Theoretical Study of Low Viscous Ionic Liquids at the Graphene

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Theoretical Study of Low Viscous Ionic Liquids at Graphene Interface Mert Atilhan, and Santiago Aparicio J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10434 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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The Journal of Physical Chemistry

Theoretical Study of Low Viscous Ionic Liquids at the Graphene Interface Mert Atilhan*a,b and Santiago Aparicio*c

a

Department of Chemical Engineering, Texas A&M University at Qatar, Doha, Qatar b

Gas and Fuels Research Center, Texas A&M University, College Station, TX, USA c

Department of Chemistry, University of Burgos, 09001 Burgos, Spain

*

Corresponding authors: [email protected] (M. A.) and [email protected] (S. A.)

ABSTRACT: The interaction of 1-ethyl-3-methylimidazolium dicyanamide ionic liquid with graphene was studied using computational chemistry methods with quantum chemistry and classical dynamics approaches. The adsorption of this ionic liquid at the graphene surface, the structure at the interfaces and layering were analysed. The arrangement of ions and composition at adsorbed layers was determined together with the strength of ion-graphene interactions. Likewise, the disrupting effect of graphene on ionic liquid structure and interionic interactions was considered. The effect of ionic liquid on graphene-graphene interactions was studied, and potential of mean force calculated, to infer the possible screening effect of this ionic liquid and its relationship with its graphene exfoliation ability.

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Introduction The properties of ionic liquids (ILs) at the graphene interface and in general of graphene/ILs mixed materials or nanocomposites have attracted relevant attention for their applicability in technologies such as graphite to graphene exfoliation in liquid phase,1-3 nanolubrication,4,5 analytical sensors,6,7 supercapacitors,8 electrocatalysis,9 or wettability control.10,11 For the development of these applications, it is fundamental a knowledge of the graphene – IL interactions, i.e. the nature of ILs structuring at the graphene interface. For this purpose, experimental12-15 and theoretical16-21 studies have been reported for a reduced number of types of ILs, with special attention to alkylimidazolium-based ILs. The available literature studies have confirmed the development of dense, solid-like, layers of adsorbed ILs on top of graphene,22-27 because of the strong affinity of ILs toward graphene surface. This is especially relevant for alkylimidazolium-based ILs in which imidazolium rings lie parallel to graphene surface.16-29 The perturbating effect of graphene on ILs structure is extended wellbeyond the first adsorbed layer, leading to several highly structured IL layers above the layer in direct contact with graphene.27,30,31 This strong interfacial layering leads to the dynamics of ions in these layers being remarkably slower than those in bulk liquid phases.19,26 The literature studies show contradictory results on the composition of adsorbed ILs layers,14 with some authors reporting cation (alkylimidazolium) rich layers,15,53 and others reporting densification formed by ionic pairs.12 Nevertheless, in spite of the pivotal role of the cation on the ILs structuring at the graphene interface, especially for alkylimidazolium – based ILs, the anion should also affect the properties of the interfaces. The anion effect would depend on its affinity regarding to graphene surface, and thus on the possibility of competing with alkylimidazolium cations for adsorption sites on top of graphene, but it has been scarcely studied. The main advantage for the development of task-specific ILs is the large number of anion-cation combinations leading to ILs, which can be as large as the widely cited number of 1018 possible ILs when multi-ionic ILs (mixed) are considered.32 Nevertheless, despite the large number of compounds leading to ILs, several drawbacks have been raised for the use of ILs at industrial level33 for the wide collection of technologies that ILs have been considered. Relevant concerns about the

