Polarization Effects in Binary - ACS Publications - American Chemical

Subscriber access provided by - Access paid by the | UCSB Libraries

B: Liquids; Chemical and Dynamical Processes in Solution +

-4

Polarization Effects in Binary [BMIM][BF ] / 1,2Dichloroethane, Acetone, Acetonitrile, and Water Electrolytes Jesse Gatten McDaniel J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01714 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36 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

The Journal of Physical Chemistry

Polarization Effects in Binary [BMIM+][BF− 4] / 1,2-Dichloroethane, Acetone, Acetonitrile, and Water Electrolytes. Jesse G. McDaniel∗ School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 E-mail: [email protected] Phone: +1 (404) 894-0594

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract Organic electrolytes are unique in that their constituent solvents may be of much lower dielectric strength than water. This is because the ions of organic electrolytes are often those that comprise room-temperature ionic liquids, which are surprisingly miscible with many different organic solvents of low dielectric strength. Strong ion correlation results from the relatively low dielectric screening of the solvent, resulting in properties that can substantially deviate from Debye-Huckel descriptions; in addition remains the fundamental question of why the ionic liquids and low dielectric solvents are even miscible in the first place. In this work, we study electrolyte mixtures composed of the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM+ ][BF− 4 ] mixed with four solvents of systematically varying dielectric strength, 1,2-dichloroethane, acetone, acetonitrile, and water. We show that miscibility with the lowest dielectric solvent, dichloroethane, is directly attributed to ion solvation enhancement mediated by both electronic and conformational polarization of the solvent molecules. A strong dielectric enhancement results from the ∼ 70-80 % increase in solvent dipole moments at high ion content, providing significantly better solvation than predicted by the bulk solvent dielectric strength. This implies a general mechanism for miscibility of ionic liquids with low dielectric solvents, through which ions effectly create their own solvation dipoles by polarizing the local environment. From a purely computational perspective, our results imply that explicitly polarizable force fields are essential for modeling many of the organic electrolytes that are used in electrochemical applications.

1

Introduction

Electrolytes are a fundamental component of all electrochemical systems including electrochemical energy storage devices. While aqueous electrolytes 1,2 and solid-state electrolytes 3–6 have important applications, organic liquid electrolytes have become increasingly widespread due to the renaissance of room-temperature ionic liquids 7–10 in the past twenty years. Be2


Page 2 of 36

Page 3 of 36 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


cause ionic liquids are tunable and miscible with a wide variety of solvents, 11–18 such organic electrolytes can be specifically tailored for particular technologies such as batteries 19 and supercapacitors, 20–23 which is expected to be essential for meeting increasing energy storage demands. 24 An intrinsic determiner of the electrolyte properties is the dielectric strength of the organic solvent, which can span a wide range; for example ∼ 5 (chloroform 13 ) to ∼ 65 (propylene carbonate 11 ). The low-dielectric solvent limit poses particular difficulty for theoretical treatment, as reduced screening results in substantial ion pairing and correlation, challenging the mean-field ansatz of Debye-Huckel and related theories. It is expected that electronic polarization of solvent molecules becomes increasingly important when organic ions are mixed with low-dielectric solvents. Although low-dielectric solvents are generally characterized by small molecular dipole moments (normalized by molecular volume), they can be highly polarizable, as polarizability is primarily determined by electron count and conjugation, and is unrelated to the static moments of the molecule. These solvents may thus exhibit significant dynamic polarization in response to strong local electric fields from solvated ions, and one cannot expect the ion screening to be solely mediated by the static dielectric constant of the bulk solvent. The importance of solvent polarization in such electrolytes is not only intuitive, but is also strongly suggested by experimental measurements of the enthalpies of solution of organic solutes in ionic liquids. 25,26 These experiments have found that solvation energies become more favorable for conjugated solutes, e.g. alkenes compared to alkanes, styrene compared to ethylbenzene, 25 or heavier solutes (more electrons), e.g. chloroform, 26 indicating that polarization (ion-induced dipole interactions) significantly enhances the interaction strength between ionic liquids and organic solutes. This physics will naturally carry over to organic electrolytes of varying ionic concentration. Characterizing polarization effects is not only important for the fundamental understanding of organic electrolytes, but is essential for computational modeling, as the explicit incorporation of polarization into molecular models is still often neglected. Such an evaluation of

