Structure of Quaternary Ammonium Ionic Liquids at Interfaces: Effects

Article pubs.acs.org/JPCC

Structure of Quaternary Ammonium Ionic Liquids at Interfaces: Effects of Cation Tail Modification with Isoelectronic Groups Shobha Sharma and Hemant K. Kashyap* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India S Supporting Information *

ABSTRACT: Herein we use molecular dynamics simulations to investigate the effects of cation tail modification with isoelectronic groups on interfacial structure of room-temperature ionic liquids (RTILs) when confined between two oppositely charged or two neutral graphene sheets. The RTILs chosen in this regard are triethyloctylammonium bis(trifluoromethylsulfonyl)imide (N2228+/ NTf 2 − ) and (2-ethoxyethoxy)ethyltriethylammonium bis(trifluoromethylsulfonyl)imide (N222(2O2O2)+/NTf2−). For all the systems studied, we determined number density, free energy, electric field and electrostatic potential profiles along the axis normal to graphene sheet plane. Our results predict that in both the systems, the positively charged graphene sheets are screened predominantly by anionic atoms and cationic tail groups that are embedded in the cavities formed by the anions in the interfacial region, and the extent of screening for the RTIL containing alkyl-substituted cations is slightly higher than that in the diether-substituted analog. Near the negatively charged sheet, while the probability of finding cation tails is relatively enhanced in the diether system, increased density augmentation of cation head groups in the N2228+ system is observed in comparison with that in the N222(2O2O2)+ system. We observe both perpendicular and parallel orientations of cation tail near the negatively charged wall, but it appears that the probability of parallel orientation for N222(2O2O2)+ is higher than that for N2228+. In the case of neutral sheets, the anionic distribution at the interface is similar in both the cases and cation tail enrichment in the first layer close to the neutral wall is increased in the system with a non-polar tail. The probability of finding cation head groups in the vicinity of neutral wall is slightly larger in the case of diether system. The simulated zone-resolved tangential radial distribution functions (TRDFs) in planar slabs parallel to the sheets show that the cation heads (or anions) in the interfacial region closest to the negatively (or positively) charged sheet are ordered more than that in the corresponding bulk liquid. For both the isoelectronic analogs, the overall charge density fluctuations and electric field oscillations near the positively charged walls are larger than that near the negatively charged walls.

1. INTRODUCTION Room-temperature ionic liquids (RTILs) have recently revolutionized a broad area of scientific research and industrial applications. Ionic liquid (IL) that is composed only of ions and is liquid below 100 °C is now widely accepted as roomtemperature ionic liquid. RTILs are equipped with several exquisite properties, such as high thermal stability, low volatility, substantial ionicity, wide electrochemical window, tunable hydrophobicity and hydrophilicity, tunable Lewis acidity and non-flammability.1,2 In past few years, RTILs have also been thought of as good electrolytes for their uses in batteries, photo-electrochemical cells, electroplating, and capacitors.3,4 Properties of RTILs can be changed by modifying the chemical nature of constituent cation and/or anion. This makes RTILs stand in the category of promising solvents and increases the area of their applications. These applications may also include their uses in biological research, synthesis, catalysis and heat storage. Until very recently, RTILs composed of cations with non-polar alkyl tail of different length were most studied and thoroughly understood.5−7 Only a few endeavors © XXXX American Chemical Society

can be found on RTILs with cation consisting of polar functional group(s).8−12 RTILs with polar tails appear to be less viscous and better ionic conductors.13 Experimentally it has been observed that such modification of cationic tail improves the kinetic performance of RTILs and thus make these RTILs to be used as electrolytes in energy efficient devices, like electrochemical cells, dye sensitized solar cells and super capacitors.3,4,14−19 However, such potential applications of these novel category of RTILs warrant a molecular-level understanding of their structural organization and dynamics not only in bulk phase but also at RTIL/electrode interfaces.20,21 In this work, we are aiming to characterize the interfacial properties of ammonium-based RTILs with a polar and a non-polar cationic tail. The attention is on understanding the effects of cation tail mutation with isoelectronic groups on the structural patterning of RTILs in the interfacial region close Received: July 6, 2015 Revised: September 24, 2015

