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Interfacial Structures of TrihexyltetradecylphosphoniumBis(mandelato)borate Ionic Liquid Confined Between Gold Electrodes Yonglei Wang, Mikhail Golets, Bin Li, Sten Sarman, and Aatto Laaksonen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14429 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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ACS Applied Materials & Interfaces

Interfacial Structures of TrihexyltetradecylphosphoniumBis(mandelato)borate Ionic Liquid Confined Between Gold Electrodes Yong-Lei Wang,∗,†,¶ Mikhail Golets,†,§ Bin Li,‡ Sten Sarman,† and Aatto Laaksonen† †Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden ‡Theoretical Chemistry, Chemical Center, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden ¶Present address: Department of Chemistry, Stanford University, Stanford, CA 94305, United States §Present address: TS&d Mining, Akzo Nobel Surface Chemistry AB, SE-444 85, Stenungsund, Sweden E-mail: [email protected] Phone: (1) 650-785-3771

Keywords: Trihexyltetradecylphosphonium-bis(mandelato)borate ionic liquid, gold electrodes, atomistic simulations, interfacial structures, molecular arrangements

1 ACS Paragon Plus Environment


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Abstract Atomistic molecular dynamics simulations have been performed to study microscopic interfacial ionic structures, molecular arrangements and orientational preferences of trihexyltetradecylphosphonium-bis(mandelato)borate ([P6,6,6,14 ][BMB]) ionic liquid confined between neutral and charged gold electrodes. It is found that both [P6,6,6,14 ] cations and [BMB] anions are coabsorbed onto neutral electrodes at different temperatures. The hexyl and tetradecyl chains in [P6,6,6,14 ] cations lie preferentially flat on neutral electrodes. The oxalato and phenyl rings in [BMB] anions are characterized by alternative parallel-perpendicular orientations in the mixed innermost ionic layer adjacent to neutral electrodes. An increase in temperature has marginal effect on interfacial ionic structures and molecular orientations of [P6,6,6,14 ][BMB] ionic species in confined environment. Electrifying gold electrodes leads to peculiar changes in interfacial ionic structures and molecular orientational arrangements of [P6,6,6,14 ] cations and [BMB] anions in negatively and positively charged gold electrodes, respectively. As surface charge density increases but lower than 20 µC/cm2 , the layer thickness of mixed innermost interfacial layer gradually increases due to a consecutive accumulation of [P6,6,6,14 ] cations and [BMB] anions to negatively and positively charged electrodes, respectively, before the formation of distinct cationic and anionic innermost layers. In the meantime, molecular orientations of two oxalato rings in the same [BMB] anions change gradually from a parallel-perpendicular feature to that partially characterized by tilted arrangement with an angle of 45◦ from electrodes, and finally to dominant parallel coordination pattern along positively charged electrodes. Distinctive interfacial distribution patterns are also observed accordingly for phenyl rings that are directly connected to neighboring oxalato rings in [BMB] anions.


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Introduction Room temperature ionic liquids (ILs) refer to a subset of molten salts consisting solely of asymmetric organic cations and weakly coordinating organic/inorganic anions that exist as liquids at room temperature. 1–3 ILs have attracted significant research interest in diverse academia and industrial communities due to their striking and remarkable physicochemical characteristics, such as non-flammability, negligible volatility, reasonable viscositytemperature feature, high thermal-oxidative stability, wide electrochemical window, as well as outstanding affinities to various solid surfaces. 2–4 These fascinating characteristics can be widely tuned in a controllable fashion through systematic changes in molecular structures of their constituent ionic species, and thus render them exceptionally attractive and reliable alternatives to conventional molecular liquids and electrolytes in tribological and electrochemical applications. 5–8 Either used as neat lubricants or lubricant additives in conventional base oils in microlubrication, ILs show superior tribological performance in different mechanical systems. The flexible molecular structures and inherent polar nature of ionic species facilitate their interaction with engineering surfaces and thus promote the formation of boundary films, which not only avoid the direct contact between engineering surfaces and prevent some tribochemical reactions (corrosion, oxidation, etc.), but also reduce friction and wear between sliding surfaces. The microstructural and dynamical characteristics of ionic species experiencing in the vicinity of solid interfacial region have been shown to significantly influence their rheological, mechanical and tribological properties, and thus are critical in determining their functional performance in narrow conjunctions. 3,6,8–11 The interfacial microstructural organization and heterogeneous dynamics of absorbed ionic species on solid surfaces, especially in the vicinity of IL-gold metallic interfacial region, have been extensively explored by various experimental techniques including atomic force microscopy (AFM), 12–19 scanning tunneling microscope, 13,20–25 X-ray and neutron reflectometry techniques, 26–28 surface force apparatus (SFA) measurements, 29,30 and other advanced experimental techniques. 21,23,31–35 3 ACS Paragon Plus Environment


