Preferential Ionic Interactions and Microscopic Structural Changes

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Preferential Ionic Interactions and Microscopic Structural Changes Drive Nonideality in the Binary Ionic Liquid Mixtures as Revealed from Molecular Simulations Utkarsh Kapoor, and Jindal K Shah Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03314 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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Industrial & Engineering Chemistry Research

Preferential Ionic Interactions and Microscopic Structural Changes Drive Nonideality in the Binary Ionic Liquid Mixtures as Revealed from Molecular Simulations Utkarsh Kapoor and Jindal K. Shah∗ School of Chemical Engineering, Oklahoma State University, Stillwater, OK 74078 USA E-mail: [email protected]

Abstract Thermophysical and structural properties of two binary mixtures of ionic liquids were determined in this study using molecular dynamics simulations at 353 K. The mixtures contains common cation and different anions, combining 1-n-butyl-3-methylimidazolium [C4 mim]+ chloride [Cl]− with two other ionic liquids, namely, [C4 mim]+ methylsulfate [MeSO4 ]− and [C4 mim]+ bis(trifluoromethanesulfonyl)imide [NTf2 ]− . Each mixture was characterized in terms of thermodynamic quantities such as densities and excess molar volumes and transport properties specifically self-diffusion coefficients and ionic conductivities, using seven molar compositions (0:00, 10:90, 25:75, 50:50, 75:25, 90:10, 100:0). Excess molar volumes for the two binary ionic liquid mixtures exhibited small deviations from ideality; Cl-[MeSO4 ] system showed negative deviation while positive ∗

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deviation was observed for the Cl-[NTf2 ] system. Structural analysis, elucidated in terms of radial distribution functions, orientations of the anions around cation through angular distribution functions and spatial distribution functions, revealed that the significant changes in the ionic interactions occur within the first solvation shell of the cations and these changes are responsible for the observed nonideality. The selfdiffusion coefficients of the ions were found to decrease monotonically with Cl− concentration for both the mixtures. Further, predictions of ionic conductivities using both the Nernst-Einstein formalism and Einstein relationship pointed to the presence of correlated ionic motion which was confirmed by the long ion-pair relaxation time constants especially for the anion present as a minor component.


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Introduction Ionic liquids, solvents that comprise entirely of ions and can be designed to exist as liquids under ambient conditions, have attracted a large number of researchers to investigate thermodynamic and transport properties of a pure ionic liquid or mixtures of a single ionic liquid with gases and solvents with experiments, theory and molecular simulation methods. The continued interest in this field is due to the fact that desirable ionic liquid properties such as negligible vapor pressure, ability to dissolve large number of polar and nonpolar substances, wide electrochemical window can be easily tuned by a judicious choice of the cation, anion or substituents on the ions. The so-called task-specific ionic liquids till date have been discovered based on this principle. For example, the first generation of ionic liquids was developed for desired physical properties such as density and melting points followed by the second generation of ionic liquids with a primary focus on improved chemical properties. 1,2 Biological properties of ionic liquids have been targeted for the third generation of ionic liquids. 3,4

The physicochemical and structural properties of ionic liquids derived from a single cation and anion are governed by electrostatic interactions between the charged groups on the ions, dispersion interactions between the nonpolar regions and hydrogen bonding. These interactions lead to a rich diversity of ionic liquid structural behavior such as charge ordering and nanoscale segregation responsible for unique properties of ionic liquids. Although manipulation of the ionic liquid properties has been traditionally achieved by changing the identity of the ions, such variations are likely to modify properties only discretely. Mixing two ionic liquids which share a common ion but differing in the identity of counterions and compositions can potentially provide precise fine-tuning of desired physicochemical, structural and biological properties. According to Plechkova and Seddon, 5 there are as many as 1 billion binary combinations of ionic liquids that can be exploited for potential applications. Very recently such ionic liquid mixtures have been targeted for investigation. As with the ionic 3 ACS Paragon Plus Environment


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liquids obtained from a single cation and anion, initial research on ionic liquid mixtures has primarily focused on the thermodynamic and transport properties of these solutions and if such ionic liquids can be classified as ideal or non-ideal mixtures of the constituent ionic liquids forming the mixture.