A

DOI: 10.1021/acs.jpcc.5b06460 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

tails.11,12,57−62 In this work, we have performed molecular dynamics simulations of N2228+/NTf2− and N222(2O2O2)+/NTf2− RTILs (see Figure 1) confined between two oppositely charged

to the charged and neutral walls. This work will also provide a clear description of the charge alternation as well as the polar− nonpolar alternation at RTIL/electrode interfacial region. Theory of structural patterning to describe the electrostatic properties of diluted electrolyte solution at electrified interfaces starts with those of Gouy, Chapman, and Stern.22 It is now known that this classic theory works well for dilute electrolyte solutions, but in the cases where molecular length scales and shape are important factors, such primitive model based theories are not adequate, and concept of electrical doublelayer is also not applicable. 20,23−31 Recently, neutron reflectometry, cyclic voltammetry, and differential capacitance measurements25 have revealed that the interface between ionic liquid and positively charged gold surface was cation rich. This observation, that contradicts the classical double layer theory, indicates a non-electrostatic adsorption of cations onto gold electrode.25 Therefore, a comprehensive molecular level understanding of such interfaces is crucially important for the optimal utilization of the novel features related to such interfaces for their desired properties in interface-based applications.20,23,24,32−35 For example, the efficiency of electron transfer or redox reactions that occur in electrochemical devices is governed by the interfacial region near the electrode.36−38 In this context, molecular dynamics (MD) simulation studies done by Lynden-Bell and co-workers on representative imidazolium ionic liquids confined within charged and neutral graphene sheets were focused on elucidation of structural arrangement of ionic species near such walls.39,40 According to Wang et al., the arrangement of ionic species near interfacial region depends on the charge of ionic species, charge on wall and chemical structure of the ionic species.41 As described by Martin et al., the layered structure of the constituent ions of RTILs near charged walls could also be due to overscreening or crowding, depending upon the voltage applied. At lower voltage, overscreening is prominent, and if we keep on increasing the voltage, overscreening may turn into crowding. In the systems studied here, no sign of crowding of the counterions near the charged wall−ionic liquid interface is observed, but screening of the charged sheets can be noticed.42,43 This observation is not surprising as the charge density on graphene sheet used in this study (±1 e nm−2) is of medium strength. In pursuit of developing a concrete understanding of RTIL/ electrode interfacial structure, an extensive amount of other MD simulation studies have been done in the recent past26,44−52 and almost all of them were focused on imidazolium based RTILs. Compared to well-known imidazolium ionic liquids, quaternary ammonium ionic liquids are electrochemically more stable toward oxidation and reduction,1 and therefore can be considered as good electrolyte for high energy storage devices. Recently, Shirota et al.13 synthesized quaternary ammonium/phosphonium based RTILs along with their isoelectronic diether analogs and observed that the diether substituted RTILs possess lower viscosity.13 RTILs with lower viscosity and improved conductivity are crucially important for their uses in energy-application devices.13,18,53,54 Recently, the microscopic structure and dynamics of these RTILs were studied by using X-ray scattering,12 NMR,55 dielectric spectroscopy and quasi-elastic light scattering spectroscopy56 along with MD simulations.12,13 On the basis of these studies, it is now established that RTILs containing diether-substituted cations do not show intermediate range ordering and thus do not possess polar-nonpolar alternating domains that are often present in RTILs with moderately long alkyl-substituted cation

Figure 1. Chemical structure of (a) triethyloctylammonium (N2228+) (b) (2-ethoxyethoxy)ethyltriethylammonium (N222(2O2O2)+), and (c) bis(trifluoromethylsulfonyl)imide (NTf2−) ions. The positive charge distribution on the N2228+ cation is localized near head groups and the cation longest tail is overall electrically neutral.12 However, this is not the case for N222(2O2O2)+; the positive charge is well spread over the cation head and tail groups.12

or two neutral graphene sheets to appreciate how these RTILs respond to confinement and external stimulus such as electric field. The charge density for the charged graphene sheets were taken as ±1 e nm−2 and neutral walls possessed zero charges on the carbon atoms of the graphene sheets.

