Alkyl Methyl Imidazolium-Based Ionic Liquids at the Au(111) Surface

Article Cite This: J. Phys. Chem. C 2019, 123, 15087−15098

pubs.acs.org/JPCC

Alkyl Methyl Imidazolium-Based Ionic Liquids at the Au(111) Surface: Anions and Alkyl Chain Cations Induced Interfacial Effects Shanmugasundaram Kamalakannan,† Muthuramalingam Prakash,*,† Muneerah Mogren Al-Mogren,‡ Gilberte Chambaud,§ and Majdi Hochlaf*,§ †

Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur 603203, Tamil Nadu, India Chemistry Department, Faculty of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Kingdom of Saudi Arabia § Université Paris-Est, Laboratoire Modélisation et Simulation Multi Echelle, MSME UMR 8208 CNRS, Marne la Vallée Cedex 2 77454, France Downloaded via GUILFORD COLG on July 22, 2019 at 17:14:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The structure, stability, and adsorption mechanism on the Au(111) surface of hydrophilic/hydrophobic ionic liquids (ILs) with various lengths of the alkyl chain group in the cation were investigated using first-principles approaches including electrostatic potential (ESP), electron density analysis, and dispersion-corrected density functional theory (DFT-D3) methods. A suitable selection of ILs is considered. Indeed, we treat ILs of general formula [CnMIm]+[X]−, where X = Cl−, PF6−, and TFSA−, MIm = methylimidazolium and where we varied the alkyl chain (Cn) length for n = 0, 2, 4, 6, 8, and 10. We found that the adsorption energies (Eads’s) of fluorinated ILs are lower (by ∼30%) than those of nonfluorinated ILs. Computations show that the nature of the anion within the ILs and the alkyl chain length play a very important role to alter the interfacial interactions between ILs and gold surface. Indeed, the alkyl groups of the cation affect the cation−anion interaction strength within ILs because of specific modes of adsorption of alkyl chains on the Au(111) surface. Also, structural-induced adsorption effects are observed during the variation of alkyl chain and anions. These are due to the modification of the local properties of the gold surface upon IL adsorption, where we identify, in some specific cases, an enhancement of the charge transfers between the ILs and the Au(111) surface through van der Waals and dispersive interactions.

1. INTRODUCTION Hybrid inorganic−organic systems are used for various applications such as catalysis, energy storage and conversion, photovoltaics, molecular sensors, switches, skeletal tissues and biominerals, drug delivery systems, and polymer nanocomposites.1−11 We refer to the recent review by Heinz and Ramezani-Dakhel7 for a detailed presentation of examples ranging from biomolecules on nanostructured metals up to building materials. Particularly, molecular adsorption with coinage metals is receiving a widespread attention. For instance, small molecules adsorbed on gold surface devices are important for the development of novel functional materials for optical, photovoltaic, and electrode applications. To this end, the study of the adsorption mechanism and reactivity of molecules at different surfaces and solid−liquid interfaces is required. Several experimental and theoretical techniques have been devoted to understand the adsorption and reaction mechanisms of organic molecules interacting with a metal surface.1,12−16 For instance, Koch et al. investigated the adsorption of organic compounds [pentacene (PEN) and © 2019 American Chemical Society

perfluoropentacene (PFP)] on the Cu(111) surface using photoelectron spectroscopy, X-ray standing wave, and scanning tunneling microscopy. They also complemented their work by theoretical modeling using density functional theory (DFT) (PBE)-based method. They pointed out that PEN is closer to the Cu(111) surface than PFP because of weak intramolecular dipole interactions between PFP and the Cu(111) surface.1 Recent theoretical reports stated that N-based nucleophiles and aromatic π-systems form a regium bond with coinage metals.17,18 These studies revealed that gold complexes present a stronger binding with N-based nucleophiles than the other metals. Let’s also cite the work of Tkatchenko and co-workers dealing with the electronic charge rearrangement at the metal/ organic interfaces using PBE and PBE + vdW methods.19 These authors found that there are three plausible mechanisms for the charge rearrangement: (i) the Pauli pushback; (ii) charge transfer between the surface and the monolayer; and Received: April 8, 2019 Revised: May 31, 2019 Published: May 31, 2019 15087

DOI: 10.1021/acs.jpcc.9b03242 J. Phys. Chem. C 2019, 123, 15087−15098

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The Journal of Physical Chemistry C

interactions between the adsorbate substrate and also from the H-bonds between the anthraquinone cores.34 In 2018, Chinwangso et al., used X-ray photoelectron spectroscopy to study a series of unsymmetrical spiroalkanedithiols adsorbed at a gold surface by increasing the methylene units. A similar effect was observed because of the enhancing of the alkyl chain dispersion interaction with the metal surface.35 Prior to that, Li and co-workers36 established the influence of the alkyl chain length and anion species on the interfacial nanostructure of ILs at Au(111). They showed that the 1ethyl-3-methylimidazolium [EMIm]+ and 1-hexyl-3-methylimidazolium [HMIm]+ present enhanced interfacial structures than the 1-butyl-3-methylimidazolium [BMIm]+ because [EMIm]+ adsorbs strongly through cation···π stacking with a parallel carbon chain to the metallic surface and because [HMIm]+ has longer alkyl chain tails. Nevertheless, these adsorption-induced effects by the alkyl chains of the cation of ILs on the gold surface are still not understood. Besides, Pensado and Padua have studied the molecular interactions, solvation, and ordering of ILs around ruthenium nanoparticles using MD simulations. The radial and spatial distribution function analyses reveal that nonpolar groups and side chains were preferentially directed away from the surface. Also, they concluded that the nanoparticle stabilization does not depend on the electrostatic double layer and steric effect of the alkyl chain in the cations.39 Our work aims to understand the effects induced when an alkyl chain-based ILs are close to a gold surface. To the best of our knowledge, this is the first theoretical study to interpret the role of alkyl chain lengths and of anions at the Au(111) surface. We have also studied the orientation and interactions between the cations and anions at this interface. Our study should be useful for designing suitable IL-gold interfacial materials and for improving the differential capacitance of the composite materials devoted for energy storage applications.