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The combination of these experimental characterizations and available computational investigations 36–40 reveals the formation of boundary ionic solvation layers adjacent to gold metallic surfaces. 12,13,34,41,42 Three structurally distinct regions are identified in confined environment: the innermost interfacial region that is composed of ionic species in direct contact with electrodes; the transition region in which the pronounced interfacial layer structures decay to bulk morphologies; and the bulk liquid region where ionic structures decisively depend on peculiar amphiphilicities of ionic species. In the innermost interfacial region, the intrinsic microstructural organization and interfacial ordering features of boundary ionic layers are dependent on specific types of cations and anions, and delicate interactions between interfacial ionic species and detailed atomic compositions constituting gold surface. The cohesive interactions, such as Coulombic, van der Waals (vdW), hydrogen bonding and solvophobic interactions, between confined ionic species further result in well-defined ionic structural organization in interfacial region due to clustering of like molecular groups. Additionally, it is found that the application of external electric fields on gold electrodes is an effective way to alert molecular position fluctuations in confined environment, leading to the accumulation of specific ionic species in interfacial region, and finally resulting in reconstruction of interfacial layering structures. 11,15,18,25,35,38,43–48 The innermost interfacial layer is usually enriched with ionic groups carrying opposite charges as that on gold electrodes due to strong electrostatic interactions between ionic species and Au atoms constituting electrodes. Multiple boundary ionic layers characterized by alternative charging features were observed in interfacial and transitional regions depending on the magnitude of applied electric fields. The intrinsic changes in interfacial chemical compositions upon applying electric fields, such as interfacial layer thickness and relative distribution and orientation of confined ionic species, contribute to ILs’ friction-reducing and antiwear quantities in different tribological systems. 12,15,41,42 These distinctive characteristics of nanoscale organization and ordering phenomena of confined ionic species occurring in interfacial region are paramount in selecting proper cation-anion pairs and in advancing their functional performance in confined


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environment. The tetraalkylphosphonium-orthoborate ILs are a special category of ILs consisting of vo-

luminous tetraalkylphosphonium cations and (chelated) orthoborate anions. Either tetraalkylphosphoniumbased or orthoborate-based ILs present superior tribological advantages compared with typical nitrogen-based ILs and can be used as alternative high-performance lubricants and lubricant additives in conventional fully formulated engine oils in nanotribology. 9,10,49–54 However, the detailed tribological mechanism of these ILs in lubricating solid surfaces is not well characterized. Even less is known on the interfacial chemical compositions, molecular microstructures, orientational arrangements, and molecular ordering features of bulky tetraalkylphosphonium cations and asymmetric orthoborate anions that are confined between neutral electrodes, as well as peculiar changes in these interfacial properties upon charging electrodes with different surface charge densities. These interfacial properties are intrinsically correlated with their macroscopic functional performance in mechanical lubricating systems, and thus deserve a detailed understanding at microscopic level. To tackle these questions, we perform extensive atomistic molecular dynamics simulations to probe interfacial ionic structures and molecular arrangements of trihexyltetradecylphosphoniumbis(mandelato)borate ([P6,6,6,14 ][BMB]) IL (chosen by tribological interest because of its excellent tribochemical properties in mechanical engineering contacts 49,52,54 ) confined between neutral and charged gold electrodes with controllable surface charge densities. Attentions are mainly focused on interfacial chemical compositions, characterized by mass, atomic number and charge density variations, and molecular orientational arrangements of confined ionic species in interfacial region. For [P6,6,6,14 ][BMB] IL confined between neutral electrodes, both [P6,6,6,14 ] cations and [BMB] anions are accumulated in IL-gold interfacial region with preferential molecular orientational patterns. As gold electrodes get electrified, distinctive interfacial ionic structures and peculiar orientational patterns of oxalato and phenyl rings in [BMB] anions are observed as surface charge density increases. These simulation results will promote our understanding of interfacial structural and molecular ordering features of



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[P6,6,6,14 ][BMB] IL confined between neutral and charged gold electrodes.

Simulation methodology The atomistic force field parameters of [P6,6,6,14 ][BMB] IL were systematically developed in our previous work based on the AMBER framework. 55 Both inter- and intramolecular interaction parameters were refined to achieve quantitative description of intermolecular ionic structures obtained from quantum chemistry ab initio calculations, and tuned to fit vibration frequency data obtained from experimental measurements, 49 respectively. The gold electrode is modelled as triple layers of gold sheets. Each gold sheet consists of 441 Au atoms arranged in a face centered cubic structure leading to electrode dimensions of 6.0 × 6.0 nm2 . The constructed gold (100) electrode, which represents a typical single crystalline electrode and has been examined in several electrochemical experiments, 20,22,38 is positioned parallel to XY plane of an externally defined Cartesian coordination system and kept frozen in all atomistic simulations. The vdW interaction parameters of Au atoms located in gold (100) crystalline plane are represented by a 12-6 Lennard-Jones potential form with cross-sectional parameters of σ = 0.3312 nm and ϵ = 22.1333 kJ/mol adopted from Refs. 38,56 These parameters were developed by Heinz et al. 56 and can quantitatively reproduce thermodynamics and mechanical properties of gold electrodes. The cross-interaction parameters between different atom types are obtained from the Lorentz-Berthelot combination rules. For charged gold electrodes, we employ a simplified and efficient approach 38,39 by assigning quantitative partial charges to all Au atoms in gold (100) sheets that are in direct contact with confined [P6,6,6,14 ][BMB] IL film. The gold anode and cathode are allocated with positive and negative partial charges, respectively, with surface charge densities ranging from 0 to ± 100 µC/cm2 . The model IL simulation system consists of 400 [P6,6,6,14 ][BMB] ion pairs confined between two gold electrodes in a tetragonal simulation box. The distance between Au atoms facing


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Figure 1: Electrostatic potential surfaces and molecular structures of [P6,6,6,14 ] cation and [BMB] anion, and representative configuration of [P6,6,6,14 ][BMB] ionic species confined between gold electrodes. IL film in anode and cathode is set to 15.7 nm along Z axis to ensure [P6,6,6,14 ][BMB] IL adopting a bulk like behavior in the central portion of confined film. The periodic distance of this Cartesian coordination system in Z axis is 50.0 nm, which is sufficiently large so that interactions between the adsorbed [P6,6,6,14 ][BMB] ion pairs and the periodic image of gold electrodes in top plane can be eliminated. A representative configuration of [P6,6,6,14 ][BMB] IL confined between gold electrodes is shown in Figure 1. Atomistic molecular dynamics simulations were performed using GROMACS 5.0.4 package. 57 The equations of motion were integrated using classical velocity Verlet leapfrog integration algorithm with a time step of 1.0 fs. The cutoff distance of vdW interactions and real-space electrostatic interactions was set to 1.6 nm. The Particle-Mesh Ewald method



