Competitive Microstructures vs. Cooperative Dynamics of Hydrogen

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Competitive Microstructures vs. Cooperative Dynamics of Hydrogen Bonding and # Type Stacking Interactions in Imidazolium Bis(oxalato)borate Ionic Liquids Yong-Lei Wang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02899 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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The Journal of Physical Chemistry

Competitive Microstructures vs. Cooperative Dynamics of Hydrogen Bonding and π Type Stacking Interactions in Imidazolium Bis(oxalato)borate Ionic Liquids Yong-Lei Wang∗ Department of Chemistry, Stanford University, Stanford, California 94305, United States E-mail: [email protected] Phone: (1) 650-785-3771

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Abstract The delicate trade-off between hydrogen bonding and π type coordinations plays a pivotal role in stabilizing molecular structures in ionic liquids bearing multiple hydrogen bonding sites and heteroaromatic ring planes. By performing extensive atomistic simulations, we have investigated the effect of aliphatic chain length in imidazolium cations on liquid morphologies, hydrogen bonding and π type structures, and the corresponding dynamical quantities in imidazolium bis(oxalato)borate ionic liquids. The liquid morphologies are characterized by segregated apolar clusters (domains) within polar framework in liquid samples with short aliphatic chains in imidazolium cations, and are transformed to sponge-like polar and apolar arrangements in liquid matrices with lengthening aliphatic chains in imidazolium cations. Such a striking evolution of liquid morphologies of imidazolium bis(oxalato)borate ionic liquids is qualitatively characterized by total and partial X-ray scattering static structural factors. Preferential hydrogen bonds and distinctive π type coordinations among imidazolium and oxalato ring planes co-exist in ionic liquid matrices. A gradual addition of methylene units to imidazolium cations leads to a substantial increase in hydrogen bonding strength, which, however, results in decreased π type coordinations between imidazolium and oxalato ring planes, indicating a distinct competitive structural characteristics between hydrogen bonding and π type associations between imidazolium and oxalato ring planes. A prevalent cooperative feature is observed in continuous and intermittent hydrogen bonding dynamics, and in translational and re-orientational dynamics of imidazolium and oxalato ring planes with lengthening aliphatic chains in imidazolium cations. The competitive structural trade-off and cooperative dynamical interplay of hydrogen bonding and π type interactions between imidazolium cations and bis(oxalato)borate anions are intrinsically correlated with short-range collective interactions between alkyl units in imidazolium cations and long-range Coulombic interactions between imidazolium and oxalato ring planes in heterogeneous ionic environments.

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I. Introduction Ionic liquids (ILs) represent an intriguing category of molten salts composed of inorganic or organic anions and, most commonly, organic cations with voluminous structures. The bulky cations usually consist of conformationally flexible aliphatic chains and heteroaromatic ring planes with delocalized charges, which enhance the entropy effect in IL matrices and thus inhibit ILs from solidification at room temperature in spite of strong Coulombic interactions among ionic species. 1–5 ILs have attracted widespread attention in diverse communities due to their remarkable physicochemical characteristics, such as non-flammability, negligible volatility, reasonable viscosity-temperature characteristics, outstanding thermaland electrochemical-oxidative stabilities, and excellent affinities to polar and apolar compounds. 1–7 These fascinating physicochemical properties can be widely tuned in a controllable fashion through a judicious combination of different cation-anion moieties, and by mutating specific atoms in constituent ionic groups. 5,8–12 These fascinating characteristics render ILs exceptionally attractive and reliable alternatives to conventional molecular liquids and electrolytes in many envisioned applications including material synthesis and catalysis, 2,6,7,13–16 micro-lubrication and nano-tribology, 17–19 gas absorption and separation (as solvents and membrane transport media), 20,21 pre-treatment of lignocellulose biomass and dissolution of simple carbohydrates, 22,23 and electrochemical devices. 24–26 The rapid upswing in academia and industrial communities on synthesis, characterizations, and applications of ILs stems from a direct consequence of intrinsic characteristics of constituent ions and peculiar intra- and inter-molecular interactions between ionic groups. 3,5 The dominant Coulombic interactions between ionic moieties and favourable van der Waals (vdW) interactions between apolar moieties are key driving forces for structuring ionic liquid organization. 5,12,17,27 In addition, the isotropic nature of Coulombic interactions allows for a considerable assortment of directional secondary intermolecular interactions, such as dipoledipole, dipole-induced dipole, dispersion interactions, delicate hydrogen bonding (HB), and possible π type coordinations between ionic species with delocalized charges in their het3



eroaromatic ring planes. 3,28–32 In a particular situation where strong Coulombic forces compete with each other and partially cancel each other out, these delicate intermolecular interactions facilitate additional stabilization and direct the formation of striking ionic structures in bulk IL matrices and in confined environments. 5,17,25,27,33 HB is a directional coordination compared with other forces (e.g. vdW and hydrophobic interactions) between liquid molecules, and is primarily characterized by electrostatic interactions with covalent and dispersion contributions. 28–30,34–36 In most IL matrices, HB interactions facilitate the preferential orientation of close-contacted ionic species in local ionic environments. 28,35,36 Multiple HB coordinations between neighboring ionic groups act cooperatively over long distances as exemplified by water’s tetrahedral network structures, which is fundamental for many physicochemical, structural, and dynamical aspects of ILs. 30,34,36 Recent theoretical and experimental investigations revealed that HB is not a binary onoff phenomenon but occurs in a graduated scale and covers an extremely wide and diverse range in ILs depending on the detailed molecular structures of constituent ionic groups. 28–30 In the context of difficulty in defining a hydrogen bond in simple molecular liquids, there is additional complexity in quantifying a hydrogen bond in IL matrices where varied type of intermolecular interactions cannot be easily separated from essential components associated with HB, and thus the definition of a hydrogen bond between ionic species can be a matter of semantics. 29,30 In concert with HB coordination, π type interaction is related to favourable electrostatic contributions and preferential dispersion interactions between heteroaromatic ring planes, such as imidazolium, pyridinium, pyrazolium, triazolium, thiazolium cations, orthoborate anions, and their derivatives despite of repulsive Coulombic forces between like charges. 31,32,36–38 The π type interaction has been recognized as a distinctive contributing factor in protein folding and ion selectivity, self-organized electronic materials, and organic nanodevices. 25,26,32,36,39,40 In IL matrices, the cation-cation π−π stacking structures are less common but have been identified in representative imidazolium based ILs. 28,32,36,41–47 The formation of

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π − π stacking imidazolium ring dimers was observed in 1-ethyl-3-methylimidazolium nitrate ([EMIM][NO3 ]), 41 1-ethyl-3-methylimidazolium sulfate ([EMIM]2 [SO4 ]), 41 dimethylimidazolium triflate ([MMIM][OTF]) 42 and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTF2 ]) 43 IL crystal structures due to a significant screening of cationic chargecharge repulsion forces that are mediated by anionic groups. Moreover, the observation of peculiar π − π stacking structures between imidazolium ring planes and benzene molecules in IL-benzene mixtures was rationalized by attractive arrangements of quadrupole moments associated with aromatic ring planes. 48,49 Both HB and π type interactions play distinctive roles in stabilizing microscopic ionic structures in IL matrices bearing multiple HB sites and heteroaromatic ring planes. However, the subtle balance between these two type coordinations is complicated, and intrinsically depends on specific molecular structures of constituent ionic groups. For imidazolium cations coupled with small anions, such as chloride (Cl), 32,36,44 thiocyanate ([SCN]), 44 and [NO3 ] 46 anions, the π − π stacking motifs have been found to co-exist alongside HB coordinations between ionic species. The synergistic feature of π type and HB coordinations contributes to the formation of prominent ordered microstructures in ILs matrices. However, in IL matrices consisting of imidazolium cations coupled with large anions, like [NTF2 ], 28,43,45 both π type and HB interactions simultaneously get weakened. Large anionic groups typically have multiple HB acceptors, and prefer to form multiple hydrogen bonds with hydrogen atoms in imidazolium cations, which promotes to the formation of hydrogen bond network within IL matrices but with decreased HB directionality and strength. Additionally, these large anionic species take preferential on-top distributions above and below imidazolium ring planes, or tilted coordinations with imidazolium ring planes in their equatorial region, leading to π type coordinations between neighboring imidazolium ring planes being partially or totally screened due to anionic size effect. As such, the formation of HB network between ionic species overtakes π type interactions between close-contacted imidazolium ring planes, and plays a dominant role in determining local microstructures and striking liquid morphologies

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of IL matrices. The delicate trade-off between HB and π type stacking interactions among neighboring ionic species, either cooperative or competitive, becomes more complicated if anionic groups are featured with ring planes, like the chelated orthoborate families. 9,11,50,51 In a prior study, we performed first-principles molecular dynamics simulations to explore striking HB and π type associations in dimethylimidazolium bis(oxalato)borate ([MMIM][BOB]) IL at elevated temperature. 47 It was revealed that: imidazolium ring planes take preferential π − π stacking distributions at short distances; the spacial coordinations between imidazolium and oxalato ring planes are characterized by short-range displaced offset stacking conformations stabilized by preferential HB interactions between [MMIM] cations and [BOB] anions, and by sharp perpendicular orientations at intermediate distances due to attractive Coulombic interactions, respectively; intermolecular oxalato ring planes are balanced by repulsive electrostatic interactions and steric hindrance effects, leading to their tilted orientations in coordinating neighboring imidazolium ring planes in local ionic environments. In present work, we extend the earlier endeavours with an in-depth and comprehensive analysis on complicated π type stacking characteristics between imidazolium and oxalato ring planes, and how these stacking characteristics compete with and compliment HB interactions in varied imidazolium bis(oxalato)borate IL matrices. Additionally, the effect of aliphatic chain length in imidazolium cations on residence lifetimes of continuous and intermittent HB dynamics, and on translational and re-orientational dynamics of imidazolium and oxalato ring planes, are systematically explored and discussed in following subsections.