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toxicity and biodegradability of ILs,34 large production costs,35 or unsuitable physicochemical properties,36,37 have decreased the expectancies of developing ILsbased application within the sustainable chemistry approach. Nevertheless, ILs properties can be tailored selecting suitable ions within the large matrix of possible molecular features,38,39 with the objective of minimizing these drawbacks and allowing to scale up ILs to industrial level. One of the main problems with the industrial scale use of ILs is the large viscosity of many of them,40-42 which would hinder mass and heat transfer operations,43 and would increase cost of operations such liquid pumping. Nevertheless, low viscous ILs can be developed by design considering the molecular level features controlling ILs viscosity.44,45,46 ILs containing dicyanamide anion ([DCA]) have very low viscosity, e.g. 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]), Figure 1, has a viscosity of only 12.93 mPa s at 303.15 K and atmospheric pressure,47 which is only slightly larger than most of the common organic molecular solvents used in the industry. Therefore, the properties of [EMIM][DCA] IL at the graphene interface are studied in this work as a representative system on the behaviour of low viscous ILs at graphene surface, considering the main microscopic level features with special attention to the mechanism of adsorption, structuring, layering and molecular orientation. Computational studies using quantum chemistry methods (based on Density Functional Theory, DFT) and classical molecular dynamics (MD) were carried out to infer the most relevant features of the studied systems. DFT studies allow an accurate characterization of the ILs – graphene interactions, regarding molecular orientation and binding energies, but are limited by the size of the systems which can be analysed because of computational constraints, whereas MD simulations leads to information on the layering and mechanisms of adsorption considering larger systems, and thus the combined use of both approaches provide complementary information48 on the [EMIM][DCA] – graphene systems.

Methods DFT calculations were carried out with the Gaussian 09 (Revision D.01) package.49 Ionic pairs in gas phase, ions on graphene and ionic pairs on graphene were optimized at wb97xd50/6-31g(d) theoretical level. The use of long-range functional with empirical 3



dispersion terms, have proved to provide an accurate description of alkylimidazoliumbased ILs.51 Likewise, the use of dispersion correction terms is pivotal for the description of ILs properties52 and ILs-graphene interaction.53,54 The reported binding energies, ΔE (defined as the difference between the energy of the cluster and those of the corresponding monomers) were corrected according to the counterpoise procedure for Basis Set Superposition Error (BSSE).55 Intermolecular interactions were also analyzed based on Bader’s theory (Quantum Theory of Atoms in Molecules, QTAIM)56 and the Reduced Density Gradient (RDG)57 methods using the Multiwfn code.58 Atomic charges and charge transfer, Δq, were calculated according to CHELPG method.59 MD simulations for pure [EMIM][DCA] and for [EMIM][DCA]-graphene systems were carried out using the MDynaMix v.5.2 program.60 The forcefield parameterization for [EMIM][DCA] ions, Table S1 (Supporting Information), was obtained from the literature.61,62 Graphene and graphene sheets were modelled as rigid entities along all the considered simulations, and thus they were described using Lennard−Jones parameters for graphene carbon atoms (graphene periodic systems, with null charges for all the atoms) and Lennard-Jones parameters for carbon and hydrogen atoms (graphene sheets hydrogen terminated, with null charges for carbon atoms in C-C bonds and charges -0.1 and +0.1 for carbon and hydrogen atoms, respectively, involved in C-H bonds), with parameters obtained from the literature.63,64 The analysis of the properties of [EMIM]-[DCA] interaction with graphene was carried out with MD using four approaches, Table S2 (Supporting Information): i) a layer of liquid [EMIM][DCA] on top of fixed periodic graphene was simulated at 298 and 373 K in the NVT ensemble, ii) a layer of liquid [EMIM][DCA] on top of three stacked fixed periodic graphenes (to mimic graphite-like configuration) was simulated at 298 and 373 K in the NVT ensemble, iii) a layer of liquid [EMIM][DCA] was confined between two fixed periodic graphenes separated by 74 Å and simulated at 298 and 373 K in the NVT ensemble, and iv) two fixed squared graphene sheets (hydrogen terminated, 50×50 Å2) were placed in a parallel configuration, separated a distance R and immersed into liquid [EMIM][DCA], being simulated at 298 K and 1 bar in the NPT ensemble for different values of R.

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Initial simulation boxes were prepared using Packmol65 program. 0All MD simulations were carried out starting from an equilibration step (10 ns) followed by a production run (10 ns) in the corresponding ensemble. Temperature and pressure were controlled with the Nose–Hoover method, electrostatic interactions by the Ewald method66 (15 Å for cut-off radius), and Lennard-Jones cross terms with the LorentzBerthelot mixing rules. Equations of motion were solved with the Tuckerman–Berne double time step algorithm67 (1 and 0.1 fs for long and short time steps). All systems were treated considering periodic boundary conditions.