Lopes et al. 6 investigated a number of ionic liquid mixtures containing 1-n-decyl-3-methylimidazolium [C10 mim] bis(trifluoromethanesulfonyl)imide [NTf2 ] and [Cn mim][NTf2 ] (n= 2, 4, 6 and 8) and found that the excess molar volumes were small (tenths of cc/mol) but positive and the magnitude increased with increase in the difference in the size of the cations. The authors reported similar trends for binary ionic liquid mixture with the [C4 mim]+ cation and varying compositions of the pair of the anions ([NTf2 ]− and tetrafluoroborate [BF4 ]− ; [NTf2 ]− and hexafluorphoshpate [PF6 ]− ). Nearly ideal behavior of the mixture evaluated in terms of the excess molar volume led the authors to conclude that, at molecular level, the interactions in the mixture are probably indistinguishable from those in the pure ionic liquids. Subtle changes in the structure of the nonpolar domains were provided as a possible reason for the small excess molar volumes measured for the systems in which the cation alkyl chain length was varied.

Measurements by Stoppa et al. 7 for the binary ionic liquid mixtures containing 1-ethyl3-methylimidazolium [C2 mim][BF4 ] and [C2 mim]dicyanamide [DCA] revealed that the excess molar volume were small (∼0.1%) but consistently positive over the entire composition range. However, dynamical properties such as conductivity, time constant for CC relaxation and static permittivity deviated significantly from a simple linear mixing relationship. The authors provided gradual changes in the liquid structure between those of the pure ionic liquids as an explanation for the observed mixture properties.

Pinto et al. 8 performed thermodynamic property, such as excess molar volume, measure-


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ments on the ionic liquids formed from [C2 mim] ethylsulfate [EtSO4 ] and [C2 mim][NTf2 ] and observed that the excess molar volumes showed positive deviation from ideality over the entire range of compositions and the maximum deviation was 0.8 cc/mol at a [C2 mim][NTf2 ] mole fraction of 0.4. Similar results were obtained when the ionic liquid [C4 mim][EtSO4 ] was added to [C2 mim][NTf2 ]. The excess viscosity of the binary ionic liquid mixtures with the ions [C2 mim]+ , [NTf2 ]− and [EtSO4 ]− was found to be nearly ideal. However, negative deviations as high as -60 mPa.s were reported for the mixture [C2 mim][NTf2 ] and [C4 mim][EtSO4 ] suggesting challenges in predicting nonidealities in transport properties of ionic liquid mixtures based solely on the knowledge of excess thermodynamic properties.

For more examples of the ionic liquid mixtures, the interested reader is referred to comprehensive collections of thermodynamic and transport properties of ionic liquids composed of multiple cations/anions recently published by Niedermeyer et al. 9 and Chatel et al. 10 In the latter article, the authors proposed the term “double salt ionic liquids” emphasizing the fact that once formed, it’s not possible to distinguish which ionic liquid a given ion was originally a part of. We also note that it’s not always necessary that the ionic liquids containing three or more ions be derived from mixing two or more pure ionic liquids. In fact, Davis and co-workers demonstrated that the multi-ion ionic liquids containing distinct cations could be efficiently prepared with a one-pot synthesis. 11

Molecular simulations have also been carried out to predict thermodynamic and transport properties of such ionic liquid mixtures. One of the earliest studies was performed by Gutowski and Maginn. 12 Though not explicitly geared towards computing the properties of ionic liquid mixtures, molecular dynamics (MD) simulations of an amine-functionalized ionic liquid capable of reacting with CO2 and its mixture with ionic liquids produced due to the CO2 reaction showed that excess molar volumes of the mixtures were negative and that the dynamics of the systems are severely impacted due to the formation of hydrogen-



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bonded network that permeates through the entire system. Subsequently, Hao et al. 13 used MD simulations to investigate thermodynamic, transport and structural properties of the ionic liquid tetrabutylphosphonium [P4444 ] 2-cyanopyrrolide [2-CNPyr] and its product with CO2 reaction [P4444 ][2-CNPyr-CO2 ]. The authors reported that the density of the system increased linearly with the increase in the [P4444 ][2-CNPyr-CO2] content in the system. The self-diffusion coefficients of all the species, however, were found to be nearly independent of the extent of the CO2 reaction with the parent ionic liquid.

Shimizu et al. 14 studied equimolar mixture of the ionic liquids [C2 mim][NTf2 ] and [C6 mim][NTf2 ] using MD simulations and concluded that the absence of cross-interactions between two different species of the mixture lead to nearly ideal behavior of the mixture. From a technical perspective, the work also highlighted the fact that excess molar volumes are challenging to compute from simulations as it involves the difference between two large quantities.