II. Ionic Models and Computational Methodology Six imidazolium cations with different aliphatic chains including [EMIM], 1-butyl-3methylimidazolium ([BMIM]), 1-hexyl-3-methylimidazolium ([HMIM]), 1-octyl-3-methylimidazolium ([OMIM]), 1-decyl-3-methylimidazolium ([DMIM]), and 1-dodecyl-3-methylimidazolium ([DdMIM])

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Figure 1: Schematic molecular structures and representative atom types in imidazolium cations and [BOB] anion. The studied cations include [EMIM], [BMIM], [HMIM], [OMIM], [DMIM], and [DdMIM] with n = 0, 1, 2, 3, 4, and 5. The imidazolium and oxalato ring planes refer to those formed by CR-N-CW-CW-N atoms, and by B-OB-C-C-OB atoms in consecutive orders, respectively. In the representative orientation of imidazolium-oxalato ring planes, r is the radial distance between center-of-mass of imidazolium and oxalato ring planes. n ¯ and m ¯ represent the normal vectors to imidazolium and oxalato ring planes, respectively. θ is the angle between two normal vectors, and also the angle between imidazolium and oxalato ring planes. This representative conformation of imidazolium-oxalato ring planes is reprinted with permission from Ref. 47 Copyright (2017) American Chemical Society. are considered in present work. The molecular structures of imidazolium cations and [BOB] anion, and the representative atom types in these ionic species are present in Fig. 1. Atomistic interaction parameters for imidazolium bis(oxalato)borate ILs stem from a systematically developed force field in previous studies based on the AMBER framework. 9,52 The cross interaction parameters between different atom types are obtained from the Lorentz-Berthelot combination rules. During force field development, the imidazolium cations and [BOB] anion were described by unity charges and all atomic partial charges were derived by fitting molecular electrostatic potential generated from ab initio calculations of individual ions using restraint electrostatic potential fitting approach. 9,52 It has been suggested in earlier studies that the down-scaling of atomic partial charges is an effective way to account for polarization and charge transfer effects between ionic species, and thereby can improve the reliability of calculations of dynamical and transport quantities of ILs. 53–56 It was identified in a recent study that a

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charge scaling factor of 0.8 could provide reliable liquid viscosities of trihexyltetradecylphosphonium bis(mandelato)borate ([P6,6,6,14 ][BMB]) and [P6,6,6,14 ]Cl ILs as those obtained from experimental measurements within a wide temperature interval of 373 − 463 K. 57 Therefore in present work, the fractional charges for all atoms in imidazolium cations and [BOB] anion are uniformly re-scaled using a factor of 0.8 from unity ion charges. It should be noted that even the adopted charge scaling factor of 0.8 is an empirical correction of electrostatic polarization and charge transfer effects between imidazolium cations and [BOB] anions, it gives a good performance in describing structural and dynamical quantities of imidazolium bis(oxalato)borates ILs. In present atomistic simulations, each simulation system consists of varied number of imidazolium bis(oxalato)borate ion pairs with the total atoms of approximately 32000. The detailed simulation system compositions are listed in Table 1. Atomistic molecular dynamic simulations were performed using GROMACS 5.0.4 package 58 with cubic periodic boundary conditions. The equations of motion were integrated using a classical velocity Verlet leapfrog integration algorithm with a time step of 1.0 fs. A cutoff distance of 1.6 nm was set for short-range vdW interactions and real-space electrostatic interactions between atomic partial charges. The particle-mesh Ewald summation method with an interpolation order of 5 and a Fourier grid spacing of 0.12 nm was employed to handle long-range electrostatic interactions in reciprocal space. All imidazolium bis(oxalato)borate IL systems were first energetically minimized using a steepest descent algorithm, and thereafter annealed gradually from 1000 to 323 K within 20 ns. The annealed simulation systems were equilibrated in NPT (isothermal-isobaric) ensemble for 60 ns of physical time maintained using Nosé-Hoover chain thermostat and Parrinello-Rahman barostat with time coupling constants of 0.5 and 0.2 ps, respectively, to control temperature at 323 K and pressure at 1 atm. Atomistic simulations were further performed in NPT ensemble for 80 ns for all imidazolium bis(oxalato)borate ILs, and simulation trajectories were recorded at an interval of 100 fs for further structural and dynamical analysis.

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Table 1: The number of imidazolium bis(oxalato)borate ion pairs and the total number of atoms in all simulation systems, and the computational liquid densities of all six imidazolium bis(oxalato)borate ILs at 323 K. ILs No. of ion pairs No. of total atoms [EMIM][BOB] 1000 32000 [BMIM][BOB] 842 31996 [HMIM][BOB] 728 32032 [OMIM][BOB] 640 32000 [DMIM][BOB] 572 32032 [DdMIM][BOB] 516 31992

Liquid densities (kg/m3 ) 1354.51±7.08 1306.86±8.65 1261.04±9.03 1212.13±9.45 1165.43±9.53 1126.81±9.34

III. Computational Results and Discussion A. Liquid densities The liquid densities of six imidazolium bis(oxalato)borate ILs calculated from current atomistic simulations at 323 K are listed in Table 1. Due to limited experimental liquid densities of imidazolium bis(oxalato)borate ILs in literature, it is difficult to make a systemic comparison with current atomistic simulation results, but can validate some computational density data at specific conditions. The liquid densities of [BMIM][BOB] and [HMIM][BOB] ILs are (1306.86 ± 8.65) and (1261.04 ± 9.03) kg/m3 , respectively, at 323 K. These computational results are quite consistent with available experimental density data of 1295.1 and 1239.5 kg/m3 of these two ILs determined at the same temperature. 50,59 In addition, the experimental density of [DMIM][BOB] is 1174 kg/m3 at 293 K and 1119 kg/m3 at 363 K, respectively, which are compatible with the corresponding computational result obtained at 323 K as listed in Table 1. The agreement between these experimental data and current atomistic simulation results is remarkably good with a maximum deviation of approximately 2.1%. Such a small deviation in liquid densities of neat IL samples, as specified in previous studies, 10,12 presents negligible impact on thermodynamics and microstructural characterizations of ILs, and on dynamical properties of representative groups in ionic environments. Additionally, the liquid densities of six imidazolium bis(oxalato)borate ILs decrease linearly with lengthening aliphatic chains in imidazolium cations, and with increasing temperatures 9



Figure 2: Representative liquid morphologies of imidazolium bis(oxalato)borate ILs at 323 K. (A) [EMIM][BOB]; (B) [BMIM][BOB]; (C) [HMIM][BOB]; (D) [OMIM][BOB]; (E) [DMIM][BOB]; and (F) [DdMIM][BOB]. The imidazolium ring planes, H5, and H4 atoms in cations are represented by blue beads. The remaining atoms in imidazolium cations and the [BOB] anions are described by cyan and red beads, respectively. within the range of 300 − 500 K.

B. Liquid morphologies and static structural factors Representative liquid morphologies of six imidazolium bis(oxalato)borate ILs are present in Fig. 2. The overall effect of aliphatic chain length in imidazolium cations on microscopic structural ordering characteristics in bulk imidazolium bis(oxalato)borate IL matrices is quantified by the X-ray scattering static structural factor, S(q). 10,60–64 The total static structural factor of model IL simulation systems can be determined by the summation of P P atom type based partial structural factors as S(q) = ni=1 nj=1 Sij (q). The Sij (q) is the

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partial static structural factor between atoms types i and j and is given by

Sij (q) =

ρ0 xi xj fi (q)fj (q)

R L/2 0

4πr2 [gij (r) − 1] sin(qr) W (r)dr qr

P [ ni=1 xi fi (q)]2

.

