Two-Dimensional Ordering of Ionic Liquids Confined by Layered

Aug 3, 2015 - The flexible H-bond formed between Br and surface hydroxyl group at fixed d-spacing results in the liquidlike ordering that breaks down ...
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Two-Dimensional Ordering of Ionic Liquids Confined by Layered Silicate Plates via Molecular Dynamics Simulation Zhifei Yan,† Dawei Meng,*,† Xiuling Wu,*,† Xiaolong Zhang,† Weiping Liu,† and Kaihua He‡ †

Faculty of Materials Science and Chemistry and ‡Faculty of Mathematics and Physics, China University of Geosciences, Wuhan 430074, China S Supporting Information *

ABSTRACT: Recent experiments and computer simulation studies on nanoconfined ionic liquids (ILs) have shifted the focus from perpendicular to lateral distribution, the understanding of which is crucial for IL performance in the field of energy storage systems and tribology. In this article, the structure of 1-ethyl-3-methylimidazolium bromide, [Emim][Br], confined by a hydroxyl group functionalized surface of kaolinite plates has been studied by molecular dynamics simulation. Depending on the degree of confinement, the IL anion can pack into a two-dimensional (2D) ordered structure with square symmetry, coexisting liquid−solid phase, or liquidlike structure. The ordered structure arises from surface-induced ionic orientational preference and the driving force from confinement that supports the formation of the 2D planar structure. The flexible H-bond formed between Br and surface hydroxyl group at fixed d-spacing results in the liquidlike ordering that breaks down the electrostatic network in ILs. The influence of water addition varies when confining plates are treated differently, namely, forming large H-bonding network and small isolated oligomers for relaxed and fixed d-spacing, respectively. This work reveals additional information about the relative importance of factors like packing constraints, interaction within ILs, and selective attraction in determining the structure and dynamics of confined ILs.

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INTRODUCTION Ionic liquids (ILs), a class of organic substances resulting from the combination of a bulky organic cation and various anions, are finding applications in energy storage devices1 as electrolytes, in chemical reactions as catalysts,2,3 and as lubricants.4,5 In these applications, ILs form a thin film covering a high surface area solid substrate or transport under confinement of nanopores. Consequently, the properties of these systems depend critically on the interface of ILs and the solid surface, which has driven recent interest in the behavior of ILs at the solid interface6,7 and in confined geometries.8,9 Interference from contacting the surface has multifold influence over the static arrangement and dynamic behavior of ILs, including symmetry-breaking effect and additional driving forces for liquid ordering, such as overscreening and templating effects, when interaction between the IL and substrate is strong. One-dimensional (1D) ion distribution resolved in the direction normal to the solid surface has been well-documented mainly in the context of electrochemistry, i.e., the electrical double layer of ILs. On the basis of lattice-gas model and by accounting for the finite volume occupied by ions and ignoring ion correlation, Kornyshev10 proposed a mean-field theory to describe the Fermi-like distribution for ionic concentration near the surface, according to which either lattice saturation effect or overscreening would be realized depending on the surface© 2015 American Chemical Society

charge density. Alternate ion layering and oscillation in charge density profile observed in experiment11 and simulation12 at an electrified surface substantiate the above prediction. In situ Xray reflectivity study combined with molecular dynamics (MD) simulation 13 assumed the local interfacial layering at intermediate potential to be a mixture of two phases, the cation- and anion-terminated phase, with the relative coverage depending on applied potential. Because of the pronounced overscreening induced by a charged surface, IL structure adjacent to charged mica surface exhibited alternating cation− anion layering extending 3.5 nm into the bulk fluid; however, a mixed densely solidified cation−anion layering is observed for the first layer of ILs at uncharged graphene surface.14−16 The trend to turn our attention from 1D to two-dimensional (2D) aspects of ILs is spurred by the studies of ILs in narrow gaps17−21 and by the lateral arrangement of ions in the interfacial region which correlates with the peak in differential capacitance (DC) versus potential.22−24 A coarse grain model was used to accurately reproduce the hexagonal ordering for the first layer ILs at graphite−IL interface, and by combining it with importance sampling method, the authors were able to sample Received: June 16, 2015 Revised: July 31, 2015 Published: August 3, 2015 19244