2. SIMULATION DETAILS Molecular dynamics simulations for N2228+/NTf2 − and N222(2O2O2)+/NTf2− RTILs confined between either two oppositely charged or two neutral monolayered graphene walls were performed in the canonical (NVT) ensemble by using GROMACS 4.6.5 package.63,64 The chemical structures of N2228+ and N222(2O2O2)+ cations are shown in Figure 1a,b, respectively. The anion (NTf2−) is depicted in Figure 1c. The temperature of all the systems was set to 292 K using a velocityrescale65 thermostat. For all the systems studied, a previously equilibrated12 box of RTIL with 1000 ionic-couples was carefully replicated along the z-axis, leading to a rectangular parallelepiped box with 2000 ionic-couples. The dimensions of the graphene sheets, constructed by using Visual Molecular Dynamics (VMD) package,66 were chosen according to the side-lengths of RTIL boxes along x and y directions. In case of N2228+/NTf2− system, the graphene walls were placed at z = 0 and z = 18.24 nm (see Figure 2a,b). For N222(2O2O2)+/NTf2− system, the two walls were placed at z = 0 and z = 17.674 nm (see Figure 2c,d). In case of charged walls, the wall at z = 0 was positively charged with +1 e nm−2 charge density and the other wall was negatively charged with −1 e nm−2 charge density. For neutral walls, both the walls were kept at zero charges. A vacuum of half of the box-length in the z-direction was used to avoid interaction between the negative wall of one image to the positive wall of another image (Figure 2). Please see the Supporting Information for the whole set of box dimensions used in the simulations. The Lennard-Jones (LJ) and bonded parameters for the graphene sheet atoms were adapted from those given by Trinidad et al.;39 ϵ = 0.29288 kJ mol−1,σ = 0.355 nm, r0 = 0.142 nm. Parameters for bis(trifluoromethylsulfonyl) B


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Figure 2. Equilibrium snapshot for (a) N2228+/NTf2− ionic liquid confined within charged graphene walls (blue = positively charged and red = negatively charged) (b) N2228+/NTf2− ionic liquid confined within neutral graphene walls (cyan = neutral) (c) N222(2O2O2) +/NTf2− ionic liquid confined within charged graphene walls (d) N222(2O2O2) +/NTf2− ionic liquid confined within neutral graphene walls. Here the oxygen atoms of N222(2O2O2)+ cation are depicted in red. The cations and anions are shown in lime and orange colors, respectively. The dimensions of the boxes provided in the figures are in units on nanometers.

Figure 3. Number density (relative to bulk number density), as a function of distance z from the positively charged graphene wall for (a) N2228+/ NTf2− and (b) N222(2O2O2)+/NTf2− RTILs. We have used about 5000 slices to define the bin-width in the above graphs.

imide NTf2− anion, triethyloctylammonium N2228+ and (2ethoxyethoxy)ethyltriethylammonium N222(2O2O2)+ cations were used from Lopes and Pádua67,68 (CLaP) and all-atom optimized potential for liquid simulations (OPLS-AA) forcefields.69−72The full set of parameters for these RTILs can be found in the Supporting Information of ref 12 along with the Supporting Information of the present article. Keeping the spirit of OPLS-AA type combination rules, all the cross terms for LJ interactions were approximated as ϵij = (ϵiϵj)1/2 and σij = (σiσj)1/2. Proper periodic boundary conditions and minimum image convention were applied. Electrostatic interactions were computed by using particle-mesh Ewald (PME) summation73 technique together with Yeh-Berkowitz correction for the slab geometry.74 The velocity-verlet integration algorithm with a

time-step of 1 fs was used for integrating equations of motion of RTIL atoms. Atoms of graphene sheets were frozen to their equilibrium bond distance during the simulations. The cutoff radius for the short-range interaction and the real-space part of electrostatic interaction was set to 12 Å. For PME summation interpolation order of 6 and Fourier grid spacing of 0.8 Å was employed. The equilibration run for N2228+/NTf2− and N222(2O2O2)+/NTf2− systems were of 25 and 35 ns, respectively. Atomic velocities were randomly generated at 292 K from a Gaussian distribution at the very first step of the equilibration run. Finally production run of another 10 ns was carried out and the trajectories were saved at every 100 fs for computing the density profiles, radial distribution functions and other properties. C


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Figure 4. Number density (relative to bulk number density) as a function of distance z from the negatively charged graphene wall for (a) N2228+/ NTf2− and (b) N222(2O2O2)+/NTf2− RTILs. We have used about 5000 slices to define the bin-width in the above graphs.