Herein, xi and xj are the mole fractions of atoms types i and j, gij (r) is the corresponding partial radial distribution function (RDF), and fi (q) and fj (q) are correspondent X-ray atomic form factors, 65 respectively. ρ0 =

Natom

refers to the average atom number density

of simulation system and L is the simulation box length. W (r) is a Lorch window function defined as W (r) =

sin(2πr/L) , 2πr/L

which is used to minimize the effect of finite truncation of r in

the calculation of gij (r). In the current work, the total X-ray scattering static structural factor S(q) is mathematically partitioned into polar-polar and apolar-apolar subcomponent contributions, as well as their cross correlations to address polar and apolar domain alternations in IL matrices. The detailed partitioning scheme is systematically described in prior publications and references therein. 10,12,60–64 The polar domains consist of [BOB] anions, imidazolium ring planes, H5, and H4 atoms in cations. The apolar domains are occupied by the remaining hydrophobic alkyl units in imidazolium cations. Fig. 3 presents the partial static structural factors for polar-polar, apolar-apolar, and cross term (polar-apolar/apolar-polar) subcomponents at q ≤ 25.0 nm−1 , as well as the total X-ray scattering static structural factors for six imidazolium bis(oxalato)borate ILs. Two prominent peaks located at around 5.0 and 15.0 nm−1 are shown in the total static structural factors for six imidazolium bis(oxalato)borate ILs. Such a double-peak-plot is a general feature for imidazolium based ILs, and is mainly attributed to close packing structures of constituent ions in local ionic environments. 10,12,61,63,64 With some variations across static structural factor plots for imidazolium bis(oxalato)borate ILs, these peak positions are essential characteristic hallmark of their microstructural landscapes, indicating the particular microscopic ionic ordering phenomena at different length scales in IL matrices. 10,12,61,64

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Total

q)

6

Static structural functions S(

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Polar-Polar

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Polar-Apolar + Apolar-Polar

Apolar-Apolar

A: [EMIM][BOB]

B: [BMIM][BOB]

C: [HMIM][BOB]

D: [OMIM][BOB]

E: [DMIM][BOB]

F: [DdMIM][BOB]

4 2 0 -2 -4 6 4 2 0 -2 -4 0

5

10

15

-1

q (nm

)

20

25

5

10

15

-1

q (nm

20

)

25

5

10

15

-1

20

25

q (nm )

Figure 3: The computational X-ray scattering static structural factors S(q) in the range of q ≤ 25.0 nm−1 for six imidazolium bis(oxalato)borate ionic liquids at 323 K. The total static structural factors are mathematically partitioned into polar-polar, polar-apolar/apolar-polar, and apolar-apolar subcomponent contributions. The low q-peak occurs at around q ∼ 5.0 nm−1 is an indicative of mesoscopic liquid organization characterized by long range polarity ordering (or polar-apolar density alternation) in IL matrices. 10,12,61–64 The polar-apolar alternation peak is registered at 7.0 and 6.8 nm−1 in [EMIM][BOB] and [BMIM][BOB] IL matrices, respectively. These peak positions are tightly correlated with polar domains encompassing [BOB] anions, imidazolium ring planes, H5 and H4 atoms in cations and apolar domains occupied by remaining alkyl units in imidazolium cations. In these two IL systems, it is shown that the apolar-apolar subcomponents have negligible contribution to the total static structural factors, which are entirely determined from distinctive polar-polar correlations. These computational results indicate that the microscopic liquid structures are characterized by segregated small apolar clusters within a continuous polar network of ionic channels consisting of imidazolium and oxalato ring planes that are closely packed together by means of strong Coulombic interactions. The representative liquid morphologies of these two IL systems are present in panels A and B of Fig. 2. A progressive addition of methylene units to aliphatic chains in imidazolium cations re12


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sults in an aggregation of alkyl units because of their mutual hydrophobic characteristics, and thereafter promotes the formation of spatially heterogeneous apolar domains due to collective solvophobic interactions between alkyl units. 12,60,61,63,64 In [HMIM][BOB] and [OMIM][BOB] IL systems, the volumetric molecular sizes of polar and apolar domains are comparable, and thus the corresponding liquid morphologies are described by bi-continuous sponge-like arrangements with interpenetrating polar and apolar networks, as shown in panels C and D of Fig. 2. This observation is qualitatively manifested in the partial static structural factors for apolar-apolar components with q values centered at 4.6 and 3.1 nm−1 for [HMIM][BOB] and [OMIM][BOB] IL matrices, respectively. It is known that the peak positions in static structural factor plots are associated with typical inverse characteristic distances for polarity alternations through the formula of d = 2π/q, where d is the polarity alternation distance in real space. 12,61–64 The shift of peak positions from 7.0 to 6.8 nm−1 in total static structural factors for [EMIM][BOB] and [BMIM][BOB] IL matrices, and from 4.6 to 3.1 nm−1 in apolar-apolar partial static structural factors for [HMIM][BOB] and [OMIM][BOB] IL matrices, indicates that the polarity alternation behavior occurs at larger characteristic distance due to the gradual expansion of apolar domains in IL matrices with lengthening aliphatic chains in imidazolium cations. This explanation is clearly endorsed by representative liquid morphologies of imidazolium bis(oxalato)borate ILs shown in Fig. 2. The further lengthening aliphatic chains in imidazolium cations leads to an additional expansion of apolar network in IL matrices. In the meantime, the polar network becomes more permeated by expanding apolar domains, and tends to persist but has to accommodate the growing apolar domains by loosing part of its connectivity, as shown in typical liquid snapshots of [DMIM][BOB] and [DdMIM][BOB] ILs in panels E and F of Fig. 2. In these two IL systems, the polar-polar subcomponents contribute to positive intensities, which are ultimately offsetted by anti-correlations for cross term (polar-apolar + apolar-polar) subcomponents at the same q values. Such an almost complete cancellation of peaks and anti-peaks leads to the total static structural factors being mainly determined by distinctive

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2.5 2.0

A: Total

1.0

B: Polar-Polar

q)

1.5 Static structural functions S(

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5

1.0 0.5

0.0 [EMIM]

0.0 -0.5 12

14

16

-1

18

[BMIM]

-0.5 20 12

14

q (nm )

1.0

16

-1

18

20

q (nm )

C: Polar-Apolar + Apolar-Polar

1.0

[HMIM] [OMIM]

D: Apolar-Apolar

[DMIM] [DdMIM]

0.5 0.5 0.0 0.0

-0.5 -1.0 12

14

16

-1

q (nm

18

-0.5 20 12

14

)

16

-1

q (nm

18

20

)

Figure 4: The total static structural factors in the range of 12.0 ≤ q ≤ 20.0 nm−1 for six imidazolium bis(oxalato)borate ionic liquids at 323 K are mathematically partitioned into polar-polar and apolar-apolar subcomponent contributions, as well as their cross term (polar-apolar + apolar-polar) correlations. correlations from apolar-apolar subcomponents, indicating the strong apolarity ordering ionic structures in the corresponding IL matrices. The prominent peaks at high q values near 15.0 nm−1 are associated with short range adjacency correlations originated from nearest neighboring interactions between ionic species. 10,12,60–64 In the six imidazolium bis(oxalato)borate IL systems, all the polar-polar, apolar-apolar and the cross term (polar-apolar + apolar-polar) subcomponents have considerable contributions to the adjacency correlation peaks in the corresponding total static structural factor plots. However, the individual contribution of the three subcomponents is distinct depending on aliphatic chain length in imidazolium cations, as clearly shown in Fig. 4. The lengthening aliphatic chains in imidazolium cations leads to the increased intensities in partial static structural factors for apolar-apolar and cross term (polar-apolar + apolar-polar) subcomponents, as well as in the total static structural factors for six imidazolium bis(oxalato)borate IL systems with a concomitant shift of adjacency correlation peaks to lower q values. In

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the meantime, the adjacency correlation peaks in partial static structural factors for polarpolar subcomponents are also gradually shifted to lower q values but with decreased peak intensities. All these microstructural changes in imidazolium bis(oxalato)borate IL matrices are attributed to the expansion of apolar domains in heterogeneous IL matrices with a gradual addition of alkyl units to aliphatic chains in imidazolium cations. Such a striking microstructural evolution in imidazolium bis(oxalato)borate IL matrices is similar to that observed in other bulk ILs 12,66,67 and IL mixtures, 9,68–70 and can be rationalized by peculiar intermolecular competitions of persistent short-range collective hydrophobic interactions between alkyl units in imidazolium cations and favorable long-range Coulombic interactions between imidazolium and oxalato ring planes, as well as other delicate coordinations including directional HB interactions and preferential π type stacking associations between ionic species in heterogeneous IL matrices. Guided by the detailed analysis on the computational static structural factors for polar and apolar domains, and the representative liquid morphologies of imidazolium bis(oxalato)borate ILs, a comprehensive analysis of HB structures and dynamics, π type stacking structures between imidazolium and oxalato ring planes, as well as re-orientational and translational dynamics of representative ionic groups in IL matrices, is performed to address how aliphatic chains in imidazolium cations affect structural and dynamical associations of HB and π type coordinations among imidazolium cations and [BOB] anions.