Results and discussion The adsorption of ILs on graphene surfaces leads to large changes in the characteristics of anion-cation interactions, because the affinity of the involved ions toward the surface may lead to a weakening of anion-cation interactions. Likewise, the layering developed upon interaction with graphene leads to preferential orientations of ions on the surface,12-34 e.g. the parallel arrangement of alkylimidazolium ions on top of graphene,16,27,68 which develops anion-cation interactions different to those in the bulk liquid phases.54 For this purpose, the [EMIM]-[DCA] interactions were initially analysed by DFT in absence of graphene as a reference framework for comparison purposes when ions interact with the surface. Ten different positions were considered for analysing [EMIM]-[DCA] interactions, with the binding energies and charge transfer calculated for the optimized ionic pairs, Figure 2. [DCA] anion can act only as hydrogen bond acceptor whereas hydrogen bond donation by [EMIM] cation can be developed through the C2(H) (carbon atom between N atoms in imidazolium ring) or C1(H) (the other two C sites in imidazolium ring) sites. It is well known that C2(H) site leads to stronger interactions for most of the studied types of anions;69,70 nevertheless, ionic pairs reported in Figure 2 consider different interaction sites (top, bottom, front and side of imidazolium ring) following Matthews et al.70 Shakourian-Fard et al.54 studied the interaction between 1-butyl-3-methylimidazolium, [BMIM], and [DCA], considering front interaction (through C2(H) site) reporting a binding energy of -366.43 kJ mol-1 (at M06-2X-D3 level without considering BSSE) in good agreement with results in Figure 2. The ten interaction positions considered in Figure 2 can be grouped in two different

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ones: i) P2, P4 and P5 with binding energies close to 320 kJ mol-1 and ii) the remaining ones with binding energies close to 350 kJ mol-1. P3 and P5 interactions consider hydrogen bonding through C1(H) site, which are weaker than those through C2(H). Likewise, interactions through C2(H) (P3 and P10) lead to interactions with strength like those in which [DCA] anion is placed on top of imidazolium ring (P1 and P6 to P9). Therefore, [DCA] anion shows several mechanisms of interaction with [EMIM] cation with similar binding energies, which would allow an efficient interaction with graphene surface, i.e. [EMIM] can interact efficiently with the graphene surface adopting a parallel configuration with the [DCA] anion shifted outwards of the surface without weakening anion-cation interaction. To quantify the strength of ion – graphene interactions a hexagonal graphene sheet, hydrogen terminated, was considered. In a first step, the interaction of single ions with the surface was quantified as a function of distance to the sheet, d. In the case of [DCA] cation, two different orientations were considered: i) anion parallel to the surface and ii) anion perpendicular to the surface, whereas in the case of [EMIM] only parallel orientation was considered considering previous literature results,53,68 which reported the prevailing role of π-π interactions on the imidazolium – graphene interactions (Carstens et al.68 showed that π-π stacking can be up to 65 % of total interaction energy of imidazolium cation with graphene). Regarding results for [EMIM] on top of graphene, large binding energies are inferred, with minimum of -145 kJ mol-1 at d = 3.2 Å. The results for [EMIM]-graphene are slightly different to those reported by Pensado et al.,53 who reported a minimum of roughly -100 kJ mol-1 at d = 2.4 Å, which can be attributed to the different considered functionals. Nevertheless, it should be remarked that other authors such as Shakourian-Fard et al.54 have reported distances in the 3.0 to 3.2 Å range for alkylimidazolium adsorbed onto graphene surface, which is in agreement with MD simulations71 showing alkylimidazolium cations arranged parallel to the graphene surface at a distance of 3.5 Å. Regarding interaction of [DCA] with graphene, results in Figure 3 show that [DCA] lying parallel to the graphene surface leads to binding energy (-50 kJ mol-1) twice the value for anion lying perpendicular (-25 kJ mol-1) to the surface, although the minima in the interaction curves show that perpendicular arrangements allow a slightly closer approaching to

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the surface than parallel one (minima at 2.8 and 3.0 Å for perpendicular and parallel arrangements, respectively). The adsorption of [EMIM][DCA] on graphene sheets was also quantified in this work, Figure 4.