Figure 5. Number density (relative to bulk number density) as a function of distance z from the neutral graphene wall for (a) N2228+/NTf2− and (b) N222(2O2O2)+/NTf2− RTILs. Also, see Figure S2 for more details. We have used about 5000 slices to define the bin-width in the above graphs.

fluorine atoms of anion is also more than its bulk value. Beyond these peaks the triplet peaks in the range of 0.3−0.5 nm for anion nitrogen atoms can also be observed. This tells us that the first layer of the RTILs near the positively charged wall predominantly consists of anion atoms for both the systems. A careful investigation of the interaction terms for the cation and anion atoms reveals that the strong affinity of oxygen atoms toward the wall is due to both nonpolar (LJ) and electrostatic interactions. Although anion distribution near the positively charged wall looks similar in the density profiles for both the RTILs, there are differences in the cation head and tail distributions in the alkyl and diether substituted systems. It is clear that the most prominent peak for cation tail appears at ∼0.4 nm, before any peak that belongs to cation head for both the RTILs. This infers that on average there is dominance of cationic tail groups that are oriented toward the positively charged walls in both the systems. From these figures, Figure 3, parts a and b, it is clear that overall orientation of the charged species in both the systems near the positively charged wall

3. RESULTS AND DISCUSSION 3.1. Number Density Profiles. In order to analyze the ionic distribution or structural patterning at the interface between RTIL and graphene wall, in Figures 3−5 we have depicted the number density profiles, ρn(z), for cation head, cation tail and anion atoms as a function of distance (z) from the wall. The density profiles for cation head and tail were computed by using center-of-mass coordinates of the cation head and cation longest tail groups (see Figure 1 for the exact definition of cation head and tail groups) . The first feature to be observed from these figures is that for the charged systems, all the profiles show pronounced oscillations, the so-called layering, that last up to ∼1.5 nm distance from the graphene sheets. However, near neutral graphene walls these density oscillations are apparent only up to ∼1.2 nm. Parts a and b of Figure 3 show that number density augmentation for anion oxygen is maximum and is nearest to the positively charged sheet in both the systems. The first peak position for the anion oxygen atom is at around 0.3 nm from the wall. At around same distance the probability of finding D


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Figure 6. Potential of mean force, A(z), as a function of distance z from the neutral graphene wall for (a) N2228+/NTf2− and (b) N222(2O2O2)+/NTf2− RTILs.

follows the order: anion oxygen ∼ anion fluorine > anion nitrogen > cation tail > cation head. Density profiles near the negatively charged walls depicted in Figure 4, parts a and b, clearly have two dominant peaks nearest to the wall, at ∼0.4 nm and ∼0.45 nm. The closest peak belongs to cation tail (red) and the peak to it corresponds to cation head (black). As will be more clear later on, the closest peak belongs to cation tails that are parallel to the wall. The cation head peak has head facing toward the wall and the tails associated with these heads are oriented both parallel and perpendicular to the wall. One can also observe another cation tail peak at ∼0.5 nm for the diether system. A closer look of the simulation snapshots confirmed that this peak belongs to tails that are slightly tilted outward from the wall. Hence, for both the systems the first interfacial layer near the negatively charged graphene predominantly consist of cations. The subsequent layer in the alkyl-substituted system is broader and composed of cation tails and anions with no preferential orientation. But, in the case of diether system the second layer is comprised of cation head, cation tail as well as anions. The probability of finding the cation tails near negatively charged graphene sheet in the diether system is more than that in its alkyl-substituted analog. The ordering of the groups for both the systems near the negatively charged wall follows as cation tail > cation head > anion oxygen > anion fluorine > anion nitrogen. To investigate further details, number density profiles for nitrogen atom of cation head and three terminal carbon atoms of three ethyl tails were also calculated (see Figure S1). It was found that these ionic subunits are arranged near the negatively charged wall in the following order: cation head terminal carbons > cation tail > cation nitrogen > anion oxygen > anion fluorine > anion nitrogen. This implies that the approach of nitrogen atom, which is equivalent to cation head center-ofmass, of cation head toward negatively charged wall is hindered by three ethyl tails of the cation. The appearance of cation tail peak before cation head peak in the number density profiles is, therefore, due to steric shielding effects. For the systems confined between neutral walls (Figure 5a,b) one can observe that even in absence of external electric field, neutral graphene surfaces lead to certain structural patterning of