(iii) the intrinsic electronic dipole of the monolayer. The induced van der Waals (vdWs) interactions enhance the Pauli pushback effect and at the same time reduce the charge transfer at the interface. Metal surface−ionic liquid (IL)-based interface materials are used for energy storage applications.20−22 ILs are liquid at temperature below 100 °C. ILs have an advantage over conventional organic solvents because of their wide electrochemical window, great tenability, high charge density, high thermal stability, and low volatility. The structural orientation and the nature of cations and anions of ILs define the specific capacitance of the supercapacitor and of various energy storage materials.23 All of these properties make them well-recognized materials for electrolytes in batteries and supercapacitors and as catalytic conversion materials.24 Compared to conventional liquids, ILs have well-structural order at the interface9,25 because of their vdWs, H-bonding, and coulombic interactions26 either mutually or with the metal surface (e.g., Fe, Ag, and Cu). For instance, Heinz and coworkers studied the molecular ordering of ILs at metal surfaces using DFT and molecular dynamics (MD) simulations. They investigated the structure and energetics of the self-assembly of 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][ES]) on the crystallographic {111}, {100}, and {110} facets of gold. Single ion pairs to multilayers were considered. These authors showed that the adsorption mechanism is controlled by the interplay of soft epitaxy, ionic interactions, induced charges, and steric effects in connection with the geometry of the cation and the anion within the IL pair.14 Using surface force apparatus and atomic force microscopy, Lui et al. characterized electrical double-layer structures of ILs at the metal−IL interface. They found that increasing the size of the nonhalogen anion leads to decreasing the columbic, H-bond, and interaction strength of ILs at the interface. The electrical double layer (EDL) formation is largely influenced by Hbonding, steric effects, and molecular interactions.27 More generally, these interfacial interactions decide on the performance of the electrochemical systems.28 In interfacial molecule− substrate systems, interactions occur through various kinds of covalent and noncovalent bonding.29,30 At the microscopic level, noncovalent interactions are very difficult to describe both experimentally and theoretically. Recently, we used first-principles methodologies to treat the adsorption mechanism of various hydrophilic and hydrophobic ILs (1-butyl 3-methylimidazolium [BMIm]+[X]−, where X = Cl−, DCA−, HCOO−, BF4−, PF6−, CH3SO3−, OTF−, and TFSA−) interacting with the Au(111) surface.31 A good agreement was found with experimental microscopic and macroscopic observations, hence validating our theoretical approach. Here, we perform a theoretical study of hydrophilic/ hydrophobic fluorinated and nonfluorinated ILs interacting with Au(111). These ILs have the general formula [CnMIm]+[X]−, where X = Cl−, PF6−, and TFSA−, MIm = methylimidazolium and where we varied the alkyl chain length (i.e., Cn) for n = 0, 2, 4, 6, 8, and 10. Earlier experimental works showed that the alkyl chain length can affect the properties of this liquid−solid model interface, such as viscosity, surface tension, and conductivity.32−38 For instance, the work of Hu et al. dealing with various alkyl chainsubstituted anthraquinones adsorbed on the highly oriented pyrolytic graphite interface revealed that the Eads increases by lengthening the alkyl chain because of strong vdWs

2. COMPUTATIONAL DETAILS We performed computations of fluorinated and nonfluorinated ILs [CnMIm]+[X]− (where n = 0, 2, 4, 6, 8, and 10; X = Cl−, PF6−, and TFSA−) interacting with an Au(111) surface. These electronic computations were done using Perdew−Burke− Ernzerhof (PBE) generalized gradient approximation DFT to define the exchange−correlation40 with and without considering Grimme’s latest version of the empirical correction term (DFT-D3) to study the effect of dispersion correction.40−42 This is mandatory to describe the interaction of the organic molecule at the metal surface and carbon-based composite materials.43 Two types of computations are done: (i) “gasphase-type computations” where isolated ILs are treated; and (ii) “liquid−solid periodic computations” to model the IL-gold interface. The comparison of both sets of data should help for understanding the gold surface ILs adsorption-induced energetic and structural effects. The structures of isolated ILs were optimized using the GAUSSIAN 16 and GaussView 6.0 software packages.44 All atoms in ILs were described with the 6-311++G** basis set.45 These geometry optimizations were done without constrains at the PBE(+D3) level, followed by frequency calculations to confirm that they correspond to a minimum on their potential energy surfaces. The binding energies (BEs) were deduced and corrected for the basis set superposition error employing the counterpoise (CP) procedure of Boys and Bernardi.46 15088


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Figure 1. Optimized structures of [CnMIm]+[Cl]− complexes as computed at the PBE + D3/6-311++G** level along with the cation−anion distances in Å.

Figure 2. Optimized structures of [CnMIm]+[PF6]− and of [CnMIm]+[TFSA]− complexes as computed at the PBE + D3/6-311++G** level along with the cation−anion distances in Å.