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composition Al2Si2O5(OH)4. The MD simulation cell consists of crystallographic unit cells of kaolinite: 8 in the x-dimension, 5 in the y, and 2 in the z. The initial d-spacing of pure kaolinite is obtained as 7.74 Å. Each IL pair consists of one 3-ethyl-1methylimidazolium cation, [Emim], and one bromide anion, [Br]. In all the simulations containing intercalated ILs, the content of IL is about 24.64% of the total mass, following previous thermogravimetric analysis measurements.32 To examine the effects of confinement on the behavior of ILs within the kaolinite nanopores, the initial d-spacing (13.75 Å) was kept fixed (system A1) or relaxed (system A2) during the simulation. Isolated kaolinite layers terminated with a plane of surface hydroxyl groups (hydroxyl surface) are used in modeling the adsorption of ILs by kaolinite surface (system A3). The model system of adsorbed ILs consists of 105 [Emim][Br] ion pairs and 1 flat kaolinite sheet. The surface sheet is constructed from an 8 × 5 × 1 two-dimensional kaolinite lattice. This kaolinite surface is set 20 nm from the periodic image of the bulk IL on the z-axis so that the distance is large enough to ignore the interaction between the IL and the other surface of kaolinite. During the simulation, the positions of all atoms of the kaolinite layer are kept fixed except the surface OH groups. For studying the effects of hydration on the interaction of ILs and kaolinite, apart from different treatment on d-spacing, i.e., fixed or unfixed, different amounts of water have also been considered, ranging from 0.1 H2O molecule (system F1 and R1), 1 H2O molecule (system F10 and R10), to 2 H2O molecules/unit cell (system F20 and R20); F and R indicate fixed model and relaxed model, respectively. Force Field. The intra- and intermolecular interactions of kaolinite and ILs are described by the INTERFACE force field,33,34 which operates as an extension of common harmonic force fields by employing the same functional form and combination rules to enable accurate simulations of inorganic− organic interfaces. Excellent agreement with experimentally obtained surface properties including surface tension, hydration energy, and surface anisotropy has been found using this set of parameters.35 Parameters for ILs rely on existing parameters in PCFF, followed by extensive revisions of atomic charges and other additional parameters; final parameters can reproduce well dipole moments, liquid density, and vaporization energy.34 The SPC model is used for water.36 Simulation Settings. All the simulations were carried out using LAMMPS.37 The equations of motion were integrated with the velocity Verlet algorithm with a time step of 1 fs. In all cases, periodic boundary conditions were applied in three directions. Simulations were performed in the NPT ensemble at P = 1.0 atm for all systems. Each simulation was performed first for at least 5 ns, i.e., annealed from 600 K for 1 ns to 350 K for 1 ns, followed by 3 ns NPT simulation at 350 K to reach equilibrium. Then a production stage of 2 ns is carried out to derive the interlayer structural and dynamic properties, and an interval of 100 fs is used for recording trajectories. All bonds involved in the imidazolium ring and water molecule are kept rigid using the SHAKE algorithm.38 The van der Waals interactions are truncated using a 12 Å cutoff, and the periodic electrostatic interactions are calculated by the Ewald summation algorithm with a 12 Å cutoff. The temperature and pressure are scaled using the Nose−Hoover thermostat and Berendsen barostat, respectively. The pressure coupling is semiisotropic in that pressures in the x- and y-directions are isotropic while those in the z-direction are different.35