the confined RTIL, indicating significance of nonpolar (LJ) interactions between the wall and constituent species of the RTIL. At least three layers of ionic species are apparent up to 1.2 nm and beyond that corresponding bulk densities are recovered. The interfacial layer in this case consists of both cations and anions. The order of closest approach to the neutral walls for both system is anion fluorine > anion oxygen > anion nitrogen > cation tail > cation head. Since LJ parameters are closely related to atomic polarizabilities and atomic charge densities, we can explain this ordering on the basis of σ, the distance of closest approach, and ϵ values of the constituent atoms. As we can see from Table S2 in the Supporting Information, the position of the first peak in the number density profiles for the RTIL atoms corresponds to their respective σ values and the peak height and area under the peak are governed by both σ and ϵ values as well as electrostatic interaction between constituent ionic species. We also notice that cationic tail modification with isoelectronic group does not render any significant effect on structural ordering of the RTILs near the neutral walls, except that the probability of cation head being near to the wall is slightly higher in the diethersubstituted system. To summarize, when we compare charged and neutral systems, there are differences in the peak positions and peak heights because the charges on the graphene walls lead to molecular recognition. In charged systems, negative species are closer to the positively charged wall; positive species are closer to negatively charged wall, for screening of charges on the walls. Near the negative/positive wall, the surplus of cationic/anionic species that counterbalances the charge on the respective electrode attracts oppositely charged ions that in turn propagate the formation of the second layer of lower peak height. This process of formation of layered structure continues almost up to 1.5 nm, after which bulk density is achieved. These observations corroborate well with other studies.26,39,40,44−51 3.2. Potential of Mean Force. The amount of energy required to bring a given group or atom of RTIL from bulk to close vicinity of wall as a function of distance can be computed using potential of mean force (PMF), A(z), as E


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Figure 7. Potential of mean force, A(z), as a function of distance z from the positively graphene wall for (a) N2228+/NTf2− and (b) N222(2O2O2)+/ NTf2− RTILs.

Figure 8. Potential of mean force, A(z), as a function of distance z from the negatively charged graphene wall for (a) N2228+/NTf2− and (b) N222(2O2O2)+/NTf2− RTILs.

A(z) = −kBT ln

interfacial region of neutral walls. Also, the distance of closest approach for cation heads or cation tails are same in both the analogs. That is the distance of closest approach to the wall is not influenced by the tail modification. But the strength of attraction and repulsion for cation head as well as cation tail are different in both systems as they approach the neutral walls. Remarkable oscillations in the PMFs of anion atoms near positive wall can be seen in parts a and b of Figure 7. Anion atoms have higher preference toward the positive wall (Figures 7a,b) in both systems and are located at approximately equal distance from the positive wall, indicating the influence of electrostatic interaction between the wall and anion atoms. The oxygen atoms of the anion is most likely to be present near the wall in comparison to other atoms of the anion. Except fluorine, all atoms of anion encounter very large barrier at around 0.5−1 nm. From Figure S4 in the Supporting Information when we compare the results for cation head and tail PMFs near the

ρn (z) ρnBulk

(1)