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was employed to handle long-range electrostatic interactions in reciprocal space with a FFT grid spacing of 0.112 nm and an electrostatic energy tolerance of 10−5 , respectively. For [P6,6,6,14 ][BMB] IL confined between neutral electrodes, molecular simulations were carried out at 373, 473, and 573 K, respectively, to investigate the dependence of molecular arrangements of [P6,6,6,14 ] cations and [BMB] anions in confined environment on temperature. Atomistic simulation analyses for [P6,6,6,14 ][BMB] IL confined between charged electrodes were sampled at 373 K with different surface charge densities. All simulation systems were first annealed gradually from 800 K down to target temperatures within 10 ns, and then equilibrated in NVT ensemble for 20 ns maintained using Nosé-Hoover chain thermostat with a time coupling constant of 500 fs to control temperature. The above equilibrated simulation systems were further simulated in NVT ensemble at target temperatures for another 40 ns, and the corresponding simulation trajectories were recorded accordingly with a time interval of 100 fs for post-processing analysis.

Analysis of Simulation Results The discussion of microscopic ionic structures of [P6,6,6,14 ][BMB] IL between gold electrodes involves two different but interrelated aspects, namely, the detailed chemical compositions and molecular distributions of ionic species in IL-gold interfacial region. The former one is used to characterize the relative enhancement and depletion of certain atoms and molecular groups in interfacial region with respect to that in bulk regime, whereas the latter one is adopted to verify the detailed molecular orientations of specific vectors fixed in ionic frameworks in confined environment, respectively. In following subsections, we first discuss the variation of interfacial properties for [P6,6,6,14 ][BMB] IL confined between neutral electrodes. Next, we describe atomistic simulation results obtained from [P6,6,6,14 ][BMB] IL confined between charged electrodes with different surface charge densities to explore the effect of applied electric potential on interfacial chemical compositions and interfacial molecular ar-


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Figure 2: Mass density profiles of [P6,6,6,14 ][BMB] IL confined between neutral electrodes at different temperatures. The vertical dash-dot lines represent the relative positions of Au atoms in triple layers constituting gold electrodes. rangements of [P6,6,6,14 ] cations and [BMB] anions adjacent to negatively and positively charged electrodes.

[P6,6,6,14 ][BMB] IL between neutral gold electrodes Figure 2 presents the symmetrical overall mass density profiles for confined [P6,6,6,14 ][BMB] IL along surface normal direction (Z axis) of neutral electrodes at different temperatures. An interfacial pronounced and a subsequent intermediate density peaks adjacent to neutral electrodes are clearly observed in all mass density profiles at different temperatures, indicating the formation of dense boundary layers near neutral electrodes. A relative uniform mass density distribution is observed in the middle part of confined [P6,6,6,14 ][BMB] IL film, such as in the region of 4.0-12.0 nm, which is considered as bulk region of [P6,6,6,14 ][BMB] IL film between neutral electrodes. The separate individual mass density profiles of [P6,6,6,14 ]



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cations and [BMB] anions (provided in Figure S1 in Supporting Information) are characterized by comparable intensities, indicating that both [P6,6,6,14 ] cations and [BMB] anions are accumulated in interfacial region. The [P6,6,6,14 ] cations are mainly surrounded by [BMB] anions, and vice versa, due to strong electrostatic attractions, which lead to the interfacial distribution of ionic species in a mixed checkerboard arrangement. Such a remarkable molecular distribution of ionic species in interfacial region is attributed to the short-range template confinement effect triggered by decisive vdW interactions between ionic species and atoms constituting solid surfaces, as indicated by other experimental and computational studies. 7,28,36–39,44,46,58–60 These dominant intermolecular interactions reduce the entropic contribution of confined ionic species, and thus strengthen their accumulations in interfacial region. The effect of temperature on the overall mass density distributions of [P6,6,6,14 ][BMB] IL depends on their relative positions within confined environment. In IL-gold interfacial region, the dependence of mass density distributions on temperature is negligible due to the decisive vdW interactions between interfacially absorbed ionic species and Au atoms in neutral gold (100) sheets, which are much stronger than systematic thermal fluctuations and thus play a critical role in the formation of stable boundary layers in interfacial region. In bulk region of IL film, the mass density profiles, either overall ones or respective counterparts, exhibit considerable perturbations at 373 K due to the formation of micro- and meso-scopic heterogeneous ionic structures with segregated polar domains within interpenetrating nonpolar matrix. 55 The formation and self-assembly of heterogeneous sponge-like morphologies are driven by competitions between favorable electrostatic interactions between central polar segments in ionic species, and persistent cohesive interactions between alkyl chains in [P6,6,6,14 ] cations and phenyl rings in [BMB] anions, as well as other delicate interactions, such as directional hydrogen bonding interactions between [P6,6,6,14 ] cations and [BMB] anions. 55,61 The increase in temperature smooths the overall mass density profiles in bulk region of IL film, and also in respective mass density profiles for [P6,6,6,14 ] cations and [BMB] anions,