Aparicio and Atilhan 15 conducted MD simulations of ionic liquid mixtures by changing the composition of the anions [BF4 ]− and [DCA]− paired with 1-n-butyl-3-methylpyridinium cation [C4 -3mpyr] and another mixture in which the anion [BF4]− was coupled with the varying amounts of the cations [C4 -3mpyr]+ and n-octyl-3-methylpyridinium [C8 -3mpyr]+ . The excess molar volumes for the former system exhibited negative deviation from ideality with maximum deviation of 1 cc/mol observed at the equimolar concentrations while those for the mixtures containing common anion were positive. The same authors also suggested, based on MD calculations, the possibility of fine-tuning interfacial behavior of ionic liquid mixtures by varying the ratios of [C4 mim]Cl and [C4 mim][NTf2 ] ionic liquids. 16

Structural features of two binary ionic liquid systems, [C4 mim][PF6 ]-[C4 mim][BF4 ] and [C4 mim][PF6 ][C4 mim]Cl, were probed by Payal and Balasubramanian. 17 Differences in the radial distribution functions for the [C4 mim][PF6 ]-[C4 mim]Cl as a function of the anion compositions


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were attributed to preferential binding of Cl− with the cation while negligible dependence of radial distribution functions for the [C4 mim][PF6 ]-[C4 mim][BF4 ] mixtures on the ratios of the anions was explained in terms of similar affinity of the anions for [C4 mim]+ .

Recently, Matthews et al. 18 also investigated structures of binary ionic liquid mixtures with a common cation [C4 mim]+ combined with several anions using 1 H and

13

C NMR and MD

simulation techniques. The authors observed preferential interactions between the most acidic hydrogen atom in the cation with the strongly hydrogen bond accepting anion. Further, weakly coordinating anions such as [NTf2 ]− were found to organize above and below the plane of the imidazolium ring promoting anion-π interactions and easily disrupting favorable cation π-π interactions established by strongly coordinating anions. However, the authors pointed out that these structural changes are subtle explaining nearly ideal behavior of binary ionic liquid mixtures. The Kirchner group 19 also reported, based on ab initio MD simulations, similar observations for the ionic liquid mixtures composed of [C2 mim]Cl and [C2 mim] thiocyanate [SCN] that cations interact via π-π stacking which is weaker in the case of [C2 mim][SCN].

Based on previous studies, it appears that ionic liquid mixtures are likely to exhibit ideal or close to ideal behavior. 7,9,10,15,20 However, the factors governing the sign of ideality are not yet completely understood. For example, Navia et al. 21 investigated a series of ionic liquid mixtures based on either a common cation or an anion and noticed that though the deviations from ideality were small, the sign of the deviation was dependent on the identities of the anions. In contrast, experiments on the binary ionic liquid mixtures comprising of [C4 mim]+ , [NTf2 ]− with fluorinated anions, cyano-based anions and those derived from organic acids consistently yielded positive deviation from ideality. 22

The present work was carried out with the objective of predicting, using MD simulation



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techniques, the magnitude and sign of the deviation from ideality of binary ionic liquid mixtures and explaining the origin of such deviations, if any, based on structural changes that might occur as the composition of the ionic liquid mixture is changed. For this purpose, two ionic liquid systems bearing the common cation [C4 mim]+ and the anion Cl− were investigated. In one system, methylsulfate [MeSO4 ]− was progressively substituted for Cl− while [NTf2 ]− was the second anion in the other system. The choice of the systems was motivated by the fact that [C4 mim]+ is probably the most preferred cation for study in the ionic liquid field; the anion were selected based on the difference in their sizes and the ability to form hydrogen bonding with the cation. 23

In the next section, details of the force field employed in this work are provided followed by the simulation protocol adopted in the study. Results and discussion section focuses on thermodynamic, structural and transport properties of the binary ionic liquids as a function of the mole ratio of the anions. A discussion is included on the origin of the nonideality observed for the two binary ionic liquid system. The final section comprises of a summary of results and conclusions obtained from the study.


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Force Field The intramolecular and intermolecular interactions were described by the united-atom force field proposed by Liu et al. 24,25 with the following function form:

Etot =

X bonds

+

X

Kr (r − r0 )2 +

X

Kθ (θ − θ0 )2

angles

Kχ [1 + cos(nχ − δχ )] +

+

i=1 i

Preferential Ionic Interactions and Microscopic Structural Changes

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