Here ρn(z) is number density of the given species as a function of distance from the graphene sheet and ρBulk is its bulk density. n It is easy to conceive from Figures 6−8 that for all the systems studied, the PMFs show oscillatory behavior in close vicinity of the walls. The cation head PMFs also show tremendously large (divergent) barriers near 0.5−0.8 nm while approaching toward the positive wall (Figure 7) and at almost the same range the anion nitrogen PMFs show similar behavior while approaching to negative wall (Figure 8). For neutral walls, parts a and b of Figure 6 show that cation head and cation tail PMF profiles show deep minima near the interfacial region in both the systems, this is not the case for the anion atoms in the region (Figure S3). This observation indicates that the cationic groups are more preferred near the neutral walls and nonpolar interactions dominate in the F


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Figure 9. Tangential radial distribution functions (TRDFs) as a function of distance r for (a) cation nitrogen atom in N2228+/NTf2− system in the first zone near the interfacial region of the negatively charged graphene wall and (b) cation nitrogen in N222(2O2O2)+/NTf2− system in the first zone near the interfacial region of the negatively charged graphene wall (c) anion nitrogen in N2228+/NTf−2 system in first zone near interfacial region of the positively charged graphene wall and (d) anion nitrogen in the N222(2O2O2) +/NTf−2 system in first zone near interfacial region of the positively charged graphene wall. Here, each TRDF is compared with its corresponding RDF obtained in the corresponding bulk RTIL.

positive wall, we find that cation tails in both the system are preferentially pulled toward the positive wall and cation heads are pushed away from the wall. This is evident as the separation between cation tail and cation head first minima of the PMFs is large near the positively charged wall. Also the potential energy barriers for the cation tail in the 0.5−0.8 nm range is lower in the alkyl system than that in the diether RTIL. Near the negatively charged graphene, we observe that the cationic groups are more preferred near the wall (8a,b) and the anion locations under shallow potential are shifted to longer distances from the wall (Figure S5). Obviously, cationic tails of diether RTIL are more likely to be present near the negatively charged sheet in comparison to that of the alky tail RTIL. 3.3. Tangential Radial Distribution Functions. To elucidate the pair correlations between co- and counterions in planar slabs parallel to the graphene sheets, we computed zoneresolved tangential radial distribution function75,76 (TRDF) using eq 2

gij(r ) =

∑i , j δ(r − rij) 2πρregion dr Δz

;

zij < Δz (2)

where ρregion = Nregion/(ΔzLxLy) is average number density in each region and rij =

xij 2 + yij 2 . The slab width Δz is usually

set to 0.5 nm for proper statistical averages.75,76 Parts a−d of Figure 9 show TRDFs for cation nitrogen and anion nitrogen atoms in the interfacial slab nearest to the negatively and positively charged walls, respectively. These TRDFs are also compared with corresponding radial distribution functions (RDFs) in the bulk RTIL.12 The slabs chosen here were centered at the corresponding number density peak with maximum heights. From Figure 9, it is clear that for both the RTILs the tangential correlations of cationic (or anionic) nitrogen atoms in the interfacial region closest to the negative (or positive) wall are stronger than that in the bulk region. This means that the interfacial region is more ordered G


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Figure 10. Snapshot representing the anions and cations of (a) N2228+/NTf2− and (b) N222(2O2O2)+/NTf2− near the positively charged graphene. Isosurfaces representing the subvolume occupied by the anionic network in N2228+/NTf2− (c) and N222(2O2O2)+/NTf2− (d) near the positively charged graphene sheets. Isosurfaces representing the subvolume occupied by the anionic network and cations of N2228+/NTf2− (e) and the same for N222(2O2O2)+/NTf2− (f) near the positively charged graphene. Here, blue colored wall is positively charged graphene. Alkyl tails of cation are shown in lime color. In the cation molecules, nitrogen atoms are highlighted as blue spheres and oxygen atoms of diether alkyl tail of N222(2O2O2) + cations are highlighted as red spheres. In anion molecules, fluorine atoms are shown in pink, nitrogen atom in blue, oxygens in red, sulfur atoms in yellow and carbon atoms are shown in cyan color. Hydrogen atoms of cations are hidden for clarity. Only a cross-section of the simulation boxes are shown here.