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Figure 3: Atomic number density profiles of representative P and B atoms embedded in [P6,6,6,14 ] cationic and [BMB] anionic frameworks between neutral electrodes at different temperatures. The vertical dash-dot lines represent the relative positions of Au atoms in triple layers constituting gold electrodes. indicating that the increased thermal motions of ionic species at elevated temperatures prefer to loosen ionic structures, and thus to considerably decrease microstructural heterogeneities of [P6,6,6,14 ][BMB] IL. To reveal more detailed microscopic atomic distributions of [P6,6,6,14 ][BMB] IL between neutral electrodes, the atomic number density profiles of phosphorus (P) atoms in [P6,6,6,14 ] cations, and boron (B) atoms in [BMB] anions are shown in Figure 3, due to their central positions in respective molecular frameworks. The detailed atomic number density distributions of P and B atoms in interfacial region are provided in Figure S2 in Supporting Information. As expected from mass density distributions, the atomic number density profiles of P and B atoms exhibit similar pronounced patterns in interfacial region and considerable damped oscillations in bulk region of confined [P6,6,6,14 ][BMB] IL film. The comparable atomic num-



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Figure 4: Variation of overall volume charge density profiles of [P6,6,6,14 ][BMB] IL confined between neutral electrodes at different temperatures. The vertical dash-dot lines represent the relative positions of Au atoms in triple layers constituting gold electrodes. ber distributions of P and B atoms in interfacial region further signify their direct depletion in confined environment. The variation of overall volume charge densities of [P6,6,6,14 ][BMB] IL between neutral electrodes are present in Figure 4. The remarkable and symmetric volume charge densities in interfacial region are attributed to peculiar accumulations and interfacial ionic structures of [P6,6,6,14 ] cations and [BMB] anions in confined environment and their respective complex ionic structures. The remarkable oscillation patterns registered in volume charge density profiles decay to normal bulk values within roughly 1.5 nm. It should be noted that the variation of individual volume charge densities of [P6,6,6,14 ] cations and [BMB] anions, which are provide in Figure S3 in Supporting Information, exhibit different characteristics in interfacial region owing to their distinctive atomic charge distributions within respective ionic frameworks. The [P6,6,6,14 ] cations are bulky and voluminous, and positive atomic charges


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are delocalized in hexyl and tetradecyl chains, leading to a pre-peak distribution in all charge density profiles adjacent to neutral electrodes before intermediate charge density oscillations in IL-gold interfacial region, as clearly shown in Figure 4. While for [BMB] anions, the negative atomic charges are mainly localized in two perpendicularly distributed oxalato rings, as shown in molecular electrostatic potential surface in Figure 1. Furthermore, the magnitude of atomic partial charges in [BMB] anions is larger than that in [P6,6,6,14 ] cationic framework. These distinct charge characteristics in [BMB] anionic framework contribute to the remarkable oscillation pattern in interfacial region. Having specified the detailed chemical compositions of [P6,6,6,14 ][BMB] IL confined between neutral electrodes, we proceed to examine the molecular orientational preferences of ionic species in interfacial region. Considering the relative symmetric distributions of P and B atoms in interfacial region and their relative central positions in respective ionic frameworks, we divide [P6,6,6,14 ][BMB] IL film into three regions based on the overall mass density profiles shown in Figure 2. The first layer is the innermost interfacial layer (0.4-1.0 and 15.4-16.0 nm), and the second one is the intermediate layer (1.0-1.4 and 15.0-15.4 nm), respectively, in IL-gold interfacial region. The bulk region corresponds to the central part of [P6,6,6,14 ][BMB] IL film, that is the range of 4.0-12.0 nm, due to less fluctuations in mass density profiles. The orientational probability distribution of alkyl chains in [P6,6,6,14 ] cations is characterized by the angle between Z axis (perpendicular to gold electrodes) and specific vectors fixed in cationic framework. Herein, the P-C6 and P-C14 vectors pointing from cental P atom to terminal carbon atoms in hexyl and tetradecyl chains in [P6,6,6,14 ] cation are considered in current atomistic simulations. The pronounced probability distribution shown in Figure 5 indicates that both hexyl and tetradecyl chains prefer to take parallel orientation along neutral electrodes in the innermost interfacial layer. Such a parallel feature is considerably weakened in the intermediate layer (transition region), and consequently featureless mean distribution is observed in bulk region of confined IL film.



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Figure 5: Normalized probability distributions of P-C6 and P-C14 vectors fixed in cationic framework with respect to surface normal direction (Z axis) for [P6,6,6,14 ] cations within the innermost, the intermediate interfacial layers, and in bulk region of confined IL film between neutral electrodes at 373 K. The inset figure defines P-C6 and P-C14 vectors in [P6,6,6,14 ] cationic framework. Due to the consecutive connections of oxalato and phenyl rings in [BMB] anions, we present two combined distribution functions (CDFs) to characterize the relative distribution and orientation of oxalato and phenyl rings in confined environment. The first CDF is the angle between (the first) oxalato ring normal and Z axis (labelled as ∡noxalato -Z) vs the angle between (the second) oxalato ring normal and Z axis (also labelled as ∡noxalato -Z) to address the relative orientation of oxalato rings that are jointed by a central B atom in each [BMB] anion. The second one is the angle of ∡noxalato -Z vs the angle between phenyl ring normal and Z axis (labelled as ∡nphenyl -Z) to specify the relative distribution of oxalato and phenyl rings that are bonded together through flexible C-C bonds in [BMB] anions. The joint effect of these two type CDFs can accurately characterize molecular arrangement and interfacial ordering feature of [BMB] anions in interfacial region.


