and chemical industries. The new technology will have to reduce the amount of organic solvents needed for these separation processes, leading lower to volatile organic compounds emissions. Among the new advances, ionic liquids (ILs) are being widely promoted as probable substitutes for traditional industrial in a host of processes.

ILs frequently combine the attractive features of excellent chemical stability, high thermal stability, and exceedingly low vapor pressure in a single solvent. ILs are commonly comprised of an asymmetric, bulky organic cation paired with a weakly coordinating anion that may be organic or inorganic in nature. The properties of the anion and the functionality presented at the cationic or anionic site offer a means to alter the specific attributes of the solvent proper, giving rise to the notion of tuning, often in stepwise fashion, the key solvent features for the task at hand. For the past decade, with the development of ionic liquids, solvent extraction has been found to be one of the fields in which the application of these families of compounds looks promising [1], [2], [3]. Numerous works have shown that a large number of ILs exhibits selectivities and capacities better than the solvents typically employed to solve industrial separation problems [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. For example, it was found that the extractive desulfurization and denitrogenation process using ionic liquids can be a complementary technology for the classical processes [14], [15], [16]. These studies indicated that the ILs have high extraction ratios and greater selectivity compared to molecular solvents because of the unique solvent characteristics of ILs. For instance, IL-assisted extractive distillation or liquid–liquid extraction forms a powerful approach in the separation of ethanol–water mixtures [17] and thiophene from aliphatic hydrocarbons [16]. The influence of the cation and anion structures of the ionic liquid but also the effect of the temperature on the selectivity and the capacity were evaluated for different separation problems [11]. The highest values of selectivity are observed with ionic liquids containing small alkyl chains, e.g. based on following cations [MMIM]+, [EMIM]+, [EPY]+, [Et3S]+ coupled to a thiocyanate group in the structure. Unfortunately, when the dialkylimidazolium based ionic liquids reveal high values of the selectivity, the capacity always takes low values.

Gas chromatography technique has been used to determine the physico-chemical properties (partition coefficients, activity coefficients, selectivity) of numerous ILs [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Different approaches based on gas chromatography technique were proposed to quantify various intermolecular solute–IL interactions. Among others, Abraham et al. have developed the linear solvation energy relationship model (LSER) allowing to correlate thermodynamic properties of phase transfer processes [18], [19], [20], [21]. The most recent representation of the LSER model is given by Eq. (1)logSP=c+eE+sS+aA+bB+lLwhere SP is a solute property related with the free energy change such as gas–liquid partition coefficient, specific retention volume or adjusted retention time at a given temperature. The capital letters represent the solutes properties and the lower case letters the complementary properties of the ionic liquids. The solute descriptors are the excess molar refraction E, dipolarity/polarizability S, hydrogen bond acidity basicity, A and B, respectively, and the gas–liquid partition coefficient on n-hexadecane at 298 K, L. The coefficients c, e, s, a, b and l are not simply fitting coefficients, but they reflect complementary properties of the solvent phase. The system constants are identified as the opposing contributions of cavity formation and dispersion interactions, l, the contribution from interactions with lone pair electrons, e, the contribution from dipole–type interactions, s, the contribution from the hydrogen-bond basicity of the stationary phase (because a basic phase will interact with an acid solute), a, and b the contribution from the hydrogen-bond acidity of the stationary phase. The system constants are determined by multiple linear regression analysis of experimental log SP (log KL in this work) values for a group of solutes of sufficient number and variety to establish the statistical and chemical validity of the model.