3



polarization is particularly important for electrolyte mixtures, due to the lack of a unified computational framework. For example, while the inclusion of polarization in biophysics simulations 27 benefits from decades of experience in the development of corresponding nonpolarizable molecular models (e.g. AMBER, 28 CHARMM, 27,29 GROMOS 30 ), this is not true for organic electrolytes for which a multitude of unique force fields have been customized for specific systems. 31–33 These numerous force fields have been developed based on vastly different philosophies (i.e. empirical or ab initio), parametrized to different types of data, employ different functional forms, and may even significantly differ in physics (vide infra), and thus it is very difficult to infer systematic trends. Currently, the most extensive polarizable force field for modeling organic electrolytes is the APPLE&P force field developed by Borodin and coworkers. 34–40 For progress in computational modeling of organic electrolytes, therefore, it is thus very important to systematically understand polarization effects in these systems. From a very general viewpoint, one would expect that electronic polarization should enhance the mixing propensity of organic ions and organic solvent. This is because ioninduced dipole interactions will stabilize the mixed state, whereas polarization is expected to be less important for the unmixed, homogeneous systems. Indeed it has been shown that the polarization contribution to the cohesive energy of pure ionic liquids is surprisingly minor due to their homogeneous environment; 33,41 in heterogeneous mixtures, however, one expects the ions to be highly polarizing. Considering a qualitative description of mixing in terms of like interactions between ions (Uii ) and solvent (Uss ) and unlike interactions between ions and solvent (Uis ), one expects polarization to contribute to Uis more than Uii or Uss , and make Uis more attractive which will promote mixing. Any property of the electrolyte that depends on mixing propensity will thus be affected, including lower critical solution behavior (LCST), 14,15,42,43 ion pairing and phase segregation, 44–46 and ion structuring at interfaces. 40,47,48 Much of this extended discussion is beyond the scope of the present work, and we will limit our focus to the most fundamental characterization of the solvent dipolar

4


Page 4 of 36

Page 5 of 36 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


response and screening of the ions in such electrolyte mixtures. Furthermore, although we characterize aqueous electrolytes, this is mainly for comparative purposes, and it is notable that water/ionic liquid mixtures have many interesting properties that are deserving of a separate and focused discussion. 45,49–52 We also note that previous work has systematically evaluated the importance of explicit polarization for modeling neat ionic liquids, 39 but we avoid such a discussion as our present focus is electrolyte mixtures. In this work, we focus on polarization effects in organic electrolytes composed of solvents of varying dielectric strength. As the intent is to compare solvents of different dielectric strength rather than different organic ions, we focus primarily on the prototypical organic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM+ ][BF− 4 ] . The solvents that we study are 1,2-dichloroethane, acetone, acetonitrile, and water, which encompass a range of dielectric strength and molecular properties. To evaluate the ion screening of the different solvents, we compute the potential of mean force (PMF) of cation/anion association at dilute concentration. As will be discussed, this PMF quantifies the free energy for ionpairing relative to independently solvated, dissociated ions, and comparison for the different solvents is thus highly illuminating. We then extensively characterize the electronic response of the solvent to varying ion concentration by computing molecular dipole distributions in these different mixtures. As we employ explicitly polarizable force fields for the simulations, these distributions directly quantify the magnitude of solvent polarization as a function of ion concentration. We find that especially for lower dielectric solvents, the dipoles of solvent molecules are significantly enhanced with increasing ion content, due to strong ioninduced dipole interactions, an effect which is particularly dramatic for dichloroethane given its additional conformational flexibility. We employ both polarizable and non-polarizable force fields to understand the influence of this physics on organic electrolyte mixing. We additionally characterize so-called “scaledcharge” models, which employ ion charges less than unity, as these force fields are popular for simulations of pure, room-temperature ionic liquids. 31 In addition to physical insight, this

5



systematic comparison of different force fields reveals important criteria for accurate computational modeling of organic electrolytes. Due to the important contribution of polarization, we find that non-polarizable force fields qualitatively fail to predict mixing of ionic liquids with low dielectric solvents, predicting phase-separation instead! While scaled-charge models predict mixing of the electrolytes, this results from incorrect physics, namely reduction in ion association energy rather than the physical enhancement of ion solvation energy due to polarization. The consequence is that variation of ion association free energies (PMFs) in different dielectric solvents is not correctly captured by scaled-charge models. Our overarching conclusion is that explicit polarization is not just desirable for quantitative accuracy, but fundamentally essential for qualitative descriptions of electrolyte mixtures consisting of low-dielectric solvents.