than the bulk for both the RTILs, i.e., the cation head (or anion) packing in the interfacial region of the negatively (or positively) charged graphene wall is more than that in the corresponding bulk liquids. 3.4. Equilibrium Snapshots and Isosurfaces. The overall structural features obtained in the density profiles and TRDFs in the previous subsections are supported and can be appreciated more clearly by looking at the isosurfaces of the ions (Figures 10 and 11). In these figures we present a pictorial demonstration of anion and cation atom distribution in the interfacial region of charged walls by using isosurfaces. As apparent from Figure 10a,b, the first layer near positively charged sheet is anion-rich and the major axis of symmetry of the anions is mostly parallel to the wall, also showing the dominance of electrostatic interactions between anion oxygen and graphene sheet. From parts c and d of Figure 10, it is

evident that the anions in the interfacial layer nearest to the positively charged sheets are not homogeneously distributed and instead they form a well connected network, wherein there are cavities of irregular shapes. Smaller cavities are observed in the case of N222(2O2O2)+/NTf2− and mostly the cation tails are embedded in these cavities in both systems (Figure 10e,f). Clearly, these anionic networks and cavities are absent near the negatively charged walls (Figure 11a,b). As can be seen in parts c and d of Figure 11, for the N2228+/NTf2− system near the negatively charged wall, most of the cation tails are perpendicular to the graphene sheet, but a non-negligible parallel orientation of the cation tail can also be seen in the snapshots. In the N222(2O2O2)+/NTf2− system, due to presence of oxygen atoms and gauche kinks12,77 in the cation tail, the tails are not perfectly perpendicular to the wall and in most cases both the oxygen atoms of the cation tail are oriented H


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Figure 11. Isosurfaces representing the subvolume occupied by the anions of N2228+/NTf2− (a) and N222(2O2O2)+/NTf2− (b) near the negatively charged graphene sheets. Snapshot representing the anions and cations of N2228+/NTf2− (c) and the same for N222(2O2O2)+/NTf2− (d) near the negatively charged graphene. Here, red colored wall corresponds to negatively charged graphene. The color coding for other atoms shown here is same as in Figure 10. Only a cross-section of the simulation boxes are shown here.

d2Ψl (z)

parallel to the positively charged graphene wall, only a few cationic tails oriented normal to the graphene sheet can be observed in the diether system (Figure 10e,f). From Figure 11 it is quite apparent that while N2228+ tails are orientated both parallel and perpendicular to the graphene sheet, N222(2O2O2)+ tails are mostly aligned with the negatively charged graphene. This observation is not surprising because of the fact that the diether tail of N222(2O2O2)+ can have several gauche kinks and the carbons atoms directly bonded to the oxygen atoms possess positive charges.12 On the other hand, the alkyl tail of N2228+ mostly renders trans dihedrals.12 We also observe that the anion density pattern near the negatively charged graphene sheet is similar for both the RTILs but cation density pattern is different. 3.5. Electrostatic Properties. Profiles for the electrostatic potential, Ψl(z), and electric field, El(z), along the direction of applied field can be computed from the liquid charge density across the box in z-direction.78,79 These calculations have to be performed with suitable boundary conditions.78−80By using the Poisson equation

dz

2

=−

ρc (z) ε0

(3)

one can estimate El(z) and Ψl(z) as z

E l (z ) =

∫0 ρc (z) dz

Ψl (z) = −

ε0 1 ε0

∫0

(4) z

d z′

∫0

z′

ρc (z″) dz″

(5)

Here ε0 is vacuum permittivity, ρc(z) is liquid charge density (in e/nm3) as a function of distance, z. El(z) and Ψl(z), respectively, are the electric field and electrostatic potential due to the charge distribution of the RTILs. The external electric field (E0) and electrostatic potential (Ψ0) due to walls |σ | |σ | can be estimated via E0(z) = εs and Ψ0(z) = − εs z = −E0z , 0

0

respectively. Here, σs is the surface charge density (in e/nm2) on the graphene walls. For the oppositely charged walls with charge density of 1 e/nm2, E0(z) = 18.09 V/nm. In parts a and b of Figure 12, shown are the electrostatic potential profiles for I