the probability distribution for some of the key variables as a continuum function of applied potential. One of the main findings is that the peak in DC is associated with the collective transition to nonstoichiometric, disordered states from the stoichiometric, latticelike ordered states arising from the correlation within interfacial IL layers, as is evidenced from the non-Gaussian features of surface charge distribution.22,23 A first-order freezing transition from liquid bilayer to liquid monolayer and finally to ordered solid layer was induced by the distance between parallel walls for [Dmim][Cl] confined between two graphite slits.20 Dependence of structural arrangement on thickness of confined film has also been reported for imidazolium-based ILs, where ions at specific loading self-assembled into a closely packed hexagonal pattern.18 This checkerboard arrangement is in accordance with X-ray photoelectron spectroscopy25 and atomic force microscopy (AFM) results.26 The interpretation of the well-defined, 2D nanostructure of ILs near a solid surface needs a balancing consideration between ion packing constraints, the cohesive interaction within ILs, and the strength of interaction between ILs and substrate including van der Waals and electrostatic attraction.19,27,28 Study on two mica sheets confining ILs with similar chemical structure but differing in hydrocarbon chain length suggests a transition from alternating cation−anion monolayer to tail-to-tail bilayers controlled by the balance between electrostatic and hydrophobic interaction.27 Another study by Madden and co-workers24 established the importance of electronic polarization of the metal electrode by charges and dipoles on the ions, which creates additional source to stabilize the formation of a crystalline layer at the anode. Following those and many more findings on the 2D lateral arrangement of ions at the interfacial region, an important issue to address is gauging the relative importance of the complex factors that are at play in determining interfacial IL behavior as the confining environment varies.17,19,21,26 In this work, we report a MD study on the interface between imidazolium-based ILs and kaolinite, a member of the layered silicates, to exam if and to what extent confinement can influence the 2D ordering of ion film near a solid surface. To this end, three kinds of confinement are considered here: first, semivacuum surface, i.e. adsorption of ILs onto kaolinite surface; second, IL films are constrained in two kaolinite plates with changeable separation; and third, distance between kaolinite plates are kept fixed to impose greater confinement. Kaolinite surface is chosen for its charge neutrality and functionalized hydroxyl groups, allowing us to focus solely on the role played by the selective interaction between ILs and the solid surface. In addition, adsorption of alkylimidazolium ILs onto kaolinite surface has been investigated experimentally mainly for its influence over terrestrial and aquatic environment,29,30 though ordinarily those works did not focus on lateral resolution of adsorbed ILs. There are also several reports concerning intercalated ILs within kaolinite interlayer, but to the authors’ knowledge, only one of them mentioned the collective arrangement of ILs which is still not a 2D lateral structure.31 Finally, we investigate the effects of water addition on IL behavior at liquid−solid surface because ILs are known to be miscible with water if the anion is Br, the one considered in the present study.

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METHODS MD Simulation. Systems. Kaolinite is a 1:1 phyllosilicate characterized by a dioctahedral structure, with the chemical 19245

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Figure 1. Number density profile perpendicular to the solid surface of (a) system A1, (b) system A2, and (c) system A3. For the cation, the density is plotted for the center of mass. C-Methyl includes carbon atoms of all methyl groups in [Emim] cation; C-ethyl refers to the carbon atoms in −CH2− of ethyl groups. Shadowed area corresponds to the solid surface.

Figure 2. RDFs of Br around (a) H atoms in ILs and (b) H atoms of surface hydroxyl group, (c) RDFs of Br−Br pairs.

Analysis. The interlayer structures of the modeled systems were visualized using the VMD software39 and analyzed with atomic density plots perpendicular to the surface, radial distribution functions (RDFs) between atoms, the distribution of the angle (ϕ) between imidazolium norm and the normal to solid surface, and the angle (η) between C4−C3 axial and surface normal. Atomic density plots of two kaolinite plates are similar (Figure S1), so only one layer of kaolinite and ILs are discussed in the main text. Distributions of η in the lower and upper portion of the layers are described separately. On the basis of atomic density profile, lower and upper portion refers, respectively, to the range of 5−8 Å and 8−13 Å in systems A1, F1, F10, and F20; 0−8 Å and 8−15 Å in system A2; 3−8 Å and 8−14 Å in system R1; 3−8 Å and 8−16 Å in system R10; and 3−8 Å and 8−15 Å in system R20. Analysis of solvation number distribution is based on the method used in ref 40. DFT Calculation. Computational Details. Density functional theory (DFT) calculations were used to evaluate effects of confinement on interaction between ILs and the surface hydroxyl group. Calculations were carried out using the CASTEP code,41 where a planewave basis set is used to construct a starting trial wave function. The exchangecorrelation energy is treated by the general gradient approximation (GGA), specifically Perdue, Burke, and Ernzerhof (PBE).42 A planewave basis set represented by a kinetic energy cutoff of 400 eV and a 2 × 2 × 2 Monkhorst− Pack grid k-point mesh sampled in the Brillouin zone integration were used. Relaxation of the structure was deemed to have converged when net forces were below 0.01 eV/Å. Other convergence details are as follows: energy change per atom, 5.0 × 10−6 eV; SCF tolerance, 2.0 × 10−6 eV/atom; changes in displacement, 5.0 × 10−4 Å. Models. A double unit cell (along the a-length) of kaolinite with one pair of [Emim] and Br was made. To model the effects of nano confinement, c-length was set to 12.0, 12.5, 13.0, 13.5, 13.75, 14.0, 14.28, and 14.5 Å and kept unchanged during simulation while atomic positions were allowed to relax under the convergence criteria. In the case of semivacuum confinement, i.e., adsorption of ILs, c-length was expanded to 30 Å,

which is assumed to be large enough to avoid undesired interaction between ILs and the other side of the slab due to the periodic boundary condition.