2

Methods

We compare several different force field models to illustrate the influence of both solvent and ion polarization on the mixture properties. For the solvent, one cannot just simply “turn-off” polarization, as this will dramatically alter the system density; hence we must compare two distinct force fields. The two force fields we compare are the (non-polarizable) OPLS-AA model, 53 and the (polarizable) “SAPT-FF” model, which we and others have developed 54–56 and is parametrized based on symmetry-adapted perturbation theory (SAPT). 57,58 Both of these models provide a good description of liquid-state properties for the different sol33,59 vents, and include consistent parametrizations for the ionic liquid [BMIM+ ][BF− of 4] ;

particular importance, SAPT-FF predicts very accurate dielectric constants for the solvents (Supporting Information). To evaluate the influence of ion polarization, we turn-off the ion polarization in the SAPT-FF model (while keeping solvent polarization), which we refer to as “SAPT-FF-noionpol”; this is a meaningful comparison at dilute concentration, for which the system density is primarily determined by the solvent. The last force field we consider is

6


Page 6 of 36

Page 7 of 36 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


a scaled charge model, “OPLS-AA-0.84Q” which employs the OPLS-AA model but charges on cations and anions are scaled to 0.84 in magnitude as suggested by prior work. 60 We note that the SAPT-FF force field parameters for [BMIM+ ][BF− 4 ] and acetone are taken from previous work, 33,55 while acetonitrile and dichloroethane potentials are constructed in this work utilizing an analogous approach. 54 For water-based electrolytes, we employ a hybrid approach, in which all water-water interactions are modeled using the extensively benchmarked SWM4-NDP model, 61 and SAPT-FF parameters are used to describe water/[BMIM+ ][BF− 4 ] cross interactions (electrostatics is treated uniformly using static charges and polarizability of SWM4-NDP model). All new force field parameters developed in this work are given in the Supporting Information, along with thermodynamic benchmarks of neat solvent properties and additional ab initio calculations to verify accuracy of ion/solvent intermolecular interactions. All quantum calculations were performed with the Psi4 62 and Molpro 63 software packages. Molecular dynamics (MD) simulations are conducted using the GPU-accelerated OpenMM software, 64,65 and run on NVIDIA Tesla P100 GPU cards. Dual Langevin thermostats are utilized to maintain the nuclear degrees of freedom at 300K, and polarization (Drude oscillators) at ∼ 0 K, with timestep of 1 fs, friction coefficients of 1 ps−1 , and mass of 0.4 au assigned to the oscillators. A Monte Carlo barostat is used to maintain the pressure at 1bar, and the particle mesh Ewald (PME) method 66 is used for long-range electrostatics with van der Waals interactions cutoff at 1.4 nm. MD simulations are conducted for systematically concentrated electrolyte mixtures, and biased (umbrella-sampling) simulations are used to construct ion-pair PMFs at the dilute limit. Specific system sizes are mixtures of 1000 solvent molecules and 0, 20, 55, 110, and 220 ion pairs (ion mole fraction χ=0.0, 0.02, 0.05, 0.1, and 0.18 respectively) for the more dilute mixtures, and 220 ion pairs and 750, 550, 350, and 200 solvent molecules ( χ=0.23, 0.29, 0.39, and 0.52 respectively) for the more concentrated mixtures. For [BMIM+ ][BF− 4 ] /dichloroethane mixtures, we conduct additional high-concentration simulations of 220 ion pairs and 100, 50, and 20 solvent molecules

7



Page 8 of 36

( χ=0.69, 0.81, and 0.92 respectively). After equilibration, all simulations are run for at least 20 ns to ensure converged statistics. For the PMF calculations, a single cation and anion pair is placed in a box of 1000 solvent molecules, and umbrella sampling is utilized with harmonic potentials to constrain the cation/anion center-of-mass (COM) separation to distances 3 ˚ A < RCOM < 20 ˚ A employing windows of 0.3 ˚ A spacing and a 20 kJ/mol/˚ A2 force constant; simulations for each window are started from equilibrated configurations from the previous window. The weighted-histogram analysis method (WHAM) is then used to reverse-bias the center-of-mass distribution and construct the PMF. 67 Simulations with SAPT-AA and SAPT-AA-noionpol force fields are run for 1ns/window, while simulations with OPLS-AA and OPLS-AA-0.84Q are run for 3ns/window to compensate for slower dynamics and reduced sampling of non-polarizable force fields; 41 this gives a total of ∼ 57ns and 170ns of production time for polarizable and non-polarizable PMF simulations respectively, which we have verified provides sufficiently converged PMFs. We note that the periodicity imposed by the Ewald sum has been shown to have only a relatively minor effect on ion/ion PMFs. 68 The free energy quantity obtained from the umbrella sampling simulation (W(r)) is related to the PMF (w(r)) by 69

w(r) = W (r) + 2kB T ln(r)

(1)

where the 2kB T ln(r) entropic term is needed so as not to double count the spherical Jacobian in a volume integration (coordination number), as the Jacobian implicitly contributes to W(r). We do not discuss this subtlety further, as it does not affect the direct comparisons among different force fields.