BE([CnMIm]+ [X]− ) = E[CnMIm]+ [X]− − (E[X]− + E[CnMIm]+)

density and pseudopotentials are expressed by a mixed GPW basis set scheme.49 All atoms, except Au, were described by the optimized MOLOPT-TZVP basis set,50 whereas we used the DZVP-MOLOPT-SR-GTH (SR denotes a short range) basis set for Au atoms. A plane wave cutoff (400 Ry) was included. Orbital transformation was also performed.51 In this study, atomic structures of the Au(111) surface were taken from the previously resolved global minimum structures.52 The slab that consisted of three layers each has 48 Au atoms. Throughout the study, we fixed the bottom layer and the upper two layers were optimized freely. Afterward, we optimized the structure of [CnMIm]+@Au(111) and [CnMIm]+[X]−@Au(111) composite materials. The adsorption energies, Eads, of [CnMIm]+ and

(1)

where E[CnMIm]+[X]−, E[X]−, and E[CnMIm]+ are the total energies of the [CnMIm]+[X]− IL complex, anion, and cation as computed in the full basis set of the IL complex. The gas-phase-optimized geometries were used as a starting point for the [CnMIm]+@Au(111) and [CnMIm]+[X]−@ Au(111) bulk phase periodic computations. We used the CP2K/Quickstep code for bulk phase optimization.47 A hybrid basis set formalism known as a Gaussian and plane wave (GPW)48 is implemented in CP2K, where the Kohn−Sham orbitals are expanded in terms of contracted Gaussian type orbitals, whereas an auxiliary plane wave basis set is used to expand the electronic charge density. Indeed, valence electron 15089


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The Journal of Physical Chemistry C Table 1. BEs (kcal/mol) of Optimized [CnMIm]+[X]− (X = Cl, PF6, and TFSA) ILs in the Gas Phase BEs n = 0−10 method/basis set PBE/6-311++G(d,p)

PBE + D3/6-311++G(d,p)

X

0

2

4

6

8

10

Cl PF6 TFSA Cl PF6 TFSA

−159.2 −89.0 −87.0 −159.8 −91.4 −90.9

−95.9 −80.3 −78.2 −97.3 −84.4 −84.0

−95.3 −79.7 −77.2 −96.9 −84.3 −83.6

−94.9 −79.4 −76.9 −96.6 −84.1 −83.5

−94.8 −79.2 −76.7 −96.5 −84.0 −83.4

−94.8 −78.9 −76.6 −96.5 −84.3 −83.3

of [CnMIm]+[X]− on the Au(111) surface were evaluated using the following equation Eads(Y@Au(111)) = E[complex] − (EAu(111) + E Y )

1.5 to 2.7 Å. The shortest H-bonding distance (∼1.493 Å, Figure 1) is for 1-methyl-imidazolium chloride [MIm]+[Cl]− IL. For fluorinated ILs, we have similar type of bonding where the F/O atoms of the corresponding anions form H-bonds with the C−Hs of the alkyl groups (Figure 2). The calculated BEs of all selected ILs [CnMIm]+[X]− (where n = 0, 2, 4, 6, 8, and 10 and X = Cl− and PF6−, TFSA−) are given in Table 1. These BEs range from −83 to −160 kcal/mol. Because of the strong H-bonding and dipole interaction within [MIm]+[Cl]− IL, the absolute value of its BE is relatively high (∼160 kcal/mol). Also, Table 1 shows that fluorinated ([CnMIm]+[PF6]− and [CnMIm]+[TFSA]−) ILs have close BEs. For instance, [MIm] + [PF 6 ] − and [MIm]+[TFSA]− have BEs of −91 and −90 kcal/mol, respectively. When one compares nonfluorinated ([CnMIm]+[Cl]−) and fluorinated ([CnMIm]+[PF6]− and [CnMIm]+[TFSA]−) ILs, the later respective BEs significantly reduce (in absolute value by ∼56%). As said above, this is due to the strong electrostatic interactions between the Cl− with the C2−H of the alkyl groups, whereas the charge is delocalized over the fluorinated anion, which reduces the corresponding anion−cation interaction. The lowest BE was observed in the case of [DMIm]+[TFSA]− because of the hydrophobic perfluro moiety and the longer alkyl part.31 Our work shows that the alkyl groups affect the ion-pair interaction energy because of the electron-donating nature of the longer alkyl chain. For n ≥ 2, a reduction of the BE is observed because of the weakening of the anion−cation bonding. The BEs follow this order: n = 0 > 2 > 4 > 6 > 8 > 10 for [CnMIm]+[Cl]−, with a slight variation for n ≥ 2. Indeed, ILs with n ≥ 2 exhibit close BEs, whatever the length of the alkyl groups. For explanation, we display in Figures S1−S4 the electrostatic potential (ESP) maps of the ILs of interest in the present study. These pictures show the electron-rich and electron-poor sites that can be used to interact with the Au(111) surface to stabilize the complex structure. In particular, we note that the negative ESPs (red regions) accumulate in the −Cl, −PF6, and −TFSA anions of the ILs, whereas higher positive charges are localized in the MIm+ (blue regions). When increasing the alkyl chain length, the positive potentials in the cationic part reduce. Because of the overall charge distribution on the ILs, the ESPs of the ILs significantly increase from −5.359 × 10−2 to −8.425 × 10−2 for the [CnMIm]+[Cl]− series. This tendency is completely reversed in the case of hydrophobic ILs such as [CnMIm]+ pairing with [PF6]− and [TFSA]−. 3.2. Structure and Energetics of [CnMIm]+ (n = 0, 2, 4, 6, 8, and 10) Cations Adsorbed on the Au(111) Surface. We performed periodic calculations to study the role of alkyl chain length in the adsorption of alkyl methyl imidazolium ([CnMIm]+, n = 0, 2, 4, 6, 8, and 10) on the Au(111) surface ([CnMIm]+@Au(111)) employing the dispersion-corrected