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RESULTS Structuring and Dynamics of Dry ILs Interfaced with Kaolinite. Figure 1 illustrates the number density profiles for [Emim] cation and Br anion along the direction normal to the kaolinite surface. The imidazolium ring group, concentrating most of the charges, and the neutral alkyl side chain are examined separately. In system A1, [Emim] and Br form two sharp separate peaks with small overlapping region, while in system A2 ILs arrange in such a way that the main [Emim] density maxima coincides with Br, Figure 1a,b. In contrast, diffusive layers are found in system A3, Figure 1c, together with another small anion-predominate peak lying at approximately 2.5 Å from the hydroxyl group surface. Interestingly, this distance is almost the same as that of the Br peak (∼6.4 Å) in system A1 and the smaller peak of Br (∼5.5 Å) in system A2 to their respective hydroxyl group surfaces, hinting at the formation of a H-bond between Br and surface hydroxyl H. This assumption is supported by the first peak of the RDFs around hydroxyl proton at about 2.5 Å, Figure 2, showing the order of H-bond strength as A1 > A3 > A2, which is consistent with relative intensity of Br peaks that participate in H-bonding formation. In cases without strong attractive interaction of one species with the surface, those structures with equal numbers of cations and anions that tend to maximize their Coulumbic interaction and salvation by counterions and to impose local charge neutrality will be favored.23 In comparison with system A1, the weak H-bond formed between Br and the hydroxyl surface in systems A2 and A3 qualifies them as systems with an absence of strong attractive interaction; thus, the significance of Br−surface interaction, though perceivable, is less important than the cation−anion attraction between ILs pairs and appears only as perturbation.43 Well-pronounced peaks in RDFs surrounding IL hydrogen, Figure 2a, signify the strong interaction within bulk ILs in system A2 and A3, which is possible because layers of cation share the same region with Br 19246

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Figure 3. Top view of Br in the layer from (a) ∼5 to ∼9 Å of system A1, (b) ∼6.0 to ∼9.7 Å of system A2, and (c) ∼7.8 to ∼13.3 Å of system A3 (upper panels). Bottom panels show the averaged structural factor of anions in the corresponding layer over 1 ns.

along the z-direction, as opposed to the negligible correlation with the solid surface. Closer inspection of the different behaviors of ILs in system A2 and A3 gives an overall understanding of the influence of different confinement on the structural arrangement of ILs. It has been known that anomalous increase of area-normalized capacitance of subnanometer pores filled with ILs can be obtained as the pore size approaches electrolyte ion size,44−46 possibly because of the constructive interference of EDLs from two opposing walls. However, beyond the practice of optimizing capacitance of a nanopore by matching the size of the counterion with nanopore width, the scaling of capacitance in pores could also be controlled by the screening of electrostatic interaction in nanopores with different width, which, in turn, changes the ion solvation structure inside the pore.44 It is tempting to apply previously established rules concerning effects of confinement on EDLs structure to our system, although the systems are relatively different. By comparing system A2 and A3, one can observe wider overlapping of Br density with [Emim] groups, which supports greater attraction due to the lack of confinement on the other side of ILs. The confinement forces the cations and anions inside the pores to arrange in a 2D planar structure and be increasingly solvated by their counterparts located near their equator, leading toward the partial desolvation of ions in confined environments, as readily seen in the distribution of Br anion solvation number around each cation (Figure S2). In system A3, the occurrence of solvation number down to 1 and up to 10 is observed with the major distribution centers around 4, while the more confined environment experienced by ions in system A2 results in less solvation with the largest solvation number decreased to 6, although the distribution remains centered around 4. Further down the line, for the largest degree of confinement of the present work in system A1, the solvation number decreased furthermore, down to 1 and up to 4, with the majority distributed at 2−3. Such a drastic desolvation has also been investigated via MD simulation of a coarse-grained IL model confined in different environments categorized as edge,