3

Results

Much of the direction of this work is motivated by our initial observation that non-polarizable force fields fail to predict mixing of imidazolium-based ionic liquids and lower dielectric sol8


Page 9 of 36 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


vents, specifically acetone and dichloroethane. This is shown in Figures 1 and 2, which depict equilibrated simulation snapshots of [BMIM+ ][BF− 4 ] /dichloroethane and 1-ethyl3-methylimidazolium tetrafluoroborate ([EMIM+ ][BF− 4 ] )/acetone mixtures respectively at χ=0.18 ion fraction (∼ 40% ion volume fraction, 220 ion pairs and 1000 solvent molecules). We note that [EMIM+ ][BF− 4 ] was chosen to illustrate general trends, and we find simi− + lar results for [BMIM+ ][BF− 4 ] /acetone mixtures. Experimentally, both [EMIM ][BF4 ] and

[BMIM+ ][BF− 4 ] are fully miscible in both dichloroethane and acetone solvents, however simulations employing the non-polarizable OPLS-AA force field predict phase separation in these systems, as is clearly seen in Figures 1a and 2a. The fully-polarizable SAPT-FF force field correctly predicts mixing in these systems (Figures 1b and 2b); we thus conclude that mixing is strongly affected by electronic polarization, which will be further illustrated by ion association PMF calculations (Figure 3) as well as a systematic analysis of induced dipole moments of solvent molecules (Figures 6, 8). We note that the qualitative failure of OPLS-AA to predict mixing in the low dielectric solvents is not due to parametrization deficiencies but rather the fundamental neglect of explicit polarization; indeed similar physics was demonstrated by Jorgensen in the context of modeling dipolar solutes in low-dielectric solvents (dichloroethane). 70 We will show that for low-dielectric solvents, polarization enables dynamic ion solvation by local solvent dipole moments that are effectively much larger than predicted based on bulk solvent dielectric strength.

9



(a)

Page 10 of 36

(b)

Figure 1: Mixture of [BMIM+ ][BF− 4 ] /dichloroethane at χ=0.18 ion fraction as simulated by a) nonpolarizable OPLS-AA and b) polarizable SAPT-FF forcefields.

(a)

(b)

Figure 2: Mixture of [EMIM+ ][BF− 4 ] /acetone at χ=0.18 ion fraction as simulated by a) nonpolarizable OPLS-AA and b) polarizable SAPT-FF forcefields.

10


Page 11 of 36 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


The role of polarization omission in the artificial phase separations shown in Figures 1 and 2 is understood through analysis of ion-pairing free energies. This is because the propensity for ion/solvent mixing in the organic electrolytes is largely dependent on the magnitude of ion solvation energies relative to the cohesive energy of the neat ionic liquid. The cohesive energies of ionic liquids are ∼ 400-500 kJ/mol per ion pair (∼ 485 kJ/mol 33 for [BMIM+ ][BF− 4 ] ), which are roughly an order of magnitude larger than the cohesive

energies of organic solvents. Therefore ion association and ion solvation processes dominate the mixing energetics in the organic electrolyte. A fundamental descriptor of these relative energetics is the free energy of association of cation/anion pairs in the solvent at dilute concentration. This free energy (ion association PMF) dictates the propensity for ion-pairing, clustering, and mixing within the electrolytes. To analyze this physics, we calculate PMFs of ion association in the different dielectric solvents. In Figure 3, these PMFs are plotted as a function of BMIM+ and BF− 4 center-of-mass (RCOM ) separation distance in dichloroethane, acetone, acetonitrile, and water solvents, as computed with the four different force fields discussed in Section 2. All force fields (except OPLS-AA-0.84Q) predict the expected qualitative trend that the free energy of ion association decreases (becomes more favorable) with decreasing dielectric strength of the solvent. The solvent dielectric decreases in order water > acetonitrile > acetone > dichloroethane, and the propensity for ion pair association increases in order water < acetonitrile < acetone < dichloroethane, as lower dielectric solvents are less able to effectively solvate the isolated ions. Notably, the force fields exhibit significant differences in predicted ion-pair attraction free energies, the origin of which is predominantly due to their different treatments of electrostatics and polarization.