(2)

where E[complex], EAu(111), and EY are the total energies of Y@ Au(111), of EAu(111), and of Y [= [CnMIm]+ or [CnMIm]+[X]− (where n = 0, 2, 4, 6, 8, and 10)], respectively. For rationalization, we generated a cube density file from the optimized coordinates at the PBE + D3/TZVP level to discuss the charge distribution within the ILs and upon adsorption of [CnMIm]+ or [CnMIm]+[X]− species on the gold surface (cf. Supporting Information Figures S1−S8). We also performed a Löwdin charge-transfer analysis of the complexes interacting with Au(111). Löwdin charge analysis shows the effect of the involvement of the anions within the complex and the possible enhancement of the charge-transfer process in the local environment. The charge density difference is expressed by the following equation Δρ = ρ(Y@Au(111)) − ρ(Au(111)) − ρ(Y) +

(3) +

−

where Y denotes [CnMIm] or [CnMIm] [X] (n = 0, 2, 4, 6, 8, and 10), ρ(Y@Au(111)), ρY, and ρ(Au(111)) are the total charge density of the system, charge densities of ILs, and charge density of the gold surface, respectively. The corresponding data are given in Tables S1 and S2 of the Supporting Information.

3. RESULTS 3.1. Equilibrium Geometries of ILs in the Gas Phase. The optimized structures of ILs [CnMIm]+[X]− (where n = 0, 2, 4, 6, 8, and 10 and X = Cl− and PF6−, TFSA−) were computed at the PBE/6-311++G** and PBE + D3/6-311+ +G** levels of theory. They are shown in Figures 1 and 2 along with the distances between the cation and the anion (Å). In the following, the cationic part of the ILs is denoted as MIm for n = 0, EMIm for n = 2, BMIm for n = 4, HMIm for n = 6, OMIm for n = 8, and DMIm for n = 10 (cf. Figures 1 and 2). There are four different interionic interactions possible in various hydrophobic/hydrophilic ILs in the gas phase. These are H-bonding, vdWs, dispersion, and Coulomb forces.53−55 For instance, both hydrophilic and hydrophobic ILs can form multiple C−H donor sites within the ILs to stabilize these species.56,57 Because dispersion correction is important to describe these interactions, we will focus hereafter on discussing only PBE + D3 data. Calculations show that hydrophilic [Cl]− anions are directly H-bonded with the C2−H position of the cation (i.e., first carbon attached to MIm) because of the strong electrostatic and dispersive mode of interaction with the anion (Figure 1). Whereas hydrophobic fluorinated anions are weakly interacting with the cation (Figure 2). The ion-pair distances vary from 15090


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[MIm]+ (i.e., center) and the Au(111) surface gradually decreases. This is due to the involvement of −CH2 group(s) of alkyl chain which form(s) several C−H···Au H-bonded interaction with the Au(111) surface. The calculated adsorption energies (Eads’s) vary from −110 to −183 kcal/mol with strong variation when the alkyl chain is lengthened. For instance, [HMIm]+ has a stronger interfacial structure than [BMIm]+ and [EMIm]+ because of the enhancement of the adsorption strength between the alkyl tails and gold surface (via weak interactions). For n = 4 and 6, the respective alkyl end groups are protrude away from the surface. For n > 6, the alkyl groups are lying parallel to the surface and strongly adsorb on the gold surface. We found that the entire corresponding alkyl groups predominantly interact at top/bridge sites of the surface. Consequently, Eads of [DMIm]+ is significantly higher (−183.2 kcal/mol) than those for the others. Anyway, the longer alkyl chains gain further stabilization (larger Eads in the absolute value) because of the stronger adsorption of alkyl chains on the surface. 3.3. Structure and Energetics of [CnMIm]+[X]−@Au(111) Surface (Where n = 0, 2, 4, 6, 8, and 10; X = Cl, PF6, and TFSA). Using the PBE + D3/TZVP method, we performed periodic computations of [CnMIm]+[X]− attached to the Au(111) surface. We have selected hydrophilic [Cl]− and hydrophobic [PF6]− and [TFSA]− anions. The optimized geometries of the [CnMIm]+[X]−@Au(111) (where n = 0, 2, 4, 6, 8, and 10; X = Cl, PF6, and TFSA) ILs are depicted in Figures 4 and 5. Figure 4 presents the optimized structures of [CnMIm]+[Cl]− adsorbed on the Au(111) surface. Computations show that these ILs are strongly adsorbed on the surface where the [MIm]+ aromatic ring is parallel to the surface. These ILs have structures similar to those described above for gas-phase-respective species. We also observe alkyl chain −CH2 groups involved in AAHBs with the surface. As for [CnMIm]+@Au(111), this is mainly due to the stabilization of the [MIm]+ ring on the gold surface through cation···π stacking interactions and the subsequent induced AAHBs with the metal surface. As said above, this is in line with previous surface-enhanced Raman spectroscopy (SERS) measurements and computations of methylimidazole containing compound interacting with (111) surfaces.14,58−61 It is also observed that the long-chain alkyl groups are interacting with a parallel configuration to the surface (bridge/ top sites) via AAHB with the gold surface. The [Cl]− anions are strongly chemisorbed to the gold surface because of columbic and electrostatic interactions between the anion and the gold surface. Interestingly, we found that the adsorption

