Development of Abraham model correlations for short-chain glycol-grafted imidazolium and pyridinium ionic liquids from inverse gas-chromatographic measurements

doi:10.1016/j.molliq.2020.113983

Journal of Molecular Liquids

Volume 317, 1 November 2020, 113983

https://doi.org/10.1016/j.molliq.2020.113983 Get rights and content

Highlights

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Infinite dilution activity coefficients determined for organic solutes dissolved in 2 ILs
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Gas-to-IL partition coefficients determined for organic solutes dissolved in 2 ILs
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Excess thermodynamic properties reported for organic solutes dissolved in 2 ILs
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Abraham model correlations derived for describing solute transfer into 2 ILs

Abstract

Infinite dilution activity coefficients and gas-to-liquid partition coefficients are herein reported for more than 45 organic solutes of varying polarity and hydrogen-bonding character dissolved within the ether-grafted ionic liquids 1-ethyl-3-(2-methoxyethyl)imidazolium bis(trifluoromethylsulfonyl)imide and N-(2-methoxyethyl)pyridinium bis(trifluoromethylsulfonyl)imide. Experimental values were determined in 10 K intervals from 323.15 to 373.15 K using the method of inverse gas chromatography. Measured infinite dilution activity coefficients were then used to determine the partial molar excess Gibbs free energies, enthalpies, and entropies associated with the dissolution of these model solutes into these two short-chain glycol-grafted ionic liquids. Finally, based on the measured infinite dilution activity coefficient data, Abraham model correlations have been established for solute transfer into these ionic liquids, including the calculation of cation-specific equation coefficients. The derived Abraham model correlations were found to back-calculate the observed partition coefficients to within 0.09 to 0.13 log units.

Introduction

Ionic liquids (ILs) hold remarkable potential in chemical manufacturing processes as solvent media in the synthesis and purification (e.g., extraction) of organic compounds. In synthetic processes, ILs serve as a liquid media for mass and heat transfer and, in select applications, they have been linked to increased product yields and reduced reaction times. Separation of synthesized chemical products from unreacted starting materials and unwanted reaction byproducts can often be achieved through liquid-liquid extraction or, in the case of volatile chemical products, via fractional distillation. Depending upon the cation-anion combination, ILs will form an immiscible biphasic liquid system with water or with select organic solvents. Judicious selection of cation-anion pairing allows one to fine-tune the degree of immiscibility, as well as to alter the partitioning behavior of the desired chemical product with respect to unwanted chemical impurities. ILs also been used as stationary phases in gas-liquid chromatographic separations. Thermal stability over a broad temperature range, combined with (generally) low vapor pressures, allows chromatographic separations to be performed at relatively high temperatures. Numerous publications [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]] have examined the selectivity of ILs in regards to specific chemical separations.

The ion-specific equation coefficient version of the Abraham solvation parameter model [[19], [20], [21]] permits estimation of the logarithm of the solute's water-to-IL transfer partition coefficient, log P: $log P = c_{p, cation} + c_{p, anion} + (e_{p, cation} + e_{p, anion}) E + (s_{p, cation} + s_{p, anion}) S + (a_{p, cation} + a_{p, anion}) A + (b_{p, cation} + b_{p, anion}) B + (v_{p, cation} + v_{p, anion}) V$ as well as the logarithm of the solute's gas-to-IL partition coefficient, log K: $log K = c_{k, cation} + c_{k, anion} + (e_{k, cation} + e_{k, anion}) E + (s_{k, cation} + s_{k, anion}) S + (a_{k, cation} + a_{k, anion}) A + (b_{k, cation} + b_{k, anion}) B + (l_{k, cation} + l_{k, anion}) L$ by adding the appropriate cation-specific (c_p,cation, e_p,cation, s_p,cation, a_p,cation, b_p,cation, v_p,cation, c_k,cation, e_k,cation, s_k,cation, a_k,cation, b_k,cation, l_k,cation) and anion-specific (c_p,anion, e_p,anion, s_p,anion, a_p,anion, b_p,anion, v_p,anion, c_k,anion, e_k,anion, s_k,anion, a_k,anion, b_k,anion, l_k,anion) Abraham model equation coefficients in accordance with Eqs. (1), (2). The 65 cation-specific and 20 anion-specific equation coefficients that we have published thus far [1,[3], [4], [5], [6], [7],[22], [23], [24], [25], [26], [27], [28]] enables one to construct IL-specific Abraham model correlations for log P and log K values for solutes dissolved in 1300 (i.e., 65 × 20) different IL solvents that contain solute descriptor values (E, S, A, B, V and L): $log P = c_{p, il} + e_{p, il} \cdot E + s_{p, il} \cdot S + a_{p, il} \cdot A + b_{p, il} \cdot B + v_{p, il} \cdot V$ $log K = c_{k, il} + e_{k, il} \cdot E + s_{k, il} \cdot S + a_{k, il} \cdot A + b_{k, il} \cdot B + l_{k, il} \cdot L$

It is important to note that this number of predictive correlations far exceeds the 120 or so IL-specific correlations that have been determined based on actual experimental partition coefficient data for solutes dissolved in specific ILs.