C. Hydrogen bonding structures In imidazolium cations, the hydrogen atoms can be sorted into three categories depending on their relative positions in cationic framework. The H5 atom is a primary hydrogen bond donating site and contributes to remarkable HB features with specific atoms in anions. 29,32,46,52,55 The H4 atoms at rear part of imidazolium ring planes, and H1 atoms in methyl and methylene groups that are attached to imidazolium ring planes via nitrogen atoms, are considered as secondary hydrogen bond donors in stabilizing ionic structures 15



Page 16 of 50

in local environments. 29,30,32,36 The third type is the hydrogen atoms in the remaining hydrophobic alkyl units, which are less active and rarely contribute to HB characteristics with neighboring anions, and thus their HB features are not discussed in present work. Concerning the quantification of a hydrogen bond, a sophisticated angle-distance criterion was proposed by Hunt and coworkers through extensive first-principles calculations. 29 It addresses that the angle of ]X-H· · · Y (X-H acts as a proton donor to Y) falls within the range of 120 − 180◦ is more appropriate than the empirical angle of 150 − 180◦ chosen for HB interactions in simple molecular liquids like water. In current work, we adopt this angledistance criterion and a hydrogen bond is present when H· · · O distance (H corresponds to H5, H4, and H1 atoms in imidazolium cations, and O corresponds to OC and OB atoms in [BOB] anions, respectively) is less than the radial distance of the first minima in the corresponding H-O RDFs, and the angle of ]C-H· · · O lies within the range of 120 − 180◦ . Fig. 5 presents site-site H5-OC/OB RDFs in varied IL matrices, as well as representative combined distribution functions (CDFs) of ]CR-H5· · · OC/OB vs. rH5···OC/OB in [BMIM][BOB] and [DMIM][BOB] ILs. The strong coordination pattern is registered at a domain with radial distance rH5···OC of 0.21 nm and with angle ]CR-H5· · · OC of 110◦ -170◦ , corresponding to a preferential H5-OC HB coordination. The other three noticeable peaks in H5-OC RDF plots are resolved at 0.48, 0.58 and 0.82 nm, respectively, which attribute to weak interactions of H5 atoms with other three OC atoms in the same anionic framework. Accordingly, there are four peaks in H5-OB RDF plots in Fig. 5D, which correspond to striking coordination of H5 atoms with four OB atoms in neighboring [BOB] anions. The H5 and OC atoms are the most active atoms in imidazolium cations and [BOB] anions, respectively, and thus present decent HB characteristics. The H5-OB coordination is considerably suppressed due to a distinct local chemical environment of OB atoms in anionic framework, and thus the first peak intensities in H5-OB RDFs are overwhelmingly weaker than those in H5-OC RDFs. Additionally, the radial distances of the last three peaks in H5-OB RDFs are shifted to shorter distances due to the relative distribution of four OB atoms in comparison with

16


Page 17 of 50

180

8

Angles of CR-H5---OC/OB (degree)

[BMIM] [HMIM]

4

[OMIM] [DMIM] [DdMIM]

2 0

A

3 2 1 0 0.0

4.0E-02

150

[EMIM]

6 Radial distribution functions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120

3.0E-02

90

2.0E-02

60

1.0E-02

30 180

0.0E+00

C

B

2.0E-02

150 120

1.5E-02

90

1.0E-02

60

5.0E-03

30 D

E

0.2

0.4

0.6

0.8

H5-OC/OB distance (nm)

1.0

0 0.2

0.4

0.6

F

0.8 0.2

0.0E+00

0.4

0.6

0.8

H5-OC/OB distance (nm)

Figure 5: Radial distribution functions for H5 atoms in imidazolium cations and OC/OB atoms in [BOB] anions in varied IL matrices, and representative combined distribution functions of ]CR-H5· · · OC/OB vs. rH5···OC/OB in [BMIM][BOB] and [DMIM][BOB] ILs. (A) H5-OC RDFs; (B) ]CR-H5· · · OC vs. rH5···OC in [BMIM][BOB] IL; (C) ]CR-H5· · · OC vs. rH5···OC in [DMIM][BOB] IL; (D) H5-OB RDFs; (E) ]CR-H5· · · OB vs. rH5···OB in [BMIM][BOB] IL; and (F) ]CR-H5· · · OB vs. rH5···OB in [DMIM][BOB] IL. that of four OC atoms in [BOB] anionic framework. These observations are corroborated by representative CDF patterns of ]CR-H5· · · OC/OB vs. rH5···OC/OB in [BMIM][BOB] and [DMIM][BOB] IL matrices as shown in Fig. 5. These computational results indicate that H5-OC coordination is the primary HB interaction in IL matrices and dominates local HB characteristics, and H5-OB association delivers a secondary level of stabilization. The lengthening aliphatic chains in imidazolium cations leads to a synergistic increase in H5-OC and H5-OB RDFs, indicating an enhanced coordination of H5 atoms with both OC and OB atoms in neighboring [BOB] anions. As discussed in Subsection III B, the liquid morphologies are gradually transformed from segregated apolar clusters (domains) within polar framework to that characterized by sponge-like interpenetrating networks in IL matrices with lengthening aliphatic chains in imidazolium cations. The permeation of apolar domains into polar network leads to a constrained distribution of [BOB] anions around imidazolium ring planes, which promotes the preferential HB interactions of H5 atoms with

17



[BOB] anions via both OC and OB atoms. By visual inspection of simulation trajectories, it can be identified that H5 atoms are mainly positioned around OC atoms in equatorial region of oxalato ring planes, contributing to primary and directional HB interactions between imidazolium cations and [BOB] anions. In addition, these H5 atoms are also localized around the central tetrahedron domains of [BOB] anions for convenience of coordinating OB atoms with secondary HB characteristics. 47 It is noteworthy that two oxygen atom types in [BOB] anions can accept not only H5 atoms, but also H4 and H1 atoms due to intrinsic ionic structures of [BOB] anions and imidazolium cations containing multiple HB acceptors and donors, respectively. The HB coordination magnitude of these hydrogen atoms with [BOB] anions via either OC or OB atoms follows an order of H5 > H4 > H1. The spatial coordinations of OC and OB atoms with H4 and H1 atoms resemble their counterparts with H5 atoms but with decreased intensities in RDF plots and wider angular distributions in ]C-H· · · OC/OB. These computational results suggest that both H4 and H1 atoms in imidazolium cations display diminished coordinations with oxygen atoms in [BOB] anions in stabilizing local ionic structures. The existence of multiple HB donors (one H5, two H4, and five H1 atoms) in imidazolium cations and multiple HB acceptors (four OC and four OB atoms) in [BOB] anions contributes to the hydrogen bonds being distorted from linear ideal HB characteristics, and sometimes being bifurcated and chelated with varied HB strengths. These HB coordinations within IL matrices, either primary or secondary, are highly connected, facilitating the formation of striking hydrogen bond network, which plays an important role in maintaining distinctive local ionic structures and stabilizing peculiar orientations of imidazolium ring planes to neighboring oxalato ring planes, and vice versa, which will be substantiated in detailed analysis in following subsections.

18


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D. Hydrogen bonding dynamics In present imidazolium bis(oxalato)borate IL systems, a strong initial hydrogen bond (such as the one formed between H5 atoms in imidazolium cations and OC atoms in [BOB] anions) can be maintained continuously over a long period of time, but at some point this hydrogen bond breaks due to the local motion of ions or new interactions with other ionic groups. 30 The close-contacted ionic species diffuse away from each other, and thus this strong hydrogen bond is not re-established. In the other case, a weak hydrogen bond (such as the one formed between H1 atoms in imidazolium cations and OB atoms in [BOB] anions) can rapidly break, but because of “caging” effect constraining local vibrational motion of hydrogen bond donors and acceptors, this hydrogen bond can reform again a short time later. This weak hydrogen bond can form intermittently multiple times before its final breaking permanently. 30 These two dynamical quantities are described by continuous and intermittent HB dynamics. 30,71 Correspondingly, the residence lifetimes for these HB dynamics are related to how long a HB acceptor (oxygen atoms in [BOB] anions) remains within a radial cutoff of a HB donor (hydrogen atoms in imidazolium cations) for the whole time interval, and how often a HB acceptor moves in and out of the cutoff region of a HB donor (the acceptor remains in the local domain over times), respectively. 30,35,70 In current work, we investigate the peculiar dependence of HB dynamics on aliphatic chain length in varied imidazolium bis(oxalato)borate IL matrices. Since the residence lifetimes are quite sensitive to short-time motion of HB sites, additional atomistic simulations were performed with an integration time step of 0.5 fs, and the other simulation parameters are the same as those described in Section II. The atomistic simulation trajectories for all imidazolium bis(oxalato)borate ILs were recorded at a time interval of 2 fs for 20 ns, which provides a very high time resolution for characterizing continuous and intermittent HB dynamics in imidazolium bis(oxalato)borate IL matrices. The HB dynamics are quantified using a pair residence lifetime correlation function C(t) '

hhij (0)hij (t)i , hhij (0)2 i

in which the

tagged pair population function hij is unity when a hydrogen bond is formed between atoms 19



1.0

A

0.8

0.6

Residence time correlation functions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 50

0.4

[EMIM]

0.2

[BMIM] [HMIM]

0.0 1.0

[OMIM]

B

[DMIM]

0.8

[DdMIM]

0.6

0.4

0.2 0

20

40

60

80

100

Time (picoseconds)

Figure 6: The computational continuous (A) and intermittent (B) correlation functions for H5-OC HB dynamics in varied imidazolium bis(oxalato)borate IL matrices at 323 K. i and j at time t = 0 and t = t with the adopted angle-distance criterion, and is zero otherwise. 30,35,70–73 The angular brackets denote an ensemble-averaged statistical analysis over all possible HB sites and multiple time origins. The computational continuous and intermittent correlation functions for H5-OC HB dynamics in varied imidazolium bis(oxalato)borate ILs C at 323 K are shown in Fig. 6. The continuous and intermittent HB residence lifetimes (τHB I and τHB ) are determined through a numerical integration of the corresponding correlation R t0 R∞ 0 C(t)dt functions as τHB = 0 C(t)dt = 1−C(t , and are listed in Table 2. 0)

In a given IL matrix, the decay of continuous correlation function for H5-OC HB dynamics is much faster than that of intermittent H5-OC HB dynamics. This observation is consistent with that obtained in ILs consisting of imidazolium cations coupled with Cl anions at elevated temperatures. 30,35 The lengthening aliphatic chains in imidazolium cations strengthens HB coordinations between hydrogen atoms in imidazolium cations and oxygen atoms in [BOB] anions, and thus the decay of the corresponding correlation functions for