PBE + D3 method. The optimized geometries and adsorption distances of all alkyl chain cations (in the absence of anion) on the gold surface are shown in Figure 3. The corresponding Eads

Figure 3. Optimized geometrical structures of the [CnMIm]+@ Au(111) surface as computed at the PBE + D3/TZVP level of theory and nearest interacting distances (in Å) between [CnMIm]+ and Au(111) surface.

are listed in Table 2. Figure 3 shows that all [CnMIm]+ cations lay parallel to the surface via cation···π stacking interactions established between the imidazolium aromatic ring and the Au(111) surface. This was already observed for other methylimidazole containing compounds interacting with (111) surfaces.14,58−61 For imidazole interacting with Au(111), we found however that the imidazole is orthogonal to the surface.62,63 Figure 3 also shows the further stabilization of the alkyl chains by anchor-assisted H-bonds (AAHBs) on the gold surface. This leads to relatively large adsorption energies. For instance, we compute an Eads of −110.3 kcal/mol for the [MIm]+ cation stacked on the Au(111) surface. This is associated with cationsurface distances of 2.6−3.6 Å. When the alkyl chain is longer, the calculated distance between

Table 2. Adsorption Energies (Eads, kcal/mol) of [CnMIm]+ (Alkyl Chains) and of [CnMIm]+[X]− (X = Cl, PF6, and TFSA) ILs on the Au(111) Surface adsorption energies (Eads) n = 0−10 methods PBE/TZVP

PBE + D3/TZVP

ILs@Au(111)

0

2

4

6

8

10

[CnMIm]+ [CnMIm]+[Cl]− [CnMIm]+[PF6]− [CnMIm]+[TFSA]− [CnMIm]+ [CnMIm]+[Cl]− [CnMIm]+[PF6]− [CnMIm]+[TFSA]−

−82.6 −32.9 12.5 2.2 −110.3 −50.4 −32.9 −24.2

−70.4 −25.8 12.2 8.2 −109.0 −72.1 −31.4 −29.8

−81.1 −24.0 7.1 13.07 −114.0 −81.8 −46.6 −28.6

−87.9 −31.9 22.1 20.9 −125.9 −94.3 −42.6 −42.7

−76.2 −37.6 14.3 18.2 −132.2 −96.0 −24.8 −50.0

−183.2 −38.2 13.1 8.4 −183.2 −107.3 −31.7 −53.8

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and 2) are −50 and −72 kcal/mol, respectively. Whereas the largest absolute value E ads (−107 kcal/mol) is for [DMIm]+[Cl]−. Figure 5 displays the optimized structures of fluorinated ILs interacting with Au(111). Both [C nMIm] + [PF6 ]− and [CnMIm]+[TFSA]− ILs are weakly adsorbed on the Au(111) surface. In the vicinity of Au(111), fluorinated ILs present different behaviors when compared to [CnMIm]+[Cl]− ILs. Indeed, the anions are not directly interacting with the surface. Instead, they interact with the cation, which is adsorbed on the metallic surface. This was already noticed for other hydrophobic ILs interacting with surfaces (e.g., [BMIm]+ with [X]− (BF4−, PF6−, CH3SO3−, OTF−, and TFSA−)31). In addition, it is worth noting that the equilibrium structures of [CnMIm]+[PF6]− ILs are different from those found in the gas phase. In the present case, the anions are bound to the imidazolium core by π-stacking rather than by the multiple Hbonds described above for isolated ILs. Close analysis of the [CnMIm]+[PF6]−@Au(111) structures shows that all alkyl chain end groups are not adsorbed on the surface, whereas [CnMIm]+[TFSA]− ILs behave differently at these interfaces. Moreover, we found that the [MIm]+ aromatic ring cation is in parallel stacking with the surface for [CnMIm]+[PF6]− ILs, whereas the same aromatic ring cation is slightly tilted on the surface for the [CnMIm]+[TFSA]−@ Au(111) interface. This tilting effect is due to H-bond interactions taking place between the −C2H groups of the alkyls and the oxygen atom of the TFSA− anion. Consequently, the stacking distances between the center ring of [CnMIm]+[PF6]− and Au(111) vary from ∼3.5 to 3.7 Å, whereas a significantly shorter distance (of ∼2.4−3.0 Å) is computed between the [CnMIm]+[TFSA]− ILs and Au(111). These structural effects significantly affect the ILs Eads at the gold surface. The calculated Eads values for [CnMIm]+[PF6]−@ Au(111) and [CnMIm]+[TFSA]−@Au(111) vary from −25 to −47 and −24 to −54 kcal/mol, respectively (Table 2). Therefore, these hydrophobic anions decrease the Eads energies by 30% compared to those of the [Cl]− anion-based ILs. This is due to the charge delocalized on the [PF6]− anion which can induce strong electrostatic interactions in the C2−H and alkyl chain of [MIm]+, which are dominating the ion-pair−surface interaction. For [EMIm] +[ES]− adsorbed at Au(111), adsorption energies of ∼−30 to −40 kcal/mol were computed by Heinz et al.14 These earlier data are close to those found here for the [EMIm]+[PF6]− and [EMIm]+[TFSA]− ILs interacting with Au(111). They are however smaller (in the absolute value) than those computed for Cl− containing ILs. 3.4. Epitaxial Interactions at the Au(111)−IL Interface. Previous works showed the existence of soft epitaxial interactions of ILs on gold surfaces.8,14 This is also the case for the ILs of interest in the present work. For illustration, we present in Figure 6 top views of [BMIm]+ with Cl−, or PF6− or TFSA− adsorbed at Au(111), where we highlight the different adsorption sites and the epitaxial fcc lattice contacts. In the case of hydrophilic ILs, the nitrogen atoms of the Im+ cation are strongly adsorbed on the top layer of the gold atoms. In the case of hydrophobic ILs, the nitrogen atoms of Im+ have contacts with the epitaxial sites of the gold atoms in the second layer. Furthermore, one can see that the hydrophilic Cl− anion is strongly coordinated with the epitaxial fcc lattice site, whereas the hydrophobic PF6− and TFSA− anions possess contacts with the epitaxial fcc and the top layer of the gold atoms.