planar, hollow, and pocket sites based on degree of confinement.40 The short-range order emerging near the solid surface produces both transverse and planar static order in the layered structure of ILs (Figure 3a−c). There is a clear tendency to charge alternation for cations and anions arranged in a checkerboard-like structure, Figure S3, driven by strong Coulomb forces, hinting at crystal-like ordering for the inplane distribution of ions, but concealed by sizable concentration of defects in system A3. Panels b and c of Figure 3 show top views of Br in the main layer (∼6.0 to ∼9.7 Å) of system A2 and the second layer in system A3 (∼7.8 to ∼13.3 Å), respectively, revealing the different extent of planar order. For a more precise characterization of the planar order, the in-plane anion structure factor in the snapshot is defined and described according to s(k) =

1 N−

∑e i,j

ik·rij

(1)

where the sum runs over all pairs of anions in the targeted layer and k is a wavevector that lies in the plane of the solid surface and is commensurate with the periodic boundary conditions; N− is the number of anions in the layer.23 The structural factor averaged over the last 1 ns of the simulation shows that anions form an approximate 2D square lattice in system A2, the appearance of which is also visible in system A3 but far less defined, Figure 3c. Ordered structure of anions adsorbed by the electrode has also been observed, and the latticelike ordering persists only for a certain range of applied potential.23 The ordered arrangement of anions in the present model seems to be quite stable in system A2 in the last 1 ns, Figure S4. Anions in system A3 behave differently, displaying a structure that might be better described as coexistence of solid and liquid phase, i.e., certain degree of square symmetry can be identified as is in system A2, but the presence of disorder frustrates the square lattice formation. In experiment, a similar imidazoliumbased IL adsorbed on mica substrate has been observed to 19247

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Analysis of density profile and RDFs for system A1 reveals sharp differences with its A2 counterpart, with prominent Br peak being close to the surface and H-bond formation with hydroxyl groups. The correlation between Br and surface H is surprisingly strong with fixed walls at both sides of the IL film, Figure 2b. DFT results show a general trend of shortened Br− H bond upon gradual decrease of the interlayer space from 14.28 to 12 Å, accompanied by tilting orientation of imidazolium ring at smaller d-spacing, Figure S6. Near a hydroxylated quartz surface, the greatest H-bonding capability was found for Cl, an anion very similar to the one in the present system.43 The accumulation of surface charge due to the specific OH−Br bonding is compensated by the subsequent layer of positively charged imidazolium cation, giving rise to the near destruction of the cation−anion electrostatic network and anion−anion correlation that are partly preserved in system A2, Figure 2a,c; the result is drastically accelerated diffusion for confined ions, Figure S7.9,48 The layer structure factor of Br in system A1 displays a liquidlike structure factor, Figure 3a, together with the wide distribution of imidazolium ring orientation, Figure 4. With strong correlation between ions and the surface sites, one would normally expect anions to arrange in a 2D crystal-like structure if they align with the substrate strictly.28,49 The rationalization of liquidlike structure in system A1 has several possible contributions. Although we presented the configuration of ILs confined in 13.75 Å interlayer as parallel, Figure S6, there are still many possibilities with similar energy. Three theoretically stable representative configurations, parallel (P), vertical (V), and tilted (T) configuration based on cation orientation, have been identified, Figure S8. The surface interacts with Br anion though H-bonds and with cation via C− H of either the ring or the alkyl chain in optimized configurations.50,51 In V configuration, the most acidic proton attached to the ring points downward, while it is the C−H in alkyl side chain that provides active sites for the interaction in configuration P and T. There is negligible difference in their energy but large divergence in terms of Br displacement relative to the center of three underlying surface H that may form Hbonds with them. One striking feature in the snapshot is that hydroxyl groups underneath Br tend to direct H upward for maximum acceptance of electrons, while for those OH groups sitting below [Emim] cation hydroxyl H will orient horizontally, which leads to maximum exposure of hydroxyl O to [Emim], indicative of [Emim]−surface interaction. The influence of this nonuniform orientation of surface H will be felt by the H-bonding network between neighboring hydroxyl groups and indirectly by the Br accepting protons from those OH groups. In addition, surface atoms are totally relaxed during simulation; the lateral ordering imposed by the surface would thus be weaker than if they are fixed.49 The transition from 2D ordering to liquidlike structure is a collective effect of ion-packing constraints, cohesive interaction between ions, and attraction between ions and the surface. In AFM experiments, the progressive transition from 90° symmetry in the outmost transition zone layer of a similar ILs to 60° symmetry was observed as the tip moves toward the substrate. The 60° symmetry ordering is a consequence of electrostatically bounded cations to mica charge sites, while the 90° symmetry could be induced by IL ion pairs.26 Similarly, the coarse-grained IL model confined in two plates exhibit a 2D triangular lattice when the plates are symmetrically electrified; neutralized plates, however, induce ions to form neutral layers

Figure 4. Distribution of the angle between imidazolium normal and the normal to solid surface; definition detailed in the text.