11


1

0

0

SAPT-FF

-1

-3

W(r) (kcal/mol)

1

acetone water dichloroethane acetonitrile

-2

4

6

8

10

12

14

16

Page 12 of 36

SAPT-FF-noionpol

-1


-2

18

-3

20

4

6

8

RCOM (Å)

0

0

W(r) (kcal/mol)

1

OPLS-AA acetone water dichloroethane acetonitrile

-2

-3

4

6

8

10

12

14

16

18

20

18

20

(b)

1

-1

10

RCOM (Å)

(a)

W(r) (kcal/mol)

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

W(r) (kcal/mol)


12

14

16

18

20

-1

OPLS-AA-0.84Q

-2


-3

4

6

RCOM (Å)

8

10

12

14

16

RCOM (Å)

(c)

(d)

Figure 3: Potential of mean force for separation of BMIM+ cation and BF− 4 anion in dichloroethane, acetone, acetonitrile, and water solvents predicted by force fields a) SAPTFF, b) SAPT-FF-noionpol, c) OPLS-AA, and d) OPLS-AA-0.84Q.

12


Page 13 of 36 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


We first discuss the qualitative characteristics of the PMFs, and subsequently discuss the effect of force field. As the separation distance of the ions increases from the ionpair association minimum, there are both energetic and entropic increases in free energy. The BF− 4 anion is forced farther away from its favorable interaction with the imidazolium ring, into a higher energy interaction either with the BMIM+ alkyl chain or a longer range interaction with the ring. The unfavorable entropic contribution comes from the greater excluded volume of the cation/anion pair, which persists until a separation distance at which a solvent molecule can fit between the two ions. This physics is schematically illustrated by snapshots from the [BMIM+ ][BF− 4 ] / acetone umbrella sampling simulation (SAPT-FF forcefield) for different RCOM values, shown in Figure 4. The most interesting separation distances are RCOM ∼ 7-10 ˚ A, at which the excluded volume of the ion pair increases to the point where a solvent molecule can fit between the two ions. The barrier of the PMFs in Figure 3 occurs near RCOM = 7-8 ˚ A, and the simulation snapshot at RCOM =7 ˚ A indicates that the ions are sufficiently close that no acetone molecule can fit between them (Figure 4), i.e. the effective dielectric mediating the ion-ion interaction is unity. As RCOM increases to 8-10 ˚ A, there is a minimum in the PMF (Figure 3) which corresponds to a solventmediated cation/anion interaction, as can be seen from the simulation snapshots (Figure 4); at these distances a single solvent molecule (acetone) fits between the separated ions, with its dipole aligned to favorably solvate the ions. Henceforth, we denote this minimum separation min−solvate . distance for which a solvent-mediated ion-ion interaction can occur as RCOM

13



(a)

(b)

Page 14 of 36

(c)

Figure 4: Simulation snapshots (SAPT-FF) from umbrella sampling of BMIM+ /BF− 4 ion ˚ ˚ association in acetone for center of mass constraint distance a) 7 A, b) 8 A, and c) 10 ˚ A. Coloring scheme is: Fluorine (green), oxygen (red), nitrogen (blue), carbon (silver), boron (pink), and hydrogen white. The background solvent molecules are colored tan. Based on the above discussion, solvent effects on the ion association PMF should be min−solvate separately evaluated for two distinct distance regimes. For RCOM < RCOM , there

is no dielectric screening of the cation-anion interaction, and the two solvent effects are dielectric stabilization of the ion-pair dipole and the entropic excluded volume cost (there is an additional energy cost of separating the ions that is independent of solvent and depends min−solvate on cation-anion interaction strength 33 ). For the regime, RCOM > RCOM , the ions are

separated by at least one solvent molecule, and so there is the additional solvent contribution to cation-anion dielectric screening. However, even for separations as large as RCOM =10 ˚ A, only a single solvent molecule can screen the ion-ion interaction (Figure 4), and thus a continuum dielectric model is inappropriate for the majority of the computed range of the PMFs. The significant quantitative differences between the PMFs predicted by SAPT-FF and OPLS-AA force fields explain the influence of polarization on the predicted mixing behavior of the concentrated electrolytes (Figures 1 and 2). The most striking difference is the variation in PMF minima for the different dielectric solvents; OPLS predicts minima that range from ∼ 0.5-2.0 kcal/mol ion-pair stabilization, while SAPT-FF predicts a smaller range from