Figure 4. Optimized geometrical structures of nonfluorinated [CnMIm]+[Cl]−@Au(111) surface as computed at the PBE + D3/ TZVP level of theory and nearest interacting distances (in Å) between ILs and Au(111) surface.

sites of the [Cl]− anions depend on the length of the alkyl chain. In the case of [MIm]+[Cl]−@Au(111) and of [EMIm]+[Cl]−@Au(111), Cl− anions strongly adsorb or chemisorb at the top gold surface sites. In [BMIm]+@Au(111) and [HMIm]+@Au(111), Cl− anions are chemisorbed at the bridge sites. For [OMIm]+[Cl]−@Au(111) and [DMIm]+[Cl]−@Au(111), Cl− anions are attached at the hcp (hexagonal close-packed) sites of the gold surface. These structural changes are due to local modifications of the electronic structure of the gold surface upon adsorption of the ILs, as illustrated in Figure S6. Using MD simulations, a similar adsorption mechanism was found for the adsorption of arginine and peptides in the aqueous solution on a gold(111) surface by Heinz and Ramezani-Dakhel.8 These authors showed that the adsorbate molecules strongly adsorb to the metal surface on the epitaxial sites and elude from the top sites. They also strongly adsorb on the Au(111) facet rather than Au(100) and Au(110) surfaces. For [CnMIm]+[Cl]−@Au(111), the calculated stacking distances between [MIm]+ ring and Au(111) surface vary from ∼3.5 to 3.7 Å, whereas the distances between the alkyl chains and gold surface decrease while increasing the number of methylene groups in the chain (Figure 4). The computed Eads values for the [CnMIm]+[Cl]−@Au(111) surface are given in Table 2. They range from −107 to −50.0 kcal/mol. These Eads energies significantly reduce (up to 50%) when compared to those of [CnMIm]+@Au(111) (i.e., bare cation). Again, this is because of the local charge density modification induced by the Cl− anions attached to Au(111). Table 2 shows that the shorter alkyl chains have smaller Eads energies than the longer ones, which exhibit additional vdWs and AAHB with the gold surface. For instance, Eads values of short alkyl chains (n = 0 15092


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Figure 5. Optimized geometrical structures of Fluorinated [CnMIm]+[PF6]−@Au(111) and [TFSA]@Au(111) surface as computed at the PBE + D3/TZVP level of theory and nearest interacting distances (in Å) between ILs and Au(111) surface.

epitaxial contacts of these alkyl groups at the gold surface. A similar observation was found in the adsorption of amino acids in the aqueous solution at the Au(111) surface using MD simulations.60 Among all amino acids, arginine has the strongest binding, which is attributed to the large molecular size of arginine allowing more contacts with epitaxial sites. Whereas small molecular size amino acids such as alanine interact with lesser number of epitaxial sites of the gold atoms. 3.5. Charge Population Analysis. Distributed electronic charges of the individual atoms are quantitatively identified by using the Löwdin population analysis. The charges between ion-pair of ILs and those of the gold surface are given in Table S2 of the Supporting Information. Figure 7 shows the plot of electronic charge of the Au surface after attachment of the [CnMIm]+[X]− (X = Cl−, PF6−, and TFSA−) as derived from the Löwdin charge-transfer analysis. The obtained electronic charges indicate that there is a significant amount of charge transfer from the ILs to the Au(111) surface upon adsorption. In hydrophilic ILs (with Cl−), the amount of charge transfer gradually goes up with increasing alkyl chain length from n = 0 to 10. The partial atomic charges are −1.47 a.u. for n = 0, −1.80 a.u. for n = 2, −2.11 a.u. for n = 4, −2.71 a.u. for n = 6, −2.48 a.u. for n = 8, and −2.76 a.u. for n = 10. It is interesting to note that there is a significant amount of charge transfer in the case of [DMIm]+[Cl]− (cf. Supporting Information Table S2). This is due to the chemisorption of the Cl− anion at the gold surface, which can create a local modification of the surface with a more electrostatic attractive surface favoring hence the adsorption of alkyl chain through AAHBs.

Figure 6. Snapshots representation of epitaxial interaction sites of various ILs@Au(111): (1) [BMIm]+, (2) [BMIm]+[Cl]−, (3) [BMIm]+[PF6]−, and (4) [BMIm]+[TFSA]−. The red circles highlight the epitaxial soft contacts.