Significantly, solute descriptors have been determined for more than 8000 different organic, organometallic and inorganic compounds [29], and are defined as follows: the solute excess molar refractivity expressed in units of (cm³ mol⁻¹)/10 (E); the solute dipolarity/polarizability (S); the overall or summation hydrogen-bond acidity and basicity (A and B, respectively); the McGowan volume given in units of (cm³ mol⁻¹)/100 (V); and the logarithm of the gas-to-hexadecane partition coefficient at 298 K (L). The complimentary IL solvent properties in Eqs. (3), (4) are denoted by the lowercase alphabetic characters (c_p,il, e_p,il, s_p,il, a_p,il, b_p,il, v_p,il, c_k,il, e_k,il, s_k,il, a_k,il, b_k,il, and l_k,il) and are determined by regression analysis of experimental log P and log K values for series of organic solutes and inorganic gases dissolved in a given IL solvent. The product of a solute descriptor times the complimentary IL solvent property represents a specific type of solute-IL molecular interaction believed to be present in the liquid mixture. For example, the terms a_p,il · A + b_p,il · B in Eq. (3) (and a_k,il · A + b_k,il · B in Eq. (4)) represent hydrogen-bonding interactions between the dissolved solute and the IL solvent. That is, the solute acts as the hydrogen-bond donor in the two a_il · A terms and as the hydrogen-bond acceptor in the b_il · B terms. The Abraham model is described in greater detail in several earlier publications [[30], [31], [32], [33], [34]].

Our recent research efforts have focused on determining experimental activity coefficients and partition coefficients for solutes dissolved in previously unstudied ILs using inverse gas-liquid chromatography, followed by the use of these experimental data to develop IL-specific Abraham model correlations. Of particular interest are those IL solvents containing cations and/or anions for which ion-specific equation coefficients have yet to be determined. As noted above, the cation- and anion-specific equation coefficients can be combined to arrive at Abraham model correlations with predictive power for the partitioning behavior of solutes in numerous ILs that remain unstudied. Such predictions can aid researchers in the selection of appropriate ILs for solving specific chemical separation problems. In the current communication, we have measured infinite dilution activity coefficients and gas-to-liquid partition coefficients of more than 40 different organic solutes dissolved in the glycol-pendant ILs 1-ethyl-3-(2-methoxyethyl)imidazolium bis(trifluoromethylsulfonyl)imide ([MeOCH₂CH₂EtIm][Tf₂N]) and N-(2-methoxyethyl)pyridinium bis(trifluoromethylsulfonyl)imide ([MeOCH₂CH₂Py][Tf₂N]) over the temperature range from 323.15 to 373.15 K using inverse gas chromatography. Molecular structures of these glycol-functionalized ILs are provided in Fig. 1. Our measured infinite dilution activity coefficients were used to calculate selectivity and capacity values for several representative chemical separation problems. Measured partition coefficients were further used to determine Abraham model correlations for both of these ILs, as well as to calculate the cation-specific coefficients for 1-ethyl-3-(2-methoxyethyl)imidazolium and N-(2-methoxyethyl)pyridinium, expanding the portfolio of predictive ability for Abraham model correlations by 40 additional cation-anion combinations.

Section snippets

Ionic liquid samples and solute probes

The glycol-pendant ILs 1-ethyl-3-(2-methoxyethyl)imidazolium bis(trifluoromethylsulfonyl)imide ([MeOCH₂CH₂EtIm][Tf₂N]) and N-(2-methoxyethyl)pyridinium bis(trifluoromethylsulfonyl)imide ([MeOCH₂CH₂Py][Tf₂N]) were prepared following recently reported methods [35].

The organic solutes used in the current study were purchased from Aldrich Chemical Company at a purity of 99.5 wt% and used without further purification. We note that our gas-liquid chromatographic method efficiently separates any

Theoretical basis for thermodynamic quantities

Activity coefficients at infinite dilution of solute 1 in IL (γ_{1. IL}^∞) were calculated from retention data using the expression [37,38]: $ln γ_{1, IL}^{\infty} = ln (\frac{n_{IL} RT}{V_{N} P_{1}^{o}}) - P_{1}^{o} \cdot \frac{B_{11} - V_{1}^{o}}{RT} + \frac{2 B {}_{13}- V_{1}^{\infty}}{RT} \cdot J \cdot P_{o}$ where n_IL is the mole number of the IL presents inside the column, R is the ideal gas constant, T corresponds to the oven temperature, V_N is the net retention volume and P₁⁰ is the vapor pressure of the pure solute at temperature T. B₁₁ and B₁₃ are the second virial coefficient of the solute in the gaseous

Activity coefficients and selectivity at infinite dilution

Experimental activity coefficients at infinite dilution and gas-to-IL partition coefficients calculated using Eqs. (5), (6), (7) are listed in Table 3, Table 4, Table 5, Table 6. There is a strong relationship between a solutes' solubility and its polarity, a feature related to chain length. For example, low solubility is observed for long-chain alkanes. Among the solutes investigated, the lowest solubility is observed for tetradecane in [MeOCH₂CH₂EtIm][Tf₂N] (γ_i^∞ = 503.8 at 333.15 K) and [MeOCH

Concluding remarks

In the current study, we have reported infinite dilution activity coefficients, partial molar thermodynamic excess properties, and gas-to-liquid partition coefficient data for more than 45 organic solutes of varying polarity and hydrogen-bonding character dissolved in the ILs 1-ethyl-3-(2-methoxyethyl)imidazolium bis(trifluoromethylsulfonyl)imide and N-(2-methoxyethyl)pyridinium bis(trifluoromethylsulfonyl)imide based on inverse gas chromatographic measurements performed in the temperature

CRediT authorship contribution statement

Fabrice Mutelet: Investigation, Formal analysis, Resources, Writing - original draft. Gary A. Baker: Investigation, Resources, Writing - original draft. Hua Zhao: Investigation, Resources. Brittani Churchill: Formal analysis. William E. Acree: Formal analysis, Writing - original draft.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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