Recently, Sprunger et al. [22], [23], [24], [25] modified the Abraham solvation parameter model:logKL=ccation+canion+(ecation+eanion)·E+(scation+sanion)·S+(acation+aanion)·A+(bcation+banion)·B+(lcation+lanion)·LlogP=ccation+canion+(ecation+eanion)·E+(scation+sanion)·S+(acation+aanion)·A+(bcation+banion)·B+(vcation+vanion)·Vby rewriting each of the six solvent equation coefficients as a summation of their respective cation and anion contribution. In the work of Revelli et al. [26], the cation with its alkyl chains is splitted in different contributions: (CH3, CH2, N, CHcyclic, etc.) in order to estimate the log KL and log P of organic compounds in ionic liquids at 298 K. Mutelet et al. [27] proposed a temperature-dependent linear solvation energy relationship in order to estimate the gas-to-ionic liquid partition coefficients. The model proposes to determine the LSER parameters using a group contribution method. The decomposition into groups of the ionic liquids is very easy, that is as simple as possible. No substitution effects are considered. No exceptions are defined. Large sets of partition coefficients were analyzed using Abraham solvation's model to determine the contributions of 21 groups: 12 groups characterizing the cations and 9 groups for the anions. The derived equations correlate the experimental gas-to-ionic liquid coefficient data to within 0.13 log units. This new temperature-dependent GC-LSER (TDGC-LSER) writes as follows:logKL=2.84418+i21ni×ci+i21ni×eiE+i21ni×siS+i21ni×aiA+i21ni×biB+i21ni×liLTwhere ni is the number of group i present in the ionic liquid. This model is able to determine with high accuracy partition coefficients of solutes in imidazolium based ionic liquids. Physico-chemical models based on the concept of group contribution method need a large dataset to have a powerful ability to predict properties. For instance, Paduszyński and Domanska [28] have demonstrated that the TGC-LSER overestimates the partition coefficient of organic compounds in piperidinium based ILs. This deviation is mainly due to the fact that no data of such ILs were available for this family of ILs.

The present study continues our methodical examination of the solubilizing ability of IL solvents and the development of simple correlation equations that enable one to predict gas-to-liquid partition coefficients, KL, infinite dilution activity coefficients, γ, and mole fraction solubilities of volatile organic solutes and gases dissolved in ILs. In this work, gas–liquid chromatography was used to quantify interactions between organic compounds and three ILs composed of 1-sulfobutyl-3-methylimidazolium [SO3HBMIM] and of bis(trifluoromethylsulfonyl)imide [N(Tf)2], trifluoromethanesulfonate [OTf] or hydrogen sulfate [HSO4] as anion. Their molecular structures are given in Fig. 1. The interaction between organic compounds and the ionic liquids studied are evaluated using the LSER solvation model. To complete the knowledge on these interactions, the CHELPG atomic charges of 1-(4-sulfobutyl)-3-methylimidazolium, 1-butyl-3-methylimidazolium and each anion are determined using ab initio calculations.

Section snippets

Materials and chemicals

The ionic liquids investigated here, 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (99%) and 1-(4-sulfobutyl)-3-methylimidazolium trifluoromethanesulfonate (99%) and 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate (99%) are from Solvionic (Toulouse-France). Each ionic liquid was purified by subjecting the liquid to a very low pressure of about 5 Pa at about 343 K for approximately 24 h. Next packed columns are conditioned during 12 h. Based upon our experience, we

Activity coefficients at infinite dilution of organic compounds in ionic liquids

The uncertainty of γi values may be obtained from the law of propagation of errors. The following measured parameters exhibit uncertainties which must be taken into account in the error calculations with their corresponding standard deviations: the adjusted retention time tR′, ±0.01 min; the flow rate of the carrier gas, ±0.1 cm3 min−1; mass of the stationary phase, ±2%; the inlet and outlet pressures, ±0.002 bar; the temperature of the oven, ±0.1 K. The main source of uncertainty in the

CHELPG atomic charges

To evaluate the impact of the structure of ILs on their interaction with organic compounds, the CHELPG atomic charges of 1-(4-sulfobutyl)-3-methylimidazolium, 1-butyl-3-methylimidazolium and each anion were determined as described above. Results of calculation are shown in Fig. 3, Fig. 4. Activity coefficients at infinite dilution of thiophene, benzene and methanol 1-(4-sulfobutyl)-3-methylimidazolium based ILs measured in this work and in 1-butyl-3-methylimidazolium [N(Tf)2] or [OTf] or [HSO4]

Concluding remarks

Activity coefficients at infinite dilution of various organic compounds were measured in 1-(4-sulfobutyl)-3-methylimidazolium based ILs. Introduction of polar chain in ionic liquids affects strongly the behavior of organic compounds in mixtures with the ionic liquids. LSER treatment indicates that the most dominant interaction constants for 1-(4-sulfobutyl)-3-methylimidazolium based ILs are hydrogen bond acidity and basicity but also the polarity. The CHELPG atomic charges calculated for the

Acknowledgment

The present work has been done in the framework of the international project CMEP/Tassili (CMEP 12MDU875 – Egide 26311TA).

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