14


Page 15 of 36 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


∼ 0.2-1.0 kcal/mol for the four different solvents. Clearly then, electronic polarization of solvent molecules reduces the variation in ion association free energies in the different solvents, effectively creating local dielectric environments that are more similar than predicted by the homogeneous bulk solvent dielectric strength. This physical interpretation is confirmed by explicitly computing the distribution of solvent induced dipole moments as a function of ion concentration (vide infra). The influence of polarization on the ion association PMFs (Figure 3) is more than just a quantitative effect, as it is responsible for the phase-behavior predictions in Figures 1 and 2. The effect of polarization of the ions themselves can be understood by comparing the PMFs predicted by SAPT-FF and SAPT-FF-noionpol (Figures 3a, 3b), i.e. simulations with and without polarization on the cation and anion. Interestingly, in contrast to the effect of solvent polarization, polarization of the ions increases the variation in PMF minima from ∼ 0.2-0.8 kcal/mol predicted by SAPT-FF-noionpol to the ∼ 0.2-1.0 kcal/mol range predicted by SAPT-FF for the different solvents. Ion polarization significantly increases the attraction of cation/anion pairs, 71 an effect that has been systematically demonstrated in the context of predicting heats of vaporization of neat room-temperature ionic liquids. 33,38 However, the relatively small difference in PMFs predicted by SAPT-FF and SAPT-FF-noionpol indicate that the enhancement of ion-pair attraction is largely offset by the additional polarization enhancement of isolated-ion solvation energies (i.e. at RCOM → ∞). We now discuss the effect of scaling the ion charges to less than unity values, i.e. comparing the predictions of the OPLS-AA and OPLS-AA-0.84Q force fields. Comparing Figures 3c and 3d, it is evident that the scaled-charge OPLS-AA-0.84Q force field reduces the variation in ion association free energies for the different solvents relative to OPLS-AA. This is because ion pairs bind more weakly and isolated ions are solvated more weakly with reduced charges, with the former effect dominating. Thus ion polarization (SAPT-FF/SAPT-FF-noionpol comparison) and ion scaled charges (OPLS-AA/OPLS-AA-0.84Q comparison) have opposite effects on the PMF minima, and the latter cannot be motivated by omission of the former in contrast to

15



previous claims based on simulations of neat ionic liquids 31,32,60 The consequence is that dielectric trends in the PMFs (Figure 3) are not correctly reproduced by OPLS-AA-0.84Q. We next investigate concentrated [BMIM+ ][BF− 4 ] / dichloroethane mixtures more thoroughly, as we have shown that polarization dramatically influences the phase behavior. For the remainder of the work, we focus solely on simulations conducted with the SAPT-FF force field; these explicitly incorporate electronic polarization of both solvent and ions. To preface this discussion, it is important to note that the dipole moment of dichloroethane changes significantly as the molecule converts between trans and gauche conformations; such a conformer population shift is known to provide electrostatic stabilization in the bulk solvent. 72,73 For [BMIM+ ][BF− 4 ] / dichloroethane mixtures, we therefore expect that the effective dielectric screening of the dichloroethane solvent is not constant, but rather is a function of ion concentration as solvated ions will both alter the gauche/trans conformational distribution and additionally induce larger dipole moments through electronic polarization. To explore this effect, we analyze MD simulations of 12 systematically chosen concentrations of [BMIM+ ][BF− 4 ] /dichloroethane mixtures ranging from pure dichloroethane (χ=0 ion pair mole fraction) to dilute dichloroethane in [BMIM+ ][BF− 4 ] (χ=0.92 ion pair mole fraction). In Figure 5, we plot the dichloroethane conformational distribution (defined by the φCl−C−C−Cl dihedral angle) for the 12 different electrolyte concentrations; interestingly, it is evident that the relative gauche/trans population is significantly altered with increasing ion content. For neat dichloroethane, SAPT-FF predicts nearly 50/50 gauche/trans fraction, in good agreement with both experiment and OPLS predictions. 73 However, with increasing ion content, the fraction of gauche conformers is significantly increased, driven by the enhanced dipoles of the gauche conformers which more effectively solvate the ions. Our simulations predict that the fraction of gauche conformers approaches ∼ 80-85 % for the most concentrated electrolytes studied (χ=0.8-0.9).

16


Page 16 of 36

Page 17 of 36

0.04 χ=0.0 χ=0.02 χ=0.05 χ=0.1 χ=0.18 χ=0.23 χ=0.29 χ=0.39 χ=0.52 χ=0.69 χ=0.81 χ=0.92