The highly ordered and homogeneous IL phase in system A2 compared with the coexisting solid−liquid phase in system A3 is assumed to have two origins. First, driving forces provided by two confining walls facilitate the arrangement of ions into a 2D planar structure to maximize their solvation, rather than a 3D bonding network in system A3 due to the available free space at the side opposite the wall. This is evidenced in their solvation number and narrower distribution, and smaller possible solvation numbers are identified for larger confinement,40 Figure S2. Second, the interference from the second layer of ILs in system A3 with the first ordered layer further obscures the 2D structure. Using amplitude modulated atomic force microscopy, researchers have found that the clarity to observe ILs lattice structure in the image depends on the influence of near-surface structure, with the clearest identification observed for formate ILs, which shows lower liquid cohesion and can thus be more readily displaced by cantilever, minimizing the superstructure effect.19 Also, the distribution of anion orientation in ILs/graphene systems was found to become less uniform with additional layers, an observation that agrees well with our results.18 19248

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Figure 5. Number density profile perpendicular to the solid surface of (a) system F10 and (b) system R10. Dashed purple and green lines refer to O and H atoms of water, respectively.

with square symmetry. Neutral plates bind both cations and anions so that they arrange in a rocksalt (100) square monolayer; charged plates instead selectively bind ions with opposite charge, giving rise to the observed triangular arrangement.21 Coincidentally, square ice of water confined in graphene nanocapillary has been confirmed recently, with no observation of alignment with graphene lattice because water− surface interaction in such confinement is much weaker than the interaction between water molecules.52 Though there is enough reason to speculate that the origin of the square symmetry in systems A2 and A3 is a result of interionic correlation in ILs, the effect of packing constraints emerging with two confining walls in system A2 enhances the square ordering. The local ordering of ILs has also been shown to be governed by Coulomb interactions between cations and anions under constraint from selective adsorption;33 the preference of surface OH to be H-bonded with Br in system A1 thus breaks down the electrostatic network in bulk ILs. However, the strength of such H-bonds compares only to van der Waals interaction, and is much weaker than electrostatic attraction with the charged surface. Overall, system A1, which exhibits liquidlike structure, resembles the case of confined, sterically hindered cations that can be displaced from the substrate at high sliding force because of the weak attraction.19 Effects of Water Addition on IL−Solid Surface. Water can be miscible with ILs containing Br and thus affects the IL− solid surface interaction. The general features of the density profile for ILs with or without water are similar, Figure 5a,b, except for the splitting of [Emim] peak with water addition at 1 H2O/unit cell, Figure 6. Density curves indicate the existence of complicated interaction between water, IL ions, and the surface, as prominent water density occurs both near the surface and within bulk ILs. The pervasive H-bonds in water and ILs suggests that it is possible for water to form H-bonds with Br anion and the surface.43 Similar to the dry ILs−solid interface, Br in system F10 shows greater correlation with surface H that is almost totally lost in system R10, i.e., the appearance of a dense water layer separates the direct contact of Br with the surface hydroxyl group. On the other hand, more Br forming H-bonds with surface hydroxyl group, Figure 7a,b, and large amount of water binding with the surface, Figure 7c, results in much weakened water−Br correlation compared with system R10 which shows stronger correlation for water−[Emim] pairs. Pictorially, water are more involved in the solvation of Br in system R10, forming a Br···HOH···Br H-bond network at the surface, Figure 6a,b, as also observed by MD simulation of [BMI][Cl]−Si(OH)2 interface.43 That water−Br in system F10

Figure 6. Snapshot of (a) the water-Br oligomers in system F10 and (b) the Br···HOH···Br H-bonding network in system R10.