As discussed above, the alkyl chains are adsorbed on the gold surface mainly via AAHBs. Figure 6 shows that the alkyl groups are interacting with the top and epitaxial sites of the gold atoms. The computed absolute values of the adsorption energies are larger for the long alkyl chains except for PF6− containing ILs, which is in line with the number of top and 15093


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potential distributions for [BMIm]+@Au(111), [BMIm]+[Cl]−@Au(111), [BMIm]+[PF6]−@Au(111), and [BMIm]+[TFSA]−@Au(111) complexes. One can see that the most pronounced effect is for [BMIm]+[Cl]−@Au(111) complexes. Indeed, the Cl− ion binds with different sites on the gold surface; this is due to the high electronegativity of Cl atom. This high electronegativity of Cl− ion can induce the charge transfer to the gold surface. Hence, the calculated binding strength is significantly higher (i.e., > 2 times) than the respective hydrophobic cases. 3.6. Charge Density Difference Analysis. The adsorption and desorption mechanism of ILs at the gold surface can be identified using electron density difference analysis. The electron density contour maps of the ILs@Au(111) surface are given in Figures S5−S8. The blue and red regions represent negative and positive charge density contours, respectively. Figure S5 (in the Supporting Information) shows that the adsorption of the [CnMIm]+@Au(111) surface (where n = 0, 2, 4, 6, 8, and 10) induces negative (dark blue) charge density at the carbons of the MIm+ part. When the alkyl chain length of [CnMIm]+ interacting with Au(111) increases, the positive and neutral charges accumulate however at different positions of these chains, where a spread of negative and positive charges over the alkyl chain is observed. Similar findings are observed for the cationic parts of [CnMIm]+[Cl]−@Au(111). Whereas there are significant differences for the anionic part because of the specific adsorption sites (i.e., top, bridge, and hcp) of the Cl− anion as described above. Again, this confirms the strong electrostatic interactions between surface Cl− ions and Au, which is tuned by the alkyl chain length-induced effects. For [CnMIm]+[PF6]−@Au(111), the charge density is distributed only between imidazolium ring and Au surface. For [CnMIm]+[TFSA]−@Au(111), Figure S8 also shows that the oxygen atoms of TFSA play crucial roles in sustaining electrostatic behavior throughout the molecular structure, either in the alkyl chain or in the imidazolium ring. These variations significantly affect the interfacial structure at the microscopic level and the interfacial interactions between ILs and surfaces. Similarly, previous studies of ILs at the graphite surface pointed out the occurrence of charge-transfer phenomena at the carbon-based interfacial structure.64

Figure 7. Electronic charge of the [CnMIm]+[X]−@Au(111) surface as computed by Löwdin charge-transfer analysis.

For the hydrophobic ILs@Au(111) surface (i.e., [PF6]− and [TFSA]−), the alkyl and anions induced charge modification of the surface are different. The maximum charge transfer ability of these two anions is −0.75 a.u. for [HMIm]+[PF6]−@ Au(111) and −0.98 a.u. for [DMIm]+[TFSA]−@Au(111). The minimum and maximum charge distributions are observed for [CnMIm]+[PF6]−@Au(111) when n = 0−6, respectively. For n > 6, the charge density reduces because of the hydrophobic alkyl chain not fully adsorbing on the surface and the perturbations of the MIm ring cation···π stacking. In the case of [TFSA]− containing ILs, we observe a gradual increment of the charge vs the alkyl chain variation. The various ions in the anionic parts (N, O, F, and S) can adsorb on various sites of the Au(111) surface inducing a significant amount of charge on the gold surface. This will affect the Eads of ILs at the gold surface. These findings are useful to develop composite materials for the energy storage and bio-sensor applications. The importance of induced charges was discussed in the recent work on biomolecular interactions at the metal interfaces.69 In this study, we found similar effect, which is in good agreement with the charge-transfer analysis (Figures 7 and 8) of the ILs@Au(111) complexes. For illustration, we show in Figure 9 the layer-by-layer magnitude of charge

4. DISCUSSION Our calculations reveal that the alkyl chain of ILs adsorption on the gold surface depends on the nature of the anion and on the length of its alkyl chain. Significant interfacial interaction differences are observed. We found that there are two modes of adsorption of alkyl chain on the gold surface (i) AAHBs with the involvement of cation···π stacking and (ii) anion-induced anchor-assisted H-bonds. Indeed, we found an anion-dependent behavior on the gold surface where both cation···π and AAHBs are predominant in the case of hydrophilic (i.e., [C n MIm] + [Cl] − ) ILs, whereas hydrophobic ILs (i.e., [CnMIm]+[X]−, X = PF6 and TFSA) behave differently. The PF6− anion at the gold surface induces only the cation···π stacking of the MIm+ ring, whereas TFSA− induces both interactions with a reduced magnitude of adsorption. The interplay between these two modes of interactions plays a vital role in the stability of the complexes and the electrostatic behavior of the surface. Recent report reveals that vdWs interactions enhance the Pauli pushback effect and simultaneously reduces the charger transfer at the interface. This is in very good agreement with our ILs@Au(111) models.19

Figure 8. Relation between calculated Eads with the number of carbon atoms at the [CnMIm]+[X]−@Au(111) surface by using (DFT + D3). 15094


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Figure 9. Magnitude of charge potential distribution in ILs@Au(111) complexes with layer-by-layer analysis (1) for [BMIm]+, (2) for [BMIm]+[Cl]−, (3) for [BMIm]+[PF6]−, and (4) for [BMIm]+ [TFSA]− complexes.

