Thermophysical Properties of Imidazolium-Based Binary Ionic Liquid

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Thermophysical Properties of Imidazolium-Based Binary Ionic Liquid Mixtures Using Molecular Dynamics Simulations Utkarsh Kapoor and Jindal K. Shah* School of Chemical Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, United States S Supporting Information *

ABSTRACT: Until very recently, task-specific ionic liquids have been designed by altering the chemistry of either the cation, anion, or both. An alternative approach, that is gaining considerable momentum, is to consider ionic liquids that are derived from mixing two different ionic liquids and varying the molar composition of such blends to exert precise control over the desired physicochemical and biological properties. As the number of ionic liquids that result from mixing is projected to be close to a billion, it is highly desirable to predict a priori whether ionic liquid mixtures can be considered as ideal solutions of their pure analogues. Toward this end, we employ molecular dynamics simulations to predict the density, molar volumes, excess molar volumes, self-diffusion coefficients, and ionic conductivities for 11 ionic liquid mixture systems as a function of mole fractions spanning the entire range of compositions of the constituent ionic liquids. The ionic liquid mixtures investigated here are 1-n-butyl-3-methylimidazolium [C4mim]+ chloride Cl− paired with [C4mim]+ acetate [CH3COO]−/[OAC]−, [C4mim]+ trifluoroacetate [CF3COO]−/[TFA]− and [C4mim]+ trifluoromethanesulfonate [CF3SO3]−/[TFS]−, and [C4mim][OAC] combined with [C4mim][TFA] and [C4mim][TFS]. The effect of change in the alkyl chain length on the thermophysical properties of ionic liquid mixtures containing anions as Cl−−methylsulfate [MeSO4]−, and Cl−−bis(trifluoromethanesulfonyl)imide [NTf2]− is evaluated by coupling with 1-ethyl-3methylimidazolium [C2mim]+, 1-n-hexyl-3-methylimidazolium [C6mim]+, and 1-n-octyl-3-methylimidazolium [C8mim]+ cations. The deviation of the property trend from the linear mixing rule is discussed in terms of the difference in the properties of pure ionic liquid analogues.

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the number of possible “ionic liquids” can be greatly expanded to as many as one billion.1 As for the pure ionic liquids, a complete property determination for binary ionic liquid mixtures would be required to exploit the benefits derived from such systems. From a thermodynamic point of view, it is highly desirable to a priori predict if a given binary ionic liquid system is likely to exhibit ideal behavior, so that the properties of the resulting ionic liquid can be directly obtained from the knowledge of the pure ionic liquid properties without the need for experimentation. Similarly, insight into the characteristics of the ionic liquids that lead to nonideal behavior is key to developing ionic liquid mixtures that exhibit higher self-diffusion coefficients and ionic conductivity, and lower viscosity than those of the parent ionic liquids. Toward this end, a number of binary ionic liquid mixture systems have been investigated, both, experimentally2−15 and with simulation-based5,7,16−23 approaches. For more examples of a number of ionic liquid mixture properties, the reader is directed to excellent reviews published by Welton

INTRODUCTION Since the beginning of research efforts on ionic liquids, one of the foci has always been tailoring ionic liquid properties for a given application, which has been traditionally undertaken by recognizing that the replacement of cation, anion, or pendant groups on the ionic moieties leads to modification of physicochemical, biological, and phase equilibra properties of the parent ionic liquid. This approach has successfully led to synthesis of a large number of ionic liquids, characterization of their pure component thermophysical and mixture phase equilibria properties, and investigations of ionic liquids for their suitability for a myriad of applications. In addition to their designer attribute, negligible vapor pressure has contributed immensely to the interest in ionic liquids over the last couple of decades. One of the challenges for widespread applications of ionic liquids in an industrial setting is that the strong electrostatic interactions, though beneficial for extremely low vapor pressure, manifest adversely for transport properties such as self-diffusion coefficients and viscosity. In general, fluorinated ionic liquids are attractive from this perspective, but can be more expensive than the nonflourinated counterparts. One way to address this dilemma is to combine a low-cost nonfluorinated ionic liquid with another fluorinated ionic liquid. Plechkova and Seddon estimated that, by adopting this strategy, © XXXX American Chemical Society

Special Issue: Emerging Investigators Received: November 23, 2017 Accepted: March 7, 2018

A

DOI: 10.1021/acs.jced.7b01028 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. List of Imidazolium-Based Binary Ionic Liquid Mixtures Investigateda

a

Please note that united-atom (UA) representations are used for representing the molecular structure, as per the employed UA force field.