0.03

Population

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


0.02

0.01

0 0

100

50

150

φCl-C-C-Cl

Figure 5: Population of gauche and trans dichloroethane conformers as a function of [BMIM+ ][BF− 4 ] ion pair mole fraction, χ. Due to the symmetry, we plot the distribution as a function of the absolute value of the dihedral angle for the range 0 < φ < 180. In concentrated organic electrolytes, therefore, the screening of ions by solvent molecules can be significantly greater than predicted based on the pure solvent dielectric strength alone, due to both conformational and electronic polarization. For perspective, if dichloroethane were modeled as a freely-rotating molecule (no sterics), the dielectric strength of neat dichloroethane would be ∼ 70-80 due to the large dipoles of the fully eclipsed conformers, in comparison to the physical value of 10-11! While such a limit is never reached due to steric repulsion, the transfer in population from trans to gauche conformers with increasing ion concentration (Figure 5) clearly indicates the dynamic solvation ability of dichloroethane. For further insight, it is useful to partition the dipole moment enhancement into contributions from conformational flexibility and electronic polarization. In Figure 6, we plot the molecular dipole moment distribution of dichloroethane as a function of ion content; for clarity, we only show data for three representative mixtures, spanning χ=0 (neat dichloroethane), χ=0.23, and χ=0.92 ion pair mole fraction. These distributions are bimodal, reflecting the distinct gauche/trans dichloroethane populations (Figure 5), with smaller dipoles for trans conformations and significantly larger dipoles for gauche conformations. There are two significant changes in the dipole distribution with increasing ion content. First, as the gauche/trans fraction increases with ion concentration, the average dipole moment increases

17



due to population shift to larger dipole molecules; this is, in fact, the underlying driving force for the concentration-induced change in conformational population (Figure 5). The second significant change is the shift in the distribution peak positions to larger magnitude dipoles, which indicates larger induced dipole moments for all conformers with increasing ion content. The magnitude of the induced dipole moments due to polarization is quantified by comparing Figure 6a and Figure 6b, where Figure 6b is computed by setting the induced dipole moments on all dichloroethane molecules to zero (using the same trajectories). It is notable that the peak positions of the distributions in Figure 6b do not change with ion content, verifying that the concentration-dependent shifts in peak positions in Figure 6a are solely mediated by electronic polarization. Also noteworthy is the significant increase in peak widths of the dipole distributions due to the induced dipoles, as seen by comparing Figure 6a and Figure 6b. At high ion concentration (χ=0.92), the dipoles of gauche dichloroethane molecules span ∼ 3-6 Debye, which is a significantly larger variation than that found in neat dichloroethane! Much of this variation is due to the induced dipoles, as the corresponding distribution for static dipoles (Figure 6b) spans only ∼ 2.5-3.5 Debye, independent of ion concentration. 2 0.6

1.5

0.4

Probability

χ=0.0 χ=0.23 χ=0.92

0.5

Probability

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 18 of 36

0.3 0.2

χ=0.0 χ=0.23 χ=0.92

1

0.5

0.1 0 0

1

2

3

4

5

6

0 0

1

2

3

µ (Debye)

µ (Debye)

(a)

(b)

4

5

6

Figure 6: Molecular dipole moment distribution of dichloroethane molecules in varying ion mole fraction (χ) [BMIM+ ][BF− 4 ] /dichloroethane electrolytes. a) computed with electronic polarization, and b) same trajectories with µinduced set to zero.

18


Page 19 of 36 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


Figure 7: Polarization of BMIM+ /BF− 4 /dichloroethane solvent-separated ion pair. Isosurfaces of ±0.001 a.u. depict electron density enhancement (blue) and depletion (red) relative to the isolated molecule gas-phase wavefunctions (computed at PBE0/AVDZ). Color scheme is hydrogen (white), boron (pink), carbon (cyan), nitrogen (blue), fluorine (green), chlorine (yellow), background solvent molecules (tan), electron density isosurface (red/blue). The dipole moments of dichloroethane molecules in concentrated electrolytes are thus significantly enhanced by both electronic polarization and conformational flexibility (gauche/trans population). From integration of the distributions in Figure 6, we find that the average static dipole of dichloroethane increases from ∼ 1.7 Debye to ∼ 2.6 Debye due to the trans/gauche population shift going from χ=0 to χ=0.92 ion fraction mixtures. Including the contribution from induced dipoles, however, the average total dipole moment increases from ∼ 2.2 Debye to 3.8 Debye for the same range of low (χ=0) to high (χ=0.92) ion content mixtures. Thus the maximal dipole moment enhancement occurs for χ=0.92 [BMIM+ ][BF− 4] /dichloroethane mixtures, for which there is a ∼ 1.2 Debye induced dipole contribution from polarization, in addition to the 0.9 Debye enhancement from gauche/trans population shift! In Figure 7, we show a visual depiction of this strong polarization effect for a BMIM+ /BF− 4 /dichloroethane solvent-separated ion pair in solution; this figure shows the electron density enhancement (blue) and depletion (red) relative to the isolated molecule electron densities. The dichloroethane molecule screening the counterion interaction is in 19