5. CONCLUSIONS In this work, first-principles computations are carried out to investigate the interfacial structure of alkyl chain-substituted MIm ILs adsorbed on the Au(111) surface. Calculations show that the anion plays an important role in the behavior of the alkyl group adsorption on the Au(111) surface. We found that there are two modes of adsorption of alkyl chains on the gold surface either via AAHBs with the involvement of cation···π stacking or via anion-induced AAHBs. We thus discussed the anion-dependent behavior on the gold surface. We showed that cation···π and AAHBs are predominant in the case of hydrophilic ILs (i.e., [CnMIm]+[Cl]−), whereas hydrophobic ILs (i.e., [CnMIm]+[X]−; X = PF6 and TFSA) behave differently. The PF6− anion at the gold surface induces only cation···π stacking of the MIm+ ring, whereas TFSA− induces both interactions with a reduced magnitude of adsorption. The interplay between these two modes of interactions is vital for the stability of the complexes and for the electrostatic behavior of the surface. Furthermore, we showed that the alkyl chain length affects the interfacial interactions and thus changes the Eads of these compounds on gold. This is due to the charge transfer between ion-pair and ILs@Au(111) surface. Particularly, AAHB is involved in the stability of the complexes. It is interesting to note that the [CnMIm]+ with the Cl− anion strongly chemisorbs on the Au(111) surface, whereas [PF6]− and [TFSA]− are mostly attached to the cationic part irrespectively the alkyl chain length. Therefore, the corresponding Eads is significantly depending on the ILs nature. The electron density and charge-transfer analyses are also in good accordance with our Eads values. In sum, our work disentangles the complex nature of the interactions occurring at solid−liquid interfaces. These findings should help to understand, at the microscopic level, the adsorption mechanisms of the ILs@Au(111) surface. In addition, our studies reveal that the electrostatic nature of the surface depends on the anion and the binding sites of the anion on this surface via the induced charge-transfer processes between the surface and the liquid. This potentially helps to find suitable energy storage and electrochemical devices. In

Figure 8 shows a comparison between Eads of cations and of the corresponding ILs. This figure reveals that Eads gradually increases while lengthening the alkyl chain for the bare [CnMIm]+ cation. When anions are incorporated, similar features are observed except for [CnMIm]+[PF6]−. As stressed out above, this is due to the stabilization of [CnMIm]+[PF6]− ILs away from the surface and only the central part of the [MIm]+ ring is stacked with the Au(111) surface. The analysis of the interfacial structures and energetics shows the competition between the strong electrostatic interactions within the ILs ion-pair, the dispersive and vdWs interactions, the AAHBs, and charge-transfer phenomena within the ILs and with the gold surface. These interactions are viewed to locally alter the gold surface and to depend strongly on the nature of ILs. Indeed, different structures are found for fluorinated hydrophobic ILs of interest in the gas phase and interacting with the gold surface, whereas close structures are found for [CnMIm]+[Cl]− ILs in both media. Such surface-induced structural features were already noticed for polypeptides, oligopeptides proteins, and polymers interacting with the metal surface. For instance, SERS studies revealed that di- and tripeptides adsorb onto the silver surface through the carboxyl terminus. This is due to the side chains of the amino acid residue closest to the carboxyl terminus. Whereas SERS spectra of the polypeptides and proteins are dominated by amide bands and aromatic side-chain vibrations.65 For bisphenol A-polycarbonate (BPA-PC) attached to the Ni(111) surface, Delle Site et al. found that alkylphenolic end groups repel from the surface but the benzene ring remains strongly adsorbed to the surface via the involvement of its π-cloud electron.66 In addition, they noticed an interplay between adsorption energies and conformational entropy of bisphenol A-polycarbonate (BPA-PC) at the Ni(111) surface. Later on, this group showed morphological differences at this interface, which can vary the melts at the interface.67 Through the investigations of oligopeptides interacting with the hydrophilic Pt(111) surface in the presence of water molecules, Ghiringhelli et al. showed that the nature of the functional group influences the metal surfaces and thus the surface-specific sequence design and devices.68 15095


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The Journal of Physical Chemistry C fine, our findings may be useful to develop interface materials for various applications such as catalysis, energy storage, supercapacitor, and battery technologies. This work can be extended by full consideration of the dynamics of the adsorption of ILs on this metallic surface. This should give insights on how the ILs reach the equilibrium structures at the gold surface and the adsorption sites identified by the present DFT electronic structure computations.14,69 Both approaches are complementary to understand at the microscopic level the properties of ILs close to metallic surfaces as stated in ref 14. The use of MD should compensate on some limitations of DFT on accurately describing the longrange parts of the interaction potentials, where the physically expected 1/r convergence of the image charge potential to zero at long range is not well reproduced. This is related to a strong electron delocalization error and an inaccurate description of the long-range vdWs interaction.69

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03242. Additional data of Lowdin charge-transfer analysis of ILs@Gas and Au(111) surface and ESP map of ILs@gas phase and electron density maps of the [CnMIm]+@ Au(111) surface (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. in. Phone: +91 44 2741 7686 (M.P.). *E-mail: [email protected] (M.H.). ORCID

Muthuramalingam Prakash: 0000-0002-1886-7708 Gilberte Chambaud: 0000-0002-8031-2746 Majdi Hochlaf: 0000-0002-4737-7978 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors extend their appreciation to the International Scientific Partnership Program (ISPP) at King Saud University for funding this research work through ISPP#0045. S.K. thanks SRM Institute of Science and Technology (SRM-IST) Research Fellowship for his research work. M.P. thanks the Department of Science and Technology-Science and Engineering Research Board (DST-SERB) of India for the financial support (grant number: ECR/2017/000891). The authors also thank SRM-IST for providing the supercomputing facility and financial support.

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Alkyl Methyl Imidazolium-Based Ionic Liquids at the Au(111) Surface

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