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Figure 6: Combined distribution functions of ∡noxalato -Z vs ∡noxalato -Z for oxalato rings jointed by a central B atom in each [BMB] anion that is located in the innermost (A) and intermediate (B) interfacial layers, and in bulk region (C) of confined IL film, respectively, at 373 K. Combined distribution functions of ∡noxalato -Z vs ∡nphenyl -Z for oxalato ring and the corresponding directly bonded phenyl ring in [BMB] anions that are positioned in the innermost (D) and intermediate (E) interfacial layers, and in bulk region (F) of confined IL film, respectively, at 373 K. Figure 6 presents CDFs for oxalato and phenyl rings in [BMB] anions located in the innermost and intermediate interfacial layers, and in bulk region of confined [P6,6,6,14 ][BMB] IL film. For [BMB] anions positioned in bulk region of IL film, the board and symmetric feature in CDF for oxalato rings (panel C) indicates that each oxalato ring in [BMB] anion takes random orientation, but two oxalato rings jointed by a central B atom in each [BMB] anion are essentially perpendicular to each other. As [BMB] anions approach neutral electrodes, the remarkable coordination patterns in CDF contour suggest distinct orientation and distribution of oxalato rings in interfacial region. In the innermost boundary layer of IL film (panel A), two distinctive coordinations indicate that two oxalato rings are still peculiarly perpendicular to each other, one takes parallel orientation along neutral electrodes, and the other one in the same [BMB] anion exhibits perpendicular distribution to neutral electrodes, respectively. Such a peculiar molecular arrangement suggests that the intramolecular inter-



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actions within [BMB] anions are much stronger than the corresponding intermolecular vdW coordinations between [BMB] anions and Au atoms constituting electrodes. The orientation of oxalato rings in [BMB] anions located in the intermediate interfacial layer lies between two typical orientational patterns mentioned above for [BMB] anions in the innermost boundary layer and that in bulk region of confined IL film. In [BMB] anionic framework, two phenyl rings are directly connected to neighbouring oxalato rings through flexible C-C bonds, and thus it is expected that two phenyl rings can rotate freely around the corresponding C-C axes. Interestingly, it is shown that similar coordination patterns are observed in CDFs of ∡noxalato -Z vs ∡nphenyl -Z as those of ∡noxalato -Z vs ∡noxalato -Z, but with slightly different coordination magnitude. Such a similarity indicates that phenyl rings are generally perpendicular to the corresponding oxalato rings that they are directly attached to, both in the innermost boundary layer and in bulk region of confined [P6,6,6,14 ][BMB] IL film. Combining these distinctive orientation features of oxalato and phenyl rings in [BMB] anions, we can clarify that four ring structures are characterized by alternative parallel and perpendicular arrangements as phenyl(parallel)oxalato(perpendicular)-oxalato(parallel)-phenyl(perpendicular) in the innermost interfacial layer adjacent to neutral electrodes. Such a peculiar orientational pattern represents the most probable molecular arrangement of [BMB] anions in interfacial region, and contributes directly to the pronounced volume charge densities in confined environment. As temperature increases, the thermal fluctuation in simulation system loosens the absorption of [P6,6,6,14 ][BMB] IL onto neutral electrodes, leading to slight changes in interfacial arrangements of [P6,6,6,14 ] cations and [BMB] anions in IL-gold interfacial region, as shown in typical CDF characterizations calculated at 473 and 573 K, respectively, in Figures S4 and S5 in Supporting Information.


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[P6,6,6,14 ][BMB] IL between charged gold electrodes As neutral gold electrodes get electrified, distinctive interfacial chemical compositions and interfacial molecular ordering effect are observed in IL-gold interfacial region, both in positively charged and in negatively charged electrodes. Figure 7 presents the overall and individual mass density profiles of [P6,6,6,14 ][BMB] IL between charged electrodes with different surface charge densities at 373 K. The corresponding mass density profiles for [P6,6,6,14 ][BMB] IL confined between neutral electrodes at 373 K are also provided for comparison. Both in positively charged and in negatively charged electrodes, [BMB] anions and [P6,6,6,14 ] cations exhibit pronounced mass density distributions as surface charge density increases, indicating a gradual accumulation of correspondent ionic groups in IL-gold interfacial region. On further inspection, it is identified that the enhanced mass density distributions in interfacial region come from different ionic species, as clearly shown in the individual contributions of [P6,6,6,14 ] cations (panel B) and [BMB] anions (panel C) in Figure 7. For gold electrodes 17 ACS Paragon Plus Environment


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characterized with low surface charge densities, the innermost interfacial layer is supposed to be a mixed ionic layer consisting of both [P6,6,6,14 ] cations and [BMB] anions, as verified by comparable mass density distributions in positively and negatively charged interfacial regions, respectively. An increase in surface charge density will propel [P6,6,6,14 ] cations and [BMB] anions from the mixed innermost interfacial layers adjacent to positively and negatively charged electrodes. As shown in the individual mass density profiles in simulation system with surface charge density of ± 20 µC/cm2 , there is no [P6,6,6,14 ] cations and [BMB] anions being accumulated in the innermost interfacial layers adjacent to positively and negatively charged electrodes, respectively, indicating the formation of single anionic and cationic layers in the innermost interfacial region. It is noteworthy that the further increase in surface charge density will not contribute to the formation of alternating cationic and anionic layers but just increase the innermost layer thickness. Such an observation might be attributed to the bulky and voluminous features of [P6,6,6,14 ] cations, which is not easy to form compact structures in confined environment even they response quickly to external electric fields. This can be rationalized by an overcrowding effect as indicated by Kornyshev and coworkers, 62 attributing to higher surface charge densities on gold electrodes compared with binding energies between [P6,6,6,14 ] cations and [BMB] anions in confined environment. The aggregation of [P6,6,6,14 ] cations and [BMB] anions adjacent to negatively and positively charged electrodes tends to neutralize surface charges. The higher of the applied surface charge density indicates that more ionic species carrying opposite charges will benefit energetically from adhering to charged electrodes and to be accommodated there, and accordingly, more counterions will be absorbed from bulk region to the subsequent layers. Such an interfacial charge effect is qualitatively characterized by calculating the dependence of innermost interfacial layer thickness on applied surface charge density, and the corresponding results are presented in Figure 8. In simulation systems with gold surface charge densities lower than 20 µC/cm2 , the innermost interfacial ionic layers are mixed ones generally consisting of both [P6,6,6,14 ] cations and [BMB] anions with the corresponding accumulated