20


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Table 2: The computational continuous and intermittent residence lifetimes (in picoseconds) for HB dynamics of three hydrogen atoms in imidazolium cations with oxygen atoms in [BOB] anions in varied imidazolium bis(oxalato)borate IL matrices at 323 K. ILs [EMIM][BOB] [BMIM][BOB] [HMIM][BOB] [OMIM][BOB] [DMIM][BOB] [DdMIM][BOB]

H5-OC C I τHB τHB 1.68 604.36 2.92 786.16 4.51 996.52 6.68 1254.57 9.65 1617.69 13.45 2162.51

C τHB 1.13 1.76 2.26 2.82 3.88 5.41

H4-OC I τHB 326.57 487.53 650.59 867.98 1025.69 1151.54

H1-OC C I τHB τHB 0.54 201.01 0.79 259.21 1.04 302.84 1.37 392.02 1.86 463.91 2.47 541.14

H5-OB C I τHB τHB 0.15 29.86 0.24 41.28 0.38 58.61 0.63 79.24 0.89 112.58 1.25 157.02

H4-OB C I τHB τHB 0.08 17.19 0.12 27.09 0.17 38.27 0.22 54.25 0.30 68.38 0.46 82.94

H1-OB C I τHB τHB 0.04 9.14 0.06 12.35 0.09 15.19 0.12 20.63 0.16 27.29 0.21 33.82

continuous and intermittent HB dynamics considerably slows down. For H5-OC HB interactions in [EMIM][BOB] IL matrix, the continuous and intermittent residence lifetimes are 1.68 and 604.36 ps, respectively, at 323 K. These residence lifetimes are much larger than those for HB dynamics of water (≈ 0.5 ps and 5–6 ps for continuous and intermittent residence times) and other simple molecular liquid, like methanol, ethanol, and propanol under ambient conditions. 74,75 Additionally, these lifetime constants are somewhat comparable with those for HB dynamics of H5 atoms in imidazolium cations with specific atoms in [EMIM]Cl, [EMIM][NTF2 ], and [EMIM][BF4 ] (tetrafluoroborate) ILs at elevated temperatures. 35 A similar HB dynamical feature is also observed in [BMIM][BOB] IL in comparison with atomistic computational results of [BMIM]Cl IL at 400 K, and of [BMIM][BF4 ] IL at a wide temperature range. 30,70 It it noteworthy that the asymptotic decay of these correlation functions for continuous and intermittent H5-OC HB dynamics can be approximated by stretched bi-exponential decay functions. This is distinct to the constrained motion of Cl anions around imidazolium cations in [EMIM]Cl and [BMIM]Cl IL matrices at elevated temperatures, which exhibit three decay components in correlation functions for varied H-Cl HB dynamics. 30 This observation may attribute to striking anionic structures of [BOB] compared with mono-atomic Cl anion in coordinating hydrogen atoms in imidazolium cations. 30,35,70 In the tri-mode decay of correlation functions for continuous H5-Cl HB dynamics in [EMIM]Cl and [BMIM]Cl IL matrices, the first two (fast) decay components exhibit similar timescales of picoseconds. It was

21



addressed that the librational motion of Cl anions, such as linear bond stretching vibration along C-H bond vectors, side-to-side and up-and-down angular motion round imidazolium ring planes, may contribute to the fast decay in continuous H5-Cl HB dynamics. 30 In current work, the [BOB] anion has multiple HB acceptors, which promote the formation of HB network and the constrained distribution of [BOB] anions in continuous ionic framework. For a lattice of “fixed” imidazolium cations, the re-orientational dynamics, either side-to-side or up-and-down angular motion, of [BOB] anions around imidazolium ring planes require more energy to break distinct ionic structures and hydrogen bond network, which is not favored in ionic matrices. Therefore, the fast decay of continuous and intermittent H5-OC HB dynamics in imidazolium bis(oxalato)borate IL matrices can be rationalized by a linear hydrogen bond stretching in-and-out “vibration” along C-H bond vectors in imidazolium cations. The third (slow) decay component for intermittent H5-Cl HB dynamics in [EMIM]Cl and [BMIM]Cl IL matrices presents a significant temperature dependent feature, and is given by ≈ 300 ps in [EMIM]Cl IL at 450.0 K and ≈ 6000 ps in [BMIM]Cl IL at 353.15 K, respectively. 30 Taking the temperature effect into account, these timescales are quite close to those for the slow decay of intermittent correlation functions for H5-OC HB dynamics, which is approximately of thousands of ps in varied imidazolium bis(oxalato)borate IL matrices at 323 K. In current work, the slow decay of these correlation functions is attributed to a large angular out-of-plane (both side-to-side and up-and-down) wagging motion of oxygen atoms in [BOB] anions around imidazolium ring planes. The continuous and intermittent HB residence lifetimes for three hydrogen atoms in imidazolium cations follow an order of H5 > H4 > H1, either in coordinating OC or OB atoms in [BOB] anions, which is consistent with their static HB structural characteristics. The lengthening aliphatic chains in imidazolium cations leads to a significant increase in continuous and intermittent HB residence lifetimes for all HB interactions, as shown in Table 2. The effect of aliphatic chain length on HB dynamics can be rationalized, on a global point of view, by significant changes in microscopic liquid morphologies of IL matrices as comprehensively

22


Page 22 of 50

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discussed in Subsection III B, and on a local point of view, by the formation of hydrogen bond network in IL matrices. All these structural properties contribute to constrained distributions of [BOB] anions around imidazolium ring planes, and in the meantime, lead to enhanced stability of HB structures (network) in varied imidazolium bis(oxalato)borate IL matrices with a gradual addition of methylene units to aliphatic chains in imidazolium cations.

E. π type stacking structures among imidazolium and oxalato ring planes In order to facilitate data analysis and discussion in following subsections, we define the relative distribution of neighboring ring planes, as shown in a schematic representation in Fig. 1 for imidazolium-oxalato ring planes. θ is the angle between two normal vectors n ¯ and m ¯ to center-of-mass (COM) of imidazolium and oxalato ring planes, and also the angle of these two ring planes. θ = 0◦ indicates that two ring planes under consideration take face-face π−π stacking structure or present displaced offset stacking distribution, and θ = 90◦ corresponds to a T-shaped edge-face perpendicular orientation motif, respectively. 35,38,76 The parallel displaced offset stacking structures have large dispersion and electrostatic energy contributions together with a moderate induction energy contribution, whereas the edge-face perpendicular molecular geometries have a large electrostatic energy contribution together with substantial dispersion and induction energy contributions, respectively. 32,36 These two molecular motifs have similar energies, and are the common arrangements of aromatic residues in the base stacking of DNA molecules and proteins. 35

Imidazolium-imidazolium ring planes Fig. 7 presents RDFs for COMs of imidazolium ring planes in varied imidzolium bis(oxalato)borate IL matrices. These RDF plots exhibit prepeaks at short distances (∼ 0.42 nm), preferential peaks at intermediate distances (∼ 0.72 nm), and distinct oscillations at large radial dis23


1.6

[EMIM] [BMIM]

Imidazolium-imidazolium COM RDFs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.4

[HMIM]

1.2

[DMIM]

[OMIM]

[DdMIM]

1.0 0.8 0.8

0.6 0.6

0.4

0.4

0.2 0.00.0

0.2 0.3

0.5

1.0

0.4

0.5

1.5

0.6

2.0

Imidazolium-imidazolium COM distance (nm)

Angles of imidazolium-imidazolium ring planes (degree)


Page 24 of 50

90 60 1.2E-02

30 90

B

A

8.0E-03

C

4.0E-03 60

0.0E+00

30 D

0 0.2

0.4

0.6

0.8

1.0

E

0.4

0.6

0.8

1.0

F

0.4

0.6

0.8

1.0

Imidazolium-imidazolium COM distance (nm)

Figure 7: Radial and combined distribution functions of imidazolium-imidazolium ring planes in varied imidazolium bis(oxalato)borate IL matrices at 323 K. (A) [EMIM]; (B) [BMIM]; (C) [HMIM]; (D) [OMIM]; (E) [DMIM]; and (F) [DdMIM]. tances (> 1.25 nm), respectively. These observations indicate complex liquid structures in IL matrices attributing to delicate trade-offs between strong and weak intermolecular interactions between imidazolium ring planes, and their spatial coordinations with oxalato ring planes in [BOB] anions. The detailed orientational distributions of imidazolium-imidazolium ring planes are manifested in combined angle-distance distribution functions in Fig. 7. In the prepeak region, the weak coordination patterns are mainly registered in a domain with imidazolium-imidazolium COM radial distance of 0.42 nm and with angle θ of approximately 25◦ . This distribution indicates the presence of parallel π type alignments of imidazolium ring planes, either π − π stacking conformations or displaced offset stacking structures depending on detailed HB structural analysis. 36,44,45,77 As described in Subsection III C, the H5 and H4/H1 atoms in imidazolium cations form primary and secondary hydrogen bonds with OC atoms in [BOB] anions. This microstructural characterization suggests that the equatorial region beyond C-H vectors in a central imidazolium ring plane is occupied by OC atoms, and thus the closest imidazolium ring planes have to occupy domains above and below the central imidazolium ring plane, leading to the formation of preferential π − π stacking motifs between neighboring imidazolium ring planes. 47 These stacking structures are consistent with those observed in other IL matrices consisting of imidazolium cations