and co-workers,24 and Rogers and co-workers.25 The results from these studies show that many binary ionic liquid mixtures can be considered as ideal solutions of the constituent pure ionic liquids. However, there are examples that show departure from ideal mixing behavior. Recently, Fillion and Brennecke6 reported that some reciprocal binary ionic liquids of the type [C1][A1]−[C2][A2] (C-cation and A-anion) can lead to nonideal behavior yielding a maximum in viscosity. The authors hypothesized that, in addition to nonrandom mixing of the ions, additional attractive interactions must be present to explain the viscosity maximum. Our previous study has also illustrated that dominance of the hydrogen-bonding interactions between 1-n-butyl-3-methylimidzolium [C4mim]+ and chloride Cl− leads, in its mixture with [C4mim]+ bis(trifluoromethanesulfonyl)imide [NTf2]−, to molecular structures that are not found in either pure [C4mim]Cl or [C4mim][NTf2].16 It is possible that the presence of such non-native structures is responsible for the experimental observation that CO2 solubility is in excess of the ideal mixing predictions for the binary mixtures of [C4mim]Cl−[C4mim]-

[NTf2],26 [C2mim] ethylsulfate [C2H5SO4]−[C2mim][NTf2], and [C4mim][C2H5SO4]−[C2mim][NTf2] mixtures.4,27 Welton and co-workers7 also concluded that nonideality in binary ionic liquid systems is likely to arise when there is a substantial difference in the hydrogen bonding ability of the anions, as this can result in preferential interaction at distinct cationic locations. Nonideality in terms of immiscibility of ionic liquids,28,29 electronic environment of anions,30−32 viscosity,5,8 ionic conductivity,33 and CO2 solubility27 have also been reported. This article is a continuation of our effort in employing molecular simulations to predict thermophysical properties for ionic liquid mixtures to obtain an understanding of the extent of deviation for thermophysical properties from ideal behavior. For this work we consider ionic liquid mixtures, that have not been investigated experimentally before, sharing a common cation[C][A1][A2] produced from ionic liquids with a range of molar volumes and size of anions, and report molecular simulation results for the density, (excess) molar volumes, selfdiffusion coefficients, and ionic conductivity for these ionic B



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of 11 binary ionic liquid mixtures, grouped in three sets, were simulated. The first set consisted of five ionic liquid mixtures containing the cation 1-n-butyl-3-methylimidazolium [C4mim]+ and different combinations of anions, namely chloride Cl−, acetate [CH3COO]−, trifluoroacetate [CF3COO]−, and trifluoromethanesulfonate [CF3SO3]− represented as [OAC]−, [TFA]−, and [TFS]−, respectively in this study. Henceforth, these ionic liquid mixtures are referred to as (1) [C4mim] Clx [OAC]1−x, (2) [C4mim] Clx [TFA]1−x, (3) [C4mim] Clx [TFS]1−x, (4) [C4mim] [OAC]x [TFA]1−x, and (5) [C4mim] [OAC]x [TFS]1−x. The second group of three ionic liquid mixtures was composed of mixtures of Cl− and methylsulfate [MeSO4]− with each of the cations 1-ethyl-3-methylimidazolium [C2mim]+, 1-n-hexyl-3-methylimidazolium [C6mim]+ and 1-n-octyl-3-methylimidazolium [C8mim]+. For the following discussions, these ionic liquids are abbreviated as (6) [C2mim] Clx [MeSO4]1−x, (7) [C6mim] Clx [MeSO4]1−x, (8) [C8mim] Clx [MeSO4]1−x, respectively. The third grouping of three ionic liquid mixture systems was derived from the mixtures of Cl− and [bis(trifluoromethanesulfonyl)imide [NTf 2 ] − with [C2mim]+, [C6mim]+, and [C8mim]+. For the rest of the paper, these ionic liquids are designated as (9) [C2mim] Clx [NTf2]1−x, (10) [C6mim] Clx [NTf2]1−x, and (11) [C8mim] Clx [NTf2]1−x. These anion combinations were chosen based on differences in their hydrogen bonding ability with the cation, size, and the molar volumes of pure ionic liquids.16,25,38 In addition to the pure ionic liquid systems, simulations for a total of five intermediate molar ratios (10:90, 25:75, 50:50, 75:25, and 90:10) were performed by varying the concentrations of the respective anions. PACKMOL39 was used to generate the initial configurations that contained 256 ion pairs for all the compositions while 250 ion pairs were simulated for the 10:90 and 90:10 compositions. Initial box dimensions for pure ionic liquids were estimated from the densities reported in the literature35,40 while ideal mixing behavior was assumed to calculate initial volumes for all the mixture compositions. Each ionic liquid system was subjected to a steepest descent minimization followed by a 2 ns annealing scheme, in which the temperature was increased from 353 to 553 K and then lowered to the desired temperature of 353 K. All the systems were equilibrated in canonical (NVT) ensemble for a duration of 5 ns followed by a 10 ns simulation in isothermal−isobaric (NPT) ensemble. Production runs in the NPT ensemble lasted for 40 ns, out of which the last 20 ns was used for analysis from trajectories saved every 0.4 ps. During the production run, the temperature and pressure were controlled using Nosé−Hoover thermostat and Parrinello−Rahman barostat, respectively, with coupling time constants of τT = 0.4 ps and τP = 2.0 ps while a 2 fs time step was used for integrating the equations of motion. LJ and electrostatic interactions were truncated at a distance of 12 Å. Long range corrections were applied for the LJ interactions, while the Particle Mesh Ewald (PME) method was employed to handle electrostatic interactions beyond the cutoff. Results obtained from three simulations having distinct initial configurations were used to determine the statistical uncertainties.