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Figure 9: Atomic number density profiles of representative P (A) and B (B) atoms embedded in [P6,6,6,14 ] cationic and [BMB] anionic frameworks between charged electrodes at 373 K with different surface charge densities. The vertical blue and red dash-dot lines in each panel represent the relative positions of positively and negatively charged Au atoms constituting gold electrodes in direct contact with [P6,6,6,14 ][BMB] IL, respectively. The plots obtained at high surface charge densities are vertically shifted by 20 units based on previous curves for comparative propose. As expected from overall and individual mass density profiles of [P6,6,6,14 ] cations and [BMB] anions confined between charged electrodes, the atomic number density profiles of P and B atoms, and the variation of overall and individual volume charge densities for [P6,6,6,14 ][BMB] IL exhibit similar accumulation features, as clearly shown in Figures. 9 and 10, respectively. When gold surface charge densities are lower than 20 µC/cm2 , the absorbed ionic species in interfacial region normally can screen a major part of surface charges, leading to a quick recovery of volume charge density to bulk value with the increase of their relative distance beyond electrodes. Such a charge screening phenomena highlight the critical role of surface charge induced peculiar aggregation of ionic species in interfacial region. At higher surface charge densities, the formation of exclusive cationic and anionic innermost interfacial layers adjacent to charged electrodes contributes to the remarkable charge density profiles and an overcrowding effect as discussed in previous paragraphs.


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Figure 10: Variation of overall volume charge densities of [P6,6,6,14 ][BMB] IL confined between charged electrodes at 373 K with different surface charge densities. The vertical blue and red dash-dot lines represent the relative positions of positively and negatively charged Au atoms constituting gold electrodes in direct contact with [P6,6,6,14 ][BMB] IL, respectively. Figure 11 presents the effect of gold surface charge density on the orientational probability distributions of hexyl and tetradecyl chains in [P6,6,6,14 ] cations that are located in the innermost interfacial layer adjacent to negatively charged electrodes at 373 K. Both hexyl and tetradecyl chains are elongated along charged electrodes, mainly due to the delocalized partial charge distribution within cationic framework and the saturated aggregation of [P6,6,6,14 ] cations in the innermost cationic interfacial layer adjacent to negatively charged electrodes. Distinctive orientational preferences of oxalate and phenyl rings are observed for [BMB] anions positioned in the innermost interfacial layer adjacent to positively charged electrodes with different surface charge densities. Figure 12 presents CDFs of ∡noxalato -Z vs ∡noxalato Z for two oxalato rings jointed by a central B atom within each [BMB] anion. Two remarkable coordination patterns in panel (A) indicate that two oxalato rings take parallel-perpendicular 21 ACS Paragon Plus Environment


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Figure 12: Combined distribution functions of ∡noxalato -Z vs ∡noxalato -Z for oxalato rings jointed by a central B atom within each [BMB] anion that is positioned in the innermost interfacial layer adjacent to positively charged electrodes at 373 K with different surface charge densities. (A) 0 µC/cm2 ; (B) 5 µC/cm2 ; (C) 10 µC/cm2 ; (D) 20 µC/cm2 ; (E) 40 µC/cm2 ; (F) 60 µC/cm2 ; (G) 80 µC/cm2 ; and (H) 100 µC/cm2 . orientation in the innermost interfacial layer adjacent to neutral electrodes, as we specified in previous subsection. The charging gold electrodes leads to the rotation of oxalato rings along electrodes with the perpendicular one benting down and the parallel one benting up due to stronger intramolecular interactions between oxalato rings. It should be noted that two oxalato rings within the same [BMB] anion are still perpendicular to each other in the 22 ACS Paragon Plus Environment

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mixed innermost ionic layer, as suggested from symmetrical coordination features shown in the first three panels in Figure 12. For [P6,6,6,14 ][BMB] confined between charged electrodes with surface charge density of ± 20 µC/cm2 , the peculiar coordination pattern shown in Figure 12D indicates that besides the distinctive parallel-perpendicular orientational preference, a considerable amount of [BMB] anions take constraint orientation with oxalato rings tilted approximately 45◦ with respect to positively charged electrodes. It is known that in this simulation system the positively charged electrodes are saturated with [BMB] anions, leading to the formation of anionic innermost boundary layer. The two remarkable coordination patterns shown in Figure 12D correspond to the most popular distribution of oxalato rings in the innermost anionic layer, in which the absorbed [BMB] anions interact electrostatically with positively charged electrodes. A further increase in surface charge density leads to the diminish of peculiar coordination features for oxalate rings, which take severe constraint distribution in the innermost anionic layer. In the confined simulation box with surface charge density of ± 100 µC/cm2 , a striking coordination pattern shown in Figure 12H indicates that two oxalato rings in the same [BMB] anion are preferentially paralleled along the positively charged electrodes. In this case, the electric field generated from surface charges is much stronger than the intramolecular interactions between oxalato rings, which have to take flat orientation so as to effectively neutralize surface charges. Comparing with negligible orientational changes in hexyl and tetradecyl chains in [P6,6,6,14 ] cations positioned in the innermost cationic layer in confined environment, such a distinctive parallel orientation of oxalate rings contributes to the enhanced mass, atomic number and charge density distributions of [BMB] anions in interfacial region as presented in Figures 7, 9, and 10, respectively. Figure 13 presents coordination patterns of ∡noxalato -Z vs ∡nphenyl -Z for oxalate and phenyl rings within the same anionic framework for [BMB] anions absorbed in the innermost anionic layer adjacent to positively charged electrodes at 373 K with different surface charge densities. An increase in gold surface charge density gradually loosens the peculiar parallel-perpendicular orientation of oxalato and phenyl rings adjacent to positively charged