mostly forms small individual oligomers, leaving some Br unbounded by water, contributes in part to the faster mobility of confined ILs and water than system R10, Figure S9. Factors including enhanced ionic and water−[Emim] correlation in ILs slow the dynamics of system R10, as is reflected in the extremely stable H-bond network, Figure S10. The orientation of cations in the confined layer evolves with the addition of water. This can be measured using angles such as the one between the imidazolium ring normal and the normal to the surface and the one between the ethyl C−methyl C (C4−C3) axis and the surface normal, illustrated in Figure 8a. As the amount of water addition increases, a larger proportion of the IL becomes parallel to the surface compared with the dry IL in relaxed model, though the vertical arrangement remains the same, Figure 8b. In system A2, the distribution of C4−C3 orientation displays bimodality, equally contributed from ethyl groups in the lower and upper layer, Figure 8c. The same situation occurs in systems R1, R10, and R20, but the relative weight of the lower layer contribution decreases gradually in the order of R1 > R10 > R20. This behavior is consistent with the repulsive interaction between water and hydrophobic ethyl group, driving the latter from the immediate surface as the former begins to occupy the region, Figure 5. As more water comes in, the central region in the interlayer space starts to be occupied, leading to the migration of cations to form another layer above, as is seen in cation peak splitting in Figures 5 and S11. On the other hand, cations in this new layer mainly lie flat on the surface while pointing the C4−C3 axis downward, giving rise to the large proportion of 19249

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Figure 7. RDFs of Br around H in ILs H(ILs), H in surface hydroxyl group H(surface), and H in water molecules H(water) for (a) system F10 and (b) system R10. (c) RDFs of O in water molecules around H(ILs) and H(surface).

Figure 8. Orientation of cation on the surface. (a) Definition of the angle ϕ between the imidazolium ring normal (blue line) and the normal to solid surface (black line). The angle η is defined as the one between the surface normal and the vector from ethyl carbon (C4) to methyl carbon (C3) (red line). (b) Distribution of angle ϕ in system R1 (red line), R10 (green line), and R20 (blue line); color code is the same for panel d. (c) Distribution of angle η in system A1 and A2. Distributions of η in the lower (cyan line) and upper (magenta line) portion of the layers are described separately; details can be found in Methods.

and degree of confinement influence the lateral arrangement of ILs, a hydroxyl functionalized kaolinite surface was considered. Depending on the number of confining walls (one or two) and whether they are allowed to be free to move during simulation, ILs show apparent square symmetry, coexistence of liquid− solid phase, or liquidlike structure identified by their structure factors. Nanoconfinement from both sides of an IL film forces anions to arrange into a 2D ordered structure with square symmetry. Keeping the d-spacing unchanged allows Br to form stronger H-bonds with the solid surface, but the flexibility of surface hydroxyl groups and the uncertainty of Br relative displacement to protons results in a highly mobile liquidlike structure. Finally, we showed that addition of water in the relaxed model forms a large H-bonding network between Br and water, while smaller individual oligomers appear with fixed confining walls.

parallel configuration (∼0° or 180°), Figure 8b, and the small peak for C4−C3 distribution in upper layer (∼160°), Figure 8d. In the case of fixed d-spacing, the changes in C3−C4 orientation mainly come from the upper part (∼160°), Figure S12, similar to the small rising peak in the relaxed model but to a much larger extent in fixed systems. The observation that a larger portion of cations tend to lie parallel to the surface may be due to the cations’ tendency to gather into a new layer which is hindered by the confinement, resulting in the shoulder peaks in Figure 5.

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CONCLUSION As the focus on ILs at interfaces and in nanoconfinement begins to shift from perpendicular to lateral distribution,17,18 we studied IL structure and dynamics in different constrained environments. To get an understanding of how the presence 19250

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Article

The Journal of Physical Chemistry C

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Results in the present study might help to provide more details on the origin of the 2D ordered structure of ILs in confined space and add clues to the important role played by surface functional groups.

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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.5b05776. Details of DFT calculation results; solvation number distribution in systems A1, A2, and A3; number density plots; and angle distribution in fixed model with water addition. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (41472042 and 41172051) and the National College Students’ Innovative Training Program (201510491024).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b05776 J. Phys. Chem. C 2015, 119, 19244−19252

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DOI: 10.1021/acs.jpcc.5b05776 J. Phys. Chem. C 2015, 119, 19244−19252