liquid mixtures as a function of mole fractions at 353 K. The listing of the ionic liquid mixtures is provided in Table 1. The raw data from this work are included as the Supporting Information. In the next section, details of the force field employed in this work are provided followed by the simulation protocol adopted in the study. The Results and Discussion section focuses on the thermodynamic and transport properties of the binary ionic liquid mixtures as a function of the mole ratio of the anions and comments on the various trends obtained. The final section summarizes the results and conclusions from this work.

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FORCE FIELD DESCRIPTION In this work, we have employed a united atom (UA) classical force field developed by Zhong and co-workers34,35 that has been shown to yield accurate prediction of densities, molar volumes, self-diffusion coefficientsproperties of interest in this workfor a number of imidazolium-based ionic liquids over a wide range of temperatures. In this force field, methyl, methylene, and trifluoromethane groups are treated as a single interaction site while heteroatoms such as oxygen and sulfur are explicitly modeled. Because of the importance of hydrogen bonding interactions of the imidazolium-ring hydrogens with the anions, their identity is retained in the description of the imidazolium cations. The functional form of this UA model is given by eq 1 Etot =

∑

K r(r − r0)2 +

bonds

+

∑

K θ(θ − θ0)2

angles

∑

K χ [1 + cos(nχ − δχ )]

dihedrals

+

∑

K ψ [1 + cos(nψ − δψ )]

impropers

⎧ ⎡ ⎫ 12 ⎛ σij ⎞6 ⎤ ⎛ qiqj ⎞⎪ ⎪ ⎢⎛ σij ⎞ ⎥ ∑ ⎨4ϵij⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥ + ⎜⎜ ⎟⎟⎬ ⎝r ⎠ ⎝ rij ⎠ ⎦ ⎝ rij ⎠⎪ i [C4mim][TFS] > [C4mim][OAC] > [C4mim]Cl. The higher ionic conductivity for [C4mim][TFA] over that of [C4mim][TFS] is opposite to the experimental observation at room temperature,51 which is possibly related to the fact that even the self-diffusion coefficients are overpredicted for the molecular model adopted in this work for [C4mim][TFA]; (2) similar to the self-diffusion coefficient, the ionic conductivity decreases with longer alkyl chains on the imidazolium cations.51,61 The ionic conductivity values for the binary ionic liquid systems, similar to the self-diffusion coefficients, fall within those for the parent ionic liquids with the exception of 10:90 and 25:75 compositions of [C4mim] [OAC]x [TFA]1−x and 90:10 mixture of [C4mim] Clx [OAC]1−x (Figure S3 of the G



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characterized by higher ionicity values in comparison to the parent ionic liquids.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b01028. Raw data at 353 K (PDF)

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REFERENCES

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CONCLUSION In this paper, we have presented density, molar volume, selfdiffusion coefficient and ionic conductivity as a function of molar composition (0:100, 10:90, 25:75, 50:50, 75:25, 90:10, and 100:0) at 353 K for 11 binary imidazolium-based ionic liquid mixtures using molecular dynamics simulations. We find that both density and molar volume increase with the increase in molecular weight of the anion and the increase in alkyl chain length on the cation for pure ionic liquids. Excess molar volumes for the binary liquid mixtures are small (absolute values less than 1 cm3/mol). However, the magnitude for the positive molar volumes is greater than that for the systems exhibiting negative deviation. In general, positive deviations are obtained for ionic liquid mixtures containing fluorinated anions when they are paired with a strongly coordinating nonfluorinated anion. We also find that the magnitude of the positive deviation is maximum at the equimolar composition and increases as the alkyl chain on the cation is increased. The difference in the coordinating ability of the anions (Cl− vs [NTf2]−) also leads to slight deviation from the linear mixing rule applied to calculate self-diffusion coefficients. Our calculations suggest that the ionic conductivity for these mixtures can be computed with sufficient accuracy using the linear mixing rule as only three binary ionic liquid mixtures out of 55 considered in this work showed exception to this rule. Overall, we find that the properties such as molar volumes, selfdiffusion coefficients, and ionic conductivities for the ionic liquid mixtures investigated in this article can be obtained with linear mixing rule, as the first approximation. However, the ionic liquid mixtures with small deviations are likely to be interesting candidates for future studies on phase-equilibria properties, as these systems may contain molecular structures that are not accessible to parent ionic liquids.6,7,16,25

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jindal K. Shah: 0000-0002-3838-6266 Funding

This material is based upon work supported by the National Science Foundation (NSF) Award Number CBET-1706978. The authors gratefully acknowledge partial funding from the Oklahoma State University. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Computational resources were provided by the High Performance Computing Center (HPCC) at the Oklahoma State University. H



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