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Figure 13: Combined distribution functions of ∡noxalato -Z vs ∡nphenyl -Z for oxalato and phenyl rings bonded together by flexible C-C bonds within the same [BMB] anions that are located in the innermost interfacial layer adjacent to positively charged electrodes at 373 K with different surface charge densities. (A) 0 µC/cm2 ; (B) 5 µC/cm2 ; (C) 10 µC/cm2 ; (D) 20 µC/cm2 ; (E) 40 µC/cm2 ; (F) 60 µC/cm2 ; (G) 80 µC/cm2 ; and (H) 100 µC/cm2 . electrodes. This observation indicates that phenyl rings, either parallel or perpendicular to gold electrodes, are gradually propelled from the innermost interfacial layer leading to their featureless orientation in confined environment, especially in simulation systems with gold surface charge densities higher than 40 µC/cm2 . Additionally, the CDFs of ∡noxalato -Z vs ∡noxalato -Z and ∡noxalato -Z vs ∡nphenyl -Z for oxalate and phenyl rings within the same anionic framework for [BMB] anions located in bulk region of confined [P6,6,6,14 ][BMB] IL film are provided in Figures S8 and S9 in Supporting Information for comparison. The board and symmetric features in all CDF plots indicate that oxalato and phenyl rings in each [BMB] anion exhibit mean distributions, but the directly connected ring sturctures, either the oxalato-oxalato or the oxalato-phenyl ring pairs, are generally characterized by perpendicular orientational patterns, which are regarded as striking ionic molecular structures of [BMB] anions as we discussed in previous paragraphs.


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Discussion and Summary The detailed analyses of atomistic simulation results indicate that the interfacial chemical compositions, molecular arrangements of [P6,6,6,14 ][BMB] IL confined between neutral and charged gold electrodes are different depending on the assigned surface charge densities to gold electrodes. For [P6,6,6,14 ][BMB] IL confined between neutral electrodes, an innermost interfacial layer and a subsequent intermediate layer are formed before reaching bulk region of confined IL film. The innermost layer consists of both [P6,6,6,14 ] cations and [BMB] anions, which take compact ionic structures and checkerboard molecular arrangement in interfacial region. In this mixed innermost layer, both hexyl and tetradecyl chains in [P6,6,6,14 ] cations lie preferentially parallel along electrodes, and the most probable configuration of oxalato and phenyl rings in [BMB] anions is characterized by consecutive parallel and perpendicular arrangement adjacent to neutral electrodes, respectively. A spatial inhomogeneity of mass, atomic number and charge densities is mainly registered in the innermost and intermediate interfacial regions, due to the most probable orientation of [P6,6,6,14 ][BMB] ionic species, and the associated intramolecular charge distributions in IL-gold interfacial region. As gold electrodes get electrified but with low surface charge densities (< 20 µC/cm2 ), the mixed innermost layer thickness gradually increases as that in surface charge density, due to a gradual accumulation of [P6,6,6,14 ] cations and [BMB] anions, and their counterions being squeezed out of the innermost layer adjacent to negatively and positively charged electrodes, respectively. The effect of charging electrodes has little influence on the molecular orientation of hexyl and tetradecyl chains in [P6,6,6,14 ] cations due to their popular elongated molecular conformation along electrodes. However, charging gold electrodes leads to new orientational patterns for oxalato rings in the same [BMB] anions from parallel-perpendicular orientation to that partially characterized by constraint molecular arrangement with tilted angle of 45◦ from positively charged electrodes. In the meantime, the molecular distribution of phenyl rings that are directly bonded to oxalato rings through flexible C-C bonds in [BMB] anions is also alerted accordingly. 25 ACS Paragon Plus Environment


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Upon further charging gold electrodes with surface charge densities equal or higher than 20 µC/cm2 , distinctive innermost interfacial layers exclusively consisting of [P6,6,6,14 ] cations and [BMB] anions are form adjacent to negatively and positively charged electrodes, respectively. This points to a templating effect in producing enriched, tightly bounded and compact innermost layers closest to charged electrodes as surface charge density increases. Such an interfacial effect will in turn alter packing structures of confined ionic species in subsequent layers, and so forth, resulting in enhanced interfacial ionic structures in confined environment. The small anionic size and localized partial charges in [BMB] anionic framework contribute to compact interfacial anionic structures consisting of electrode-absorbed anions, as verified by pronounced density distributions in the innermost anionic layer, which may exhibit a solid-like behavior. It is expected that more energies are needed for a probe to rupture, to penetrate, and to displace the innermost anionic layer due to the fact that the absorbed ionic species are strongly bounded to oppositely charge electrodes, as indicated from previous AFM experiments 12,15,16,18,19 and SFA measurements. 29,30 The orientation of oxalato and phenyl rings in [BMB] anions is described by board and featureless characteristics before distinctive coordination pattern observed for [BMB] anions adjacent to positively charged electrodes with surface charge density of 100 µC/cm2 . The hexyl and tetradecyl chains in [P6,6,6,14 ] cations are preferentially aligned along negatively charged electrodes, mainly due to their delocalized charge distribution within cationic framework and their saturated distribution in the innermost cationic layer. It is noteworthy that [P6,6,6,14 ] cations and [BMB] anions exhibit different responses to external electric field generated from charged electrodes, as specified in representative quantities including innermost interfacial layer thicknesses, microstructural arrangements, and orientational coordination patterns. The particular structural and orientational changes for [P6,6,6,14 ][BMB] ionic species in confined environment will have a profound impact on interfacial friction if this IL is used as lubricant to lubricate gold engineering surfaces. Additionally, these simulation results can further suggest that lubricating and tribological properties of