24


Page 25 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


coupled with small anions. 32,36,44–46,77 It should be noted that these π − π stacking structures are not perfect, but with a tilted angle of approximately 25◦ , which overtakes repulsive electrostatic interactions and other weak intermolecular associations in determining the relative distribution of imidazolium-imidazolium ring planes. In this prepeak region, the RDF peak intensity for COMs of imidazolium-imidazolium ring planes in [OMIM][BOB] IL matrix is stronger than those in other imidazolium bis(oxalato)borate ILs, pointing to a higher degree of π − π stacking structures between imidazolium ring planes as corroborated by distinct coordination pattern shown in the CDF contour in Fig. 7D. A rationale of this observation may attribute to the effect of aliphatic chain length in imidazolium cations. The intermediate aliphatic chains may exhibit inter-digitation structures, which facilitate the alignment of imidazolium cations such that π type stacking distributions of imidazolium-imidazolium ring planes become relevant. For imidazolium cations with long aliphatic chains, it is speculated that aliphatic chains may be not fully interdigitated in apolar domains, presumably as a consequence of local and transient net non-null charge at interfacial region between polar and apolar domains and a strong steric hinderance effect induced by imidazolium and oxalato ring planes. In the intermediate distance range, the striking coordination patterns are registered in large radial distances (0.61 − 0.82 nm) and wide angle distributions (60 − 90◦ ). For typical radial distances between COMs of imidazolium ring planes, like 0.72 nm, there is enough space for a [BOB] anion to squeeze in between two imidazolium ring planes. The strong electrostatic interactions between imidazolium and oxalato ring planes and the peculiar distribution of two oxalato ring planes in [BOB] anionic framework tend to mediate the relative distribution of imidazolium ring planes being essentially perpendicular and taking edge-face orientation to neighboring imidazolium ring planes within the first solvation shell. As shown in representative CDF contours in Fig. 7, the magnitudes of these edge-face coordination patterns first decrease from [EMIM][BOB] to [HMIM][BOB] ILs, and then increase from [HMIM][BOB] IL onward, indicating that it is not only the introduced [BOB] anions be-

25



tween imidazolium ring planes, but also the hydrophobic alkyl units in cations that are involved in mediating the relative distribution of imidazolium-imidazolium ring planes. Going to larger radial distances, the pronounced oscillations in RDF plots indicate the existence of multiple configurational structures of imidazolium ring planes with complicated orientations. Additionally, it is difficult for a central imidazolium ring plane to “sense” the other ones owing to the fact that their intermolecular coordinations are partially or totally screened due to interruption of multiple ionic species in between. It is the delicate intermolecular coordinations among imidazolium and oxalato ring planes that lead to the imidazolium-imidazolium ring planes being characterized by peculiar tilted π − π stacking structures at short distances, essential perpendicular orientations at intermediate distances, and relatively mean distributions at large distances, respectively.

Imidazolium-oxalato ring planes Fig. 8 presents RDFs for COMs of imidazolium-oxalato ring planes, and the corresponding combined angle-distance distribution functions in varied imidazolium bis(oxalato)borate IL matrices at 323 K. In each RDF plot, there is a sharp peak registered at around 0.55 nm, which is accompanied by a preferential shoulder and a secondary peak at small and large radial distances, respectively. The lengthening aliphatic chains in imidazolium cations leads to decreased peak intensities for the sharp and the secondary peaks. Accordingly, the shoulder becomes less apparent and finally disappears in RDF plot for COMs of imidazolium-oxalato ring planes in [DdMIM][BOB] IL matrix. The shoulders shown at short distances (∼ 0.40 nm) correspond to the tilted distribution of imidazolium-oxalato ring planes with a relative angle of 40 ± 20◦ between two ring planes. Considering the formation of hydrogen bonds between hydrogen atoms in imidazolium cations and oxygen atoms in [BOB] anions, the close-contacted imidazolium and oxalato ring planes should be characterized by displaced offset stacking structures or tilted conformations. 47 The lengthening aliphatic chains in imidazolium cations tends to increase

26


Page 26 of 50

Page 27 of 50

3.0 Angles of imidazolium-oxalato ring planes (degree)

90

2.5 Imidazolium-oxalato COM RDFs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[EMIM] [BMIM] [HMIM]

2.0


1.5 1.0 0.5 0.00.0

0.5

1.0

1.5

2.0

Imidazolium-oxalato COM distance (nm)

60 1.2E-02

30 90

B

A

8.0E-03

C

4.0E-03 60

0.0E+00

30 D

0 0.2

0.4

0.6

0.8

1.0

E

0.4

0.6

0.8

1.0

F

0.4

0.6

0.8

1.0

Imidazolium-oxalato COM distance (nm)

Figure 8: Radial and combined distribution functions of imidazolium-oxalato ring planes in varied imidazolium bis(oxalato)borate IL matrices at 323 K. (A) [EMIM]; (B) [BMIM]; (C) [HMIM]; (D) [OMIM]; (E) [DMIM]; and (F) [DdMIM]. the preferred angles of imidazolium-oxalato ring planes from 35◦ in [EMIM][BOB] IL, to 45◦ in [BMIM][BOB] IL, and finally to 52◦ in [HMIM][BOB] and [OMIM][BOB] IL matrices. However, in [DdMIM][BOB] IL matrix, it is difficult to observe such tilted orientational distribution of imidazolium ring planes to neighboring oxalato ring planes. The sharp peaks in RDF plots for COMs of imidazolium-oxalato ring planes corresponds to striking tilted and perpendicular orientations with a wide angle distribution of 35 − 90◦ . It is noteworthy that the shoulders and the sharp peaks in RDF plots are essentially interconnected, as verified in the corresponding angle-distance CDF contours in Fig. 8. These two prominent distributions correspond to the coordination of imidazolium ring planes to the close-contacted oxalato ring planes, whereas the association of imidazolium ring planes to the other oxalato ring planes in the same [BOB] anionic framework is manifested in the second peaks registered at around 0.80 nm in RDF plots. But the magnitude of the latter coordination pattern is overwhelming weaker than that of the close-contacted imidazolium-oxalato ring planes, and thus serves as a secondary stabilization of microscopic ionic structures in local ionic environments. Additionally, it can be further addressed that anionic groups are indispensable in mediating the relative distributions of imidazolium ring planes as the formation of peculiar π − π stacking structures for imidazolium-imidazolium ring planes requires

27



the existence of anionic groups besides two imidazolium ring planes. Therefore, the imidazolium cations and the paired anionic groups should be regarded as an integration rather than separated ionic groups. The populations of these primary and secondary coordination patterns suffer progressive reduction with lengthening aliphatic chains in imidazolium cations, which is manifested in decreased peak intensities in RDF plots in Fig. 8. This striking feature is exactly opposite to prominent changes in HB structural quantities as shown in Fig. 5, indicating a distinct competitive structural interplay of HB and π type interactions between imidazolium and oxalato ring planes in imidazolium bis(oxalato)borate IL matrices. This competitive feature is essentially correlated with the gradual addition of methylene units to aliphatic chains in imidazolium cations, and the formation of heterogeneous ionic environments in imidazolium bis(oxalato)borate IL systems. In IL matrices with short aliphatic chains in imidazolium cations, like in [EMIM][BOB] IL matrix, the electrostatic interactions are much stronger than the dispersion interactions, and thus dominate the relative distributions of imidazolium and oxalato ring planes in polar domains. The radial distances between COMs of imidazolium and neighboring oxalato ring planes in the striking π-π stacking structures are shorter than those in the other imidazolium-oxalato coordinating structures. Therefore the population of π-π stacking imidazolium-oxalato ring plane structures is higher than that of the other association structures between imidazolium and oxalato ring planes. Additionally, the hydrogen atoms in imidazolium cations are coordinated with [BOB] anions via OC and OB atoms with varied HB magnitudes as we have discussed in Subsection III C. The gradual lengthening aliphatic chains in imidazolium cations leads to the progressive expansion of apolar domains in IL matrices and thereafter the permeation of apolar domains into polar network. Meanwhile, the polar network becomes more stretched, but, nevertheless, manages to preserve its continuity, and thus the polar network is characterized by (partially) interpenetrating slim ionic channels in IL matrices with long aliphatic chains in imidazolium cations, such as in the [DdMIM][BOB] IL system as corroborated by

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representative liquid morphology shown in Fig. 2F. In the constrained ionic channels, the close-contacted imidazolium and oxalato ring planes are mainly characterized by displaced offset stacking structures, which are stabilized by distinct HB interactions between hydrogen atoms in imidazolium cations and oxygen atoms in [BOB] anions. This can be rationalized by the fact that the displaced offset stacking structures have large COM radial distances between imidazolium and oxalato ring planes, which can preserve the connectivity of polar groups in ionic channels. Such a distinct transition of microscopic ionic structures from the favorable π-π stacking imidazolium-oxalato structures in [EMIM][BOB] IL matrix to the preferential displaced offset stacking imidazolium-oxalato structures stabilized by HB coordinations in [DdMIM][BOB] IL matrix attributes to the gradual lengthening aliphatic chains in imidazolium cations. From an energetic point of view, the competitive structural interplay of HB and π type interactions between imidazolium and oxalato ring planes in imidazolium bis(oxalato)borate IL matrices is intrinsically related to the competitive interactions between strong Coulombic coordinations between imidazolium and oxalato ring planes and preferential dispersion associations among hydrophobic alkyl units in imidazolium cations.