Shakourian-Fard et al.54 proposed an arrangement in which the

adsorbed ion pair is characterized by imidazolium ring lying parallel to the surface and [DCA] anion lying perpendicular to the surface with one N atom at 2.99 Å of the surface and the other one at 5.89 Å. Nevertheless, the structure considered in this work is developed with both ions lying parallel to the graphene surface, Figure 4, which leads on one side to efficient π-π stacking of the imidazolium and on the other side to stronger [DCA]-graphene interactions than for [DCA] anions lying perpendicular to the surface (Figure 3); likewise, in this arrangement the main characteristics of [EMIM]-[DCA] interactions are not disrupted (Figure 2) allowing both ion-ion and iongraphene interactions. In the parallel arrangement reported in Figure 4, the ionic pair is placed at 3.1 Å for [DCA] and 3.3 Å for [EMIM] of the graphene surface (in agreement with the preferred orientations of single ions reported in Figure 3, with [DCA] anions leading to optimal interactions at distances slightly closer to the surface than [EMIM]). The QTAIM analysis of the ionic pair – graphene interaction reported in Figures 4a and 4b shows several relevant features. The region between [EMIM] and [DCA] is characterized by the presence of bond critical points (BCPs, type (3,-1)), ring critical points (RCPs, type (3,+1)) and even one cage critical point (CCPs, type (3,+3)). These results show that [EMIM] – [DCA] interaction near the C2(H) site of the imidazolium ring is maintained upon adsorption on graphene, the presence of the CCP close to the C2(H) site (Figure 4b) show that anion-cation hydrogen bonding is not weakened upon interaction with graphene. Regarding the interaction of ions with the surface, the QTAIM analysis of the region below each ion shows the presence of RCPs and CCPs below each ion, confirming the efficient interaction with the surface. The appearance of a CCP below the imidazolium ring and centred in the ring shows the development of π-π stacking. The RDG analysis of the developed interactions confirms [EMIM] and [DCA] interacting through the C2(H) site and the large regions below both ions show the development of efficient van der Waals type interactions. Therefore, both ions can be adsorbed simultaneously without weakening anion-cation

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interactions, thus combining the strength of [EMIM]-graphene and [DCA]-graphene binding energies, Figure 3, upon adsorption of the ionic pair. The DFT results reported in the previous paragraphs showed some characteristics of the [EMIM][DCA] interactions with graphene surface, but additional features must be considered to fully characterize the behaviour of liquid [EMIM][DCA] regarding graphene, which require to consider larger number of molecules to infer long range behaviour being studied by MD simulations. The validation of the used force field was carried out by comparison of experimental and predicted MD density data.47 Results in Figure 5 show MD leading to predicted densities slightly larger than experimental values but deviations in the 1.3 to 1.6 % range confirm the validity of the applied force field for describing the behaviour of [EMIM][DCA]. In the first set of MD simulations the interaction of a liquid layer of [EMIM][DCA] with periodic graphene was considered at 298 and 373 K, Table 2 (Supporting Information). Number density profiles are reported in Figure 6a. It should be remarked that two interfaces are considered in these simulations IL-graphene and IL-vacuum, this last one on top of the IL liquid layer. Regarding the [EMIM][DCA]graphene interface, number density profiles show densification near surface, which is inferred from the large, narrow first peaks reported in Figures 6a and 6b. The maxima of these peaks appear at 3.56 Å and 3.73 Å for [EMIM] and [DCA], respectively, which are slightly larger than the values obtained from DFT (Figure 4) but in the range of previous MD results for alkylimidazolium – based ILs.22,71 Likewise, these density peaks are more intense and narrower for [EMIM] than for [DCA] but confirm that the first adsorbed layer (region zI, defined as d

Theoretical Study of Low Viscous Ionic Liquids at the Graphene

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