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Figure 12. Electrostatic potential profile, Ψ (z) as a function of distance (z) from z = 0 to z = Lz for (a) N2228+/NTf2− and (b) N222(2O2O2)+/NTf2−. Here Lz is box length (in nm) along the z-direction, Ψ0 is the electrostatic potential due to charged wall, and Ψl is the electrostatic potential due to ionic liquid. Electric field, E(z) as a function of distance (z) from z = 0 to z = Lz for (c) N2228+/NTf2− and (d) N222(2O2O2)+/NTf2−. Here E0(z) is the electric field due to charged wall, and El(z) is the electric field due to ionic liquid.

N2228+/NTf2− and N222(2O2O2)+/NTf2− RTILs when confined between charged and neutral graphene walls. These profiles are also compared with bare potentials of the applied field. For the charged walls, the overall potential drop across the box, i.e. Ψ0(Lz) + Ψl(Lz), for N2228+/NTf2− and N222(2O2O2)+/NTf2− is 7.89 and 11.01 V, respectively. The El(z) profiles in Figure 12, parts c and d, show oscillatory behavior in the interfacial region and remain constant in the bulk region for both charged and neutral walls. Pronounced oscillations in El(z) can be observed near the charged walls. These oscillatory feature of El(z) near the walls is due to structural layering of ions at the interface, (Figure S6) so as to screen the applied field. From parts c and d of Figure 12 and (Figure S6), we also observe that the electric field oscillations and overall charge density fluctuations are larger near the positively charged wall for both the RTILs.

sheets have been determined via molecular dynamics simulations. For the charged graphene, we observe that the layered structure of the RTILs lasts up to ∼1.5 nm from the sheets, beyond this length the microscopic structural organization mimics the corresponding bulk limit. In corroboration with previous studies, even between the neutral sheets each system showed layered structure of the RTILs near the sheets and the layers are composed of both the ionic species (cation and anion). We observe that the orientation of the charged species at interface near the charged walls depends on the charge of the ionic species, charge on graphene sheet, and chemical nature of the ionic species. We have shown that even though both perpendicular and parallel tail orientations of cationic longest tail near the walls exist, parallel orientation of the tail is preferred for N222(2O2O2)+ system. As shown by TRDFs and confirmed by isosurfaces, the cation heads (or anions) in the interfacial region of the negatively (or positively) charged sheet are more ordered than that in the corresponding bulk liquids. A natural continuation of this work is to study the

4. CONCLUSIONS In this study ionic distributions for N2228+/NTf2− and N222(2O2O2)+/NTf2− RTILs near charged and neutral graphene J


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kinetics of charging/discharging and differential capacitance via MD simulations and their comparison with previously proposed mean-field theories. We are currently carrying out such studies on other RTILs to see the effects of cation head atom modification on ionic distribution in interfacial region and other electrochemical properties. Further efforts to understand the effects of cation tail modification with branched or cyclic alkyl groups81 on the layering pattern of RTILs as well as mixtures of RTILs with molecular solvents at various kinds of interfaces are underway.

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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.jpcc.5b06460. Tables for the box dimensions, LJ parameters, and supporting figures showing number density profiles, atomic number density profiles, potential of mean force, and charge density profiles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.K.K.) Telephone: +91-(0) 11-26591518. Fax: +91-(0) 1126581102. E-mail: [email protected]. Notes

The authors declare no competing financial interest

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ACKNOWLEDGMENTS We sincerely thank Professor Ruth Lynden-Bell for helpful discussions and suggestions while her visit to IIT Delhi. H.K.K. thanks Professor Ranjit Biswas, Professor Claudio J. Margulis and Professor Edward W. Castner, Jr. for support and encouragement. S.S. would like to thank CSIR-UGC, India, for fellowship. We extend our thanks to CSC, IIT Delhi, for providing the HPC cluster facility. This work is supported by the Department of Science and Technology (DST), India, through a grant awarded to H.K.K. (Grant No. SB/FT/CS124/2014).

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