Intermolecular oxalato-oxalato ring planes The RDFs for COMs of intermolecular oxalato-oxalato ring planes are shown in Fig. 9. The first and second peaks, which correspond to two distinct coordination patterns between intermolecular oxalato ring planes, are essentially intensified with a gradual addition of methylene units to imidazolium cations. The detailed coordination patterns between intermolecular oxalato ring planes are manifested in the corresponding angle-distance CDF contours in Fig. 9. The first coordination pattern is registered at around 0.44 nm with dispersed angular distributions, and is bifurcated into two association types. In [EMIM][BOB] and [BMIM][BOB] IL matrices, these two association types are registered at θ ≈ 50 ± 10◦ and θ ≈ 80 − 90◦ , respectively. The lengthening aliphatic chains in imidazolium cations intensifies the second association type, as shown in panels C and D of Fig. 9 for [HMIM][BOB]

29



90 Angles of oxalato-oxalato ring planes (degree)

2.0 [EMIM] Intermolecular oxalato-oxalato COM RDFs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[BMIM] [HMIM]

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Oxalato-oxalato COM distance (nm)

Figure 9: Radial and combined distribution functions of intermolecular oxalato-oxalato ring planes in varied imidazolium bis(oxalato)borate IL matrices at 323 K. (A) [EMIM]; (B) [BMIM]; (C) [HMIM]; (D) [OMIM]; (E) [DMIM]; and (F) [DdMIM]. and [OMIM][BOB] ILs, respectively. The further addition of methylene units to imidazolium cations leads to a reverse effect. It is shown in CDF contour in Fig. 9F for [DdMIM][BOB] IL that the intermolecular oxalato ring planes take essential parallel orientations with somewhat tilted distributions. The second coordination pattern is registered at large radial distances (0.80 − 1.00 nm) with an angle distribution of θ ≈ 60 − 90◦ . These distinctive coordination patterns are rationalized by repulsive Coulombic interactions and steric hindrance effects between negatively charged oxalato ring planes, leading to their tilted orientations and somewhat perpendicular distributions in local ionic environments.

F. Re-orientational dynamics of imidazolium and oxalato ring planes In addition to π type stacking structures among imidazolium and oxalato ring planes, the re-orientational dynamics of these ring planes have been investigated by examining reorientations of representative vectors eˆ fixed in their respective molecular frameworks. 78–80 In present study, the imidazolium and oxalato ring planes are depicted by their respective normal vectors n ¯ and m, ¯ as shown in Fig. 1. The aliphatic chains in imidazolium cations are described by an unitary vector connecting two terminal carbon atoms (one is labelled as C2 and the other one is labelled as CT in Fig. 1). The correlation function, represented 30


1.0

[EMIM]

0.8

functions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reorientational correlation

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Figure 10: Re-orientational correlation functions of aliphatic chains (A) and imidazolium ring planes (B) in cations, and of oxalato ring planes (C) in [BOB] anions in varied imidazolium bis(oxalato)borate IL matrices obtained from current atomistic simulations at 323 K. by the first rank Legendre polynomial of P1 (t) = hˆ e(0) · eˆ(t)i, is adopted to characterize re-orientational dynamics of imidazolium and oxalato ring planes in ionic environments, and of aliphatic chains in apolar domains. Fig. 10 presents the re-orientational correlation functions of aliphatic chains and imidazolium ring planes in cations, and of oxalato ring planes in [BOB] anions in varied imidazolium bis(oxalato)borate IL matrices at 323 K. In a given IL matrix, the decay of reorientational correlation functions for three ionic groups is described by an order of aliphatic chains > imidazolium ring planes ≈ oxalato ring planes, which is intrinsically related to their distinct spatial distributions in heterogeneous IL matrices. A gradual lengthening aliphatic chains in imidazolium cations leads to the liquid morphologies being transitioned from dispersed apolar clusters in a continuous ionic framework in [EMIM][BOB] IL to that characterized by bi-continuous sponge-like interpenetrating polar and apolar networks in [DdMIM][BOB] IL sample. The aggregation of alkyl units in apolar domains provides an effective solvophobic environment for their re-orientational motion in hydrophobic cavities. The imidazolium and oxalato ring planes are strongly coupled together in ionic domains through decisive Coulombic interactions, preferential HB and delicate π type coordinations, which contribute to their comparable re-orientational dynamics in varied IL matrices, as shown in panels B and C of Fig. 10, respectively. It is interesting to notice that the asymptotic decay of these re-orientational correlation functions can be approximated by a stretched bi-exponential decay function with the 31


Imidazolium ring planes

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Reorientational correlation


9 8 7 6 5 4 3 2 1

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No. of methyl units in aliphatic chains in imidazolium cations

Figure 11: The fitting parameters τ1 (A) and τ2 (B), and the integrated re-orientational correlation times τ (C) of aliphatic chains and imidazolium ring planes in cations, and of oxalato ring planes in [BOB] anions in varied imidazolium bis(oxalato)borate IL matrices at 323 K. form of C(t) = c0 + c1 e−t/τ1 + c2 e−t/τ2 . The re-orientational correlation times for aliphatic chains and imidazolium ring planes in cations, and for oxalato ring planes in [BOB] anions, are determined by numerical integration of their respective correlation functions using R t0 R∞ 0 C(t)dt . The accessible fitting parameters τ1 and τ2 , and the correspondτ = 0 C(t)dt = 1−C(t 0) ing integrated correlation times τ of three ionic groups in imidazolium bis(oxalato)borate IL matrices are present in Fig. 11. In each panel, the re-orientational correlation times of imidazolium ring planes are comparable with those of oxalato ring planes, both of which are much longer than re-orientational dynamics of aliphatic chains in apolar domains. Given that aliphatic chains and imidazolium ring planes are covalently bonded within cationic species, their distinct re-orientational features indicate that the overall re-orientational dynamics of imidazolium cations are heterogeneous and are influenced by dynamical associations of polar and apolar moieties. The polar moieties (imidazolium ring planes) liberate in local ion cages in polar domains, while the apolar moieties (aliphatic chains) freely swing in hydrophobic domains in heterogeneous IL matrices. In addition, the re-orientational dynamics of apolar domains are much faster than their polar counterparts. Such a dynamical discrepancy is even more distinct with lengthening aliphatic chains in imidazolium cations. The re-orientational dynamical heterogeneities, embedded both in cationic frameworks and in the entire simulation systems, indicate that polar domains are actually difficult to rotate because of strong cation-anion 32


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association caging effect; in contrast, the apolar region owns a relatively free re-orientational motion. This observation is consistent with the computational results observed in other bulk ILs and IL mixtures. 12,70 The re-orientational correlation times, either the fitting parameters (τ1 and τ2 ) to computational correlation functions or the integrated timescales of these correlation functions, exhibit a substantial increase with lengthening aliphatic chains in imidazolium cations. This attributes to the distinctive changes in microstructures and the variations of liquid morphologies of these imidazolium bis(oxalato)borate ILs as aliphatic chains increase in cations. Additionally, the integrated re-orientational correlation times for imidazolium and oxalato ring planes are much larger than those for various HB dynamics as listed in Table 2. This is expected as the hydrogen bonds formed between hydrogen atoms in imidazolium cations and oxygen atoms in [BOB] anions should be broken before imidazolium and oxalato ring planes achieving their re-orientational randomization in ionic domains. In all imidazolium bis(oxalato)borate IL matrices, the dependence of re-orientational dynamics of imidazolium and oxalato ring planes on aliphatic chain length in cations is consistent with that for HB dynamics as shown in Fig. 6 and Table 2, indicating a prevalent cooperative effect in dynamical quantities of HB and π type coordinations between imidazolium and oxalato ring planes.