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an ionic liquid on gold electrodes can be qualitatively controlled through application of electric potential to sliding contacts, which has been reported in previous AFM and SFA experiments. These atomistic simulation results are helpful in elucidating physical changes in interfacial ionic structures and molecular arrangements of [P6,6,6,14 ][BMB] between neutral/charged gold electrodes, and may provide more physical insight in further investigation of ILs’ electrotunable friction response and lubrication mechanism before advancing their tribological performance in mechanical engineering systems.

Acknowledgement Y.-L. Wang gratefully acknowledge financial support from Knut and Alice Wallenberg Foundation (KAW 2015.0417 and 2012.0078). All atomistic simulations were performed using computational resources provided by Swedish National Infrastructure for Computing (SNIC) at PDC, HPC2N and NSC.

Supporting Information Available The following files are available free of charge. Individual mass density profiles of [P6,6,6,14 ] and [BMB] confined between neutral electrodes at different temperatures; Detailed atomic number density profiles of P and B atoms in interfacial regions at different temperatures; Variation of individual volume charge density profiles of [P6,6,6,14 ] and [BMB] confined between neutral electrodes at different temperatures; Combined distribution functions of ∡noxalato Z vs ∡noxalato -Z, and of ∡noxalato -Z vs ∡nphenyl -Z for oxalato and phenyl ring in [BMB] anions that are positioned in the innermost and intermediate interfacial layers, and in bulk region of confined IL film, respectively, at 473 and 573 K; Overall and individual mass density profiles of [P6,6,6,14 ] and [BMB] confined between charged electrodes at 373 K with different surface charge densities; Atomic number density profiles of P and B atoms between charged gold electrodes at 373 K with different surface charge densities; Combined distribution func27 ACS Paragon Plus Environment


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tions of ∡noxalato -Z vs ∡noxalato -Z, and of ∡noxalato -Z vs ∡nphenyl -Z for oxalato and phenyl ring in [BMB] anions that are positioned in bulk region of confined IL film at 373 K with different surface charge densities.

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(17) Hoth, J.; Hausen, F.; Müser, M. H.; Bennewitz, R. Force Microscopy of Layering and Friction in an Ionic Liquid. J. Phys.: Condens. Matter 2014, 26, 284110. (18) Zhong, Y.-X.; Yan, J.-W.; Li, M.-G.; Zhang, X.; He, D.-W.; Mao, B.-W. Resolving Fine Structures of the Electric Double Layer of Electrochemical Interfaces in Ionic Liquids with an AFM Tip Modification Strategy. J. Am. Chem. Soc. 2014, 136, 14682–14685. (19) Lahiri, A.; Carstens, T.; Atkin, R.; Borisenko, N.; Endres, F. In Situ Atomic Force Microscopic Studies of the Interfacial Multilayer Nanostructure of LiTFSI-[Py1,4 ]TFSI on Au(111): Influence of Li+ Ion Concentration on the Au(111)/IL Interface. J. Phys. Chem. C 2015, 119, 16734–16742. (20) Su, Y.-Z.; Fu, Y.-C.; Yan, J.-W.; Chen, Z.-B.; Mao, B.-W. Double Layer of Au(100)/Ionic Liquid Interface and Its Stability in Imidazolium-Based Ionic Liquids. Angew. Chem. Int. Ed. 2009, 121, 5148–5151. (21) Foulston, R.; Gangopadhyay, S.; Chiutu, C.; Moriarty, P.; Jones, R. G. Mono- and Multi-Layer Adsorption of an Ionic Liquid on Au(110). Phys. Chem. Chem. Phys. 2012, 14, 6054–6066. (22) Gnahm, M.; Berger, C.; Arkhipova, M.; Kunkel, H.; Pajkossy, T.; Maas, G.; Kolb, D. M. The Interfaces of Au(111) and Au(100) in a Hexaalkyl-Substituted Guanidinium Ionic Liquid: An Electrochemical and in Situ STM Study. Phys. Chem. Chem. Phys. 2012, 14, 10647–10652. (23) Uhl, B.; Cremer, T.; Roos, M.; Maier, F.; Steinrück, H.-P.; Behm, R. J. At the Ionic Liquid-Metal Interface: Structure Formation and Temperature Dependent Behavior of an Ionic Liquid Adlayer on Au(111). Phys. Chem. Chem. Phys. 2013, 15, 17295–17302. (24) Uhl, B.; Huang, H.; Alwast, D.; Buchner, F.; Behm, R. J. Interaction of Ionic Liquids with Noble Metal Surfaces: Structure Formation and Stability of [OMIM][TFSA] and


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borate Ionic Liquid Confined between Gold ... - ACS Publications

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