G. Translational diffusivity of imidazolium and bis(oxalato)borate ions Having reconciled the competitive structural characteristics and cooperative dynamics of HB and π type interactions between imidazolium and oxalato ring planes, we seek to examine the translational diffusive properties of representative ionic groups in imidazolium bis(oxalato)borate IL matrices at 323 K. Quantitative description of translational dynamics of imidazolium cations and [BOB] anions is characterized in terms of their diffusion coefficients determined from mean square displacements. 52,53,68,71,73,80,81 The translational diffusion 33


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Tail carbon atoms in aliphatic chains

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Translational diffusion coefficients (*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

m /s)


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Imidazolium ring planes

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Imidazolium cations Oxalato ring planes

12 10 8 6 4 2 2

4

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No. of methyl units in aliphatic chains in imidazolium cations

Figure 12: The translational diffusion coefficients of terminal carbon atoms in aliphatic chains, aliphatic chains, imidazolium ring planes, and COMs of imidazolium cations, as well as oxalato ring planes in varied imidazolium bis(oxalato)borate IL matrices obtained from current atomistic simulations at 323 K. coefficients of terminal carbon atoms (labelled as CT in Fig. 1) in aliphatic chains, aliphatic chains, imidazolium ring planes, and COMs of imidazolium cations, as well as oxalato ring planes in [BOB] anions are shown in Fig. 12. The translational diffusion coefficients of representative ionic groups in imidazolium bis(oxalato)borate IL matrices at 323 K fall within an order of 10−10 m2 /s, which is a common magnitude for other ionic species at a similar temperature range. 12,37,52,53,68,71,80–83 The scarcity of experimental diffusion coefficient data of these imidazolium bis(oxalato)borate ILs in literature prevents a direct comparison with our computational results. Nevertheless, in present work we mainly focus on exploring the effect of aliphatic chain length in imidazolium cations on translational diffusivity of these representative ionic groups in heterogeneous imidazolium bis(oxalato)borate IL matrices. In each IL system, the translational diffusion coefficients of specific groups follow an order of terminal carbon atoms in aliphatic chains > aliphatic chains > imidazolium cations > imidazolium ring planes ≈ oxalato ring planes. The terminal carbon atoms in aliphatic

34


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chains diffuse much faster than the other ionic groups due to their librational motion in apolar domains. The imidazolium and oxalato ring planes present comparable mobilities, and have the slowest translational dynamics compared with the other ionic groups in all IL matrices. It is the fast diffusion of terminal carbon atoms in aliphatic chains, the intermediate displacement of aliphatic chains, and the relatively slow mobility of imidazolium ring planes that lead to the intermediate translational diffusivity of imidazolium cations in heterogeneous ionic environments. In [EMIM][BOB] IL matrix, the volumetric molecular size of apolar domains composed of methyl and ethyl groups that are directly bonded to imidazolium ring planes via two nitrogen atoms is smaller than that of polar region consisting of imidazolium ring planes and [BOB] anions. This discrepancy leads to the segregated distribution of small apolar clusters in polar framework, as shown in representative snapshot of [EMIM][BOB] IL in Fig. 2A. The addition of two methylene units to the [EMIM] cation leads to a considerable expansion of apolar clusters in the [BMIM][BOB] IL matrix. However, the volumetric size of polar domains is still larger than that of the apolar clusters, which is clearly corroborated by the enhanced peak intensities in static structural factors for polar-polar subcomponent as shown in Fig. 4B. This microstructural change in the [BMIM][BOB] IL matrix contributes to the compressed translational diffusivities and re-orientational dynamics of aliphatic chains, imidazolium and oxalato ring planes in apolar and polar domains in local ionic environments. The further progressive addition of methylene units to imidazolium cations enhances vdW interactions by means of aliphatic chain-ion inductive forces and hydrophobic interactions between alkyl groups. This leads to an additional expansion of apolar domains, which will permeate into polar network, and after a threshold, will in turn compress the spatial distributions of imidazolium and oxalato ring planes in polar domains. From [HMIM] onwards, these ionic groups can be sorted into three categories depending on their translational mobilities in heterogeneous ionic environments. The translational diffusivity of terminal carbon atoms in aliphatic chains increases with cationic molecular sizes owing to the fact that

35



these terminal carbon atoms are generally located in the central part of apolar domains and are surrounded by other alkyl groups, which provide an effective environment to promote the translational diffusivity of terminal carbon atoms in aliphatic chains in local hydrophobic cavities. The whole aliphatic chains in imidazolium cations exhibit a gradual increment in their translational mobilities, however, their re-orientational dynamics considerably slow down, as shown in Fig. 10A. The former can be attributed to the relative distribution of aliphatic chains in apolar domains, which facilitates their translational motions in heterogeneous ionic environments. The latter is rationalized by the fact that the aliphatic chains are bonded to imidazolium ring planes in cationic frameworks, which restrict their free re-orientational dynamics. The longer the aliphatic chains, the harder it is for them to take re-orientational randomization in IL matrices. The lengthening aliphatic chains in imidazolium cations has negligible effect on the translational diffusion of imidazolium and oxalato ring planes, as shown in Fig. 12, which is essentially correlated with their highly constrained distributions in ionic domains. The imidazolium and oxalato ring planes, either in preferential π type stacking configurations, in tilted structures or in peculiar edge-face conformations, are tightly coordinated together in polar domains through decisive electrostatic interactions, multiple HB and favorable π type coordinations. The subtle interplay of these preferential intermolecular coordinations contributes to comparable translational mobilities and re-orientational relaxations of imidazolium and oxalato ring planes in local ionic environments.

IV. Summary and Concluding Remarks Extensive atomistic simulations have been performed to investigate the effect of aliphatic chain length in imidazolium cations on liquid morphologies, delicate structural and dynamical interplay between HB interactions and π type stacking coordinations among imidazolium and oxalato ring planes, as well as translational and re-orientational dynamics of represen-

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tative ionic groups in varied imidazolium bis(oxalato)borate IL matrices. With a gradual addition of methylene units to aliphatic chains in imidazolium cations, the liquid morphologies of these imidazolium bis(oxalato)borate ILs are transformed from segregated apolar clusters (domains) in polar framework consisting of ionic channels to bi-continuous spongelike arrangements with interpenetrating polar and apolar networks. The microstructural changes in polar and apolar domain alternations and the evolution of liquid morphologies of imidazolium bis(oxalato)borate ILs matrices are qualitatively characterized by total and partial X-ray scattering static structural factors. Preferential hydrogen bonds are formed between hydrogen atoms in imidazolium cations and oxygen atoms in [BOB] anions with varied HB characteristics. A gradual lengthening aliphatic chains in imidazolium cations leads to a synergistic increase in HB strengths for primary and secondary hydrogen bonds. The correlation functions for continuous and intermittent HB dynamics are asymptotic and can be approximated by double exponential decay functions. The decay of correlation functions for continuous HB dynamics is much faster than that for intermittent HB dynamics, both of which considerably slow down in IL matrices with lengthening aliphatic chains in imidazolium cations, and thus the corresponding HB residence lifetimes are significantly increased. Imidazolium ring planes take peculiar tilted π − π stacking structures at short distances, and exhibit complicated orientational distributions at intermediate and large radial distances due to interruption of ionic species in between. Intermolecular interactions between oxalato ring planes are balanced by repulsive Coulombic interactions and steric hindrance effects, leading to their tilted orientations and somewhat perpendicular distributions in coordinating neighboring imidazolium cations. The lengthening aliphatic chains in imidazolium cations leads to considerably enhanced coordinations for imidazolium-imidazolium ring planes and complicated variations in intermolecular oxalato-oxalato ring planes, respectively. The spatial orientations of imidazolium to neighboring oxalato ring planes are characterized by displaced offset stacking structures or tilted conformations at short distances, and by dis-

37



tinctive perpendicular orientations at intermediate distances with a wide angle distribution, respectively. The former is stabilized by directional in-plane HB interactions between hydrogen atoms in imidazolium cations and oxygen atoms in [BOB] anions, while the latter is dominated by strong Coulombic attractions between imidazolium and oxalato ring planes, respectively. The population of these striking coordination patterns for imidazolium-oxalato ring planes is significantly decreased as aliphatic chains increase from [EMIM] to [DdMIM] cations, which is exactly an opposite tendency of intensifying HB coordinations between imidazolium cations and [BOB] anions. Such a distinct change indicates a competitive interplay of HB and π type structural coordinations between imidazolium and oxalato pair planes. The translational and re-orientational dynamics of imidazolium cations are highly heterogeneous originating from distinct microstructural heterogeneities in IL matrices. The polar moieties (imidazolium ring planes) are strongly associated with oxalato ring planes in [BOB] anions through distinctive Coulombic interactions, preferential HB and striking π type coordinations in ionic environments, and the apolar moieties (aliphatic chains) are distributed in apolar domains which provide an effective solvophobic environments for their free swing and re-orientation in hydrophobic cavities, respectively. The aliphatic chains exhibit larger translational diffusion coefficients, and thus present smaller re-orientational correlation times than those for imidazolium ring planes in the same cationic framework. The translational diffusion coefficients and re-orientational correlation times of imidazolium ring planes are comparable with those for oxalato ring planes. The lengthening aliphatic chains in imidazolium cations leads to substantial changes in translational and re-orientational dynamics of representative ionic groups in imidazolium bis(oxalato)borate IL matrices, which is consistent with those for continuous and intermittent HB dynamics. This observation indicates a prevalent cooperative effect in dynamical quantities of HB and π type coordinations between imidazolium and oxalato ring planes. The competitive structural trade-off and cooperative dynamical interplay of HB and π type interactions between imidazolium cations and [BOB] anions are intrinsically correlated with short-range collective interactions

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between alkyl units in imidazolium cations and long-range Coulombic interactions between imidazolium and oxalato ring planes in heterogeneous ionic environments. These computational results may provide striking physical insights in understanding preferential HB and distinct π type stacking coordinations between ionic species bearing multiple HB sites and heteroaromatic ring planes, which will be helpful for investigating solute-based dynamics of these ionic species and understanding their functional performance in large-scale industrial applications, for example, as lubricant additives in tribology, and as alternative solvents and electrolytes in electrochemical devices.

Acknowledgement Y.-L. Wang gratefully acknowledges the financial support from the Knut and Alice Wallenberg Foundation (KAW 2015.0417). All atomistic simulations were performed using computational resources in Sherlock Cluster in Stanford University.

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Competitive Microstructures vs. Cooperative Dynamics of Hydrogen

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