the gauche conformation, and has significantly polarized electron density away from the anion towards the cation. This physical picture explains the significant shift in conformer population (Figure 5) and enhancement in dipole moments (Figure 6) as a greater fraction of dichloroethane molecules mediate counterion screening with increasing electrolyte ion content. In these electrolytes, it is important to recognize the coupling between polarization and conformational flexibility such that these mechanisms amplify each other, i.e. induced dipoles from polarization further promote population shift from trans to gauche, and vice-versa. We conduct a similar analysis for [BMIM+ ][BF− 4 ] electrolyte mixtures composed of acetone, acetonitrile, and water solvents. In Figure 8, we show the average dipole moment of solvent molecules for these different electrolyte mixtures; we note that a smaller concentration range was studied for the latter three solvents. For consistency we report the concentration dependence of both total dipoles and static dipoles for all solvents, however acetone, acetonitrile, and water are essentially rigid molecules and do not exhibit conformational polarization. It is clear that the largest change in solvent dipole moment with varying ion content occurs for dichloroethane electrolytes: Acetone molecules exhibit a smaller increase in dipole moment with increasing ion content; acetonitrile molecules have nearly constant average dipoles over the concentration range; and interestingly, the dipole moment of water molecules decreases with increasing ion concentration. This range of behaviors is due to the different static dipole moments and extent of self-polarization of the bulk solvents; for example, acetonitrile and water have larger static dipole moments (per molecular volume) and thus are significantly self-polarizing; for acetonitrile there is thus no significant change in polarization when the organic ion content is altered. Water is an extreme case, in that it is so self-polarizing that the solvent dipole moments actually decrease with increasing organic ion content! This is explained by the strong electrostatic interactions within the structured hydrogen bonding network of liquid water, which is apparently more polarizing than the relatively bulky organic ions with delocalized charge. This systematic comparison confirms

20


Page 20 of 36

Page 21 of 36

the necessity of explicit polarization for correctly modeling electrolytic mixtures that span a range of dielectric strength. 4.5

4

4 µpol µnopol

3

(Debye)

(Debye)

3.5

2.5 2 1.5 1 0

µpol µnopol

3.5 3 2.5 2

0.2

0.4

0.6

0.8

1.5 0

1

0.2

0.4

χ

(a)

0.6

0.8

1

3

5

2.5

(Debye)

µpol µnopol

4.5 4 3.5 3 2.5 0

χ

(b)

5.5

(Debye)

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


2 1.5

µpol µnopol

1 0.5

0.2

0.4

χ

0.6

0.8

1

0 0

0.2

(c)

0.4

χ

0.6

0.8

1

(d)

Figure 8: Average dipole moments of solvent molecules for varying ion mole fraction of [BMIM+ ][BF− 4 ] /solvent mixtures for a) dichloroethane, b) acetone, c) acetonitrile, and d) water solvent. Black symbols (µpol ) are with electronic polarization, and red symbols (µnopol ) are computed with µinduced set to zero.

4

Conclusions

Organic solvents and ionic liquids exhibit extremely different properties because of the large difference in their liquid cohesive energies; organic electrolytes are thus fascinating systems, as they span this full property range. The “like-desolves-like” rule that one learns in introductory chemistry seems at odds with the empirical fact that ionic liquids are miscible 21



with many organic solvents including those with relatively low dielectric constants. We have shown that electronic and conformational polarization plays an essential role in this regard, as it stabilizes the solvation of isolated ions and ion-pairs. Organic solvent molecules are generally highly polarizable, and dissolved ions induce significant dipole moments in their local coordination shells. In [BMIM+ ][BF− 4 ] /dichloroethane mixtures, the average solvent dipole moments are enhanced by up to ∼ 2.1 Debye at high ion content due to both electronic polarization and conformational changes of the molecules. Equally important is the variation in dipole moments induced by the ions, ∼ 3-6 Debye for gauche dichloroethane conformers, indicating a dynamic solvation environment which stabilizes the mixed state. We have demonstrated that the propensity for organic ion/solvent mixing is fundamentally correlated with ion pair association strengths, as quantified by the PMF for cation/anion separation at infinite dilution. We find that the polarization of solvent molecules and the polarization of ion pairs have contrasting effects on ion pair association, with the former being much more significant. Solvent polarization enhances the local dielectric strength relative to the homogeneous bulk value, enhancing the solvation of ions and reducing variation among solvent type. An important consequence is that non-polarizable force fields qualitatively fail to predict mixing of ionic liquids with low dielectric solvents, predicting phase-separation instead! The polarization of the ions themselves enhances ion pair association, with this physics analogous to the reduction in heats of vaporization in neat ionic liquids and more pronounced for low-dielectric solvents. While scaled-charge models also predict lower ion association constants compared to non-polarizable models, this is due to unphysical and artificial reduction of ion pairing energies, and results in an incorrect description of variation with solvent dielectric strength. Based on the results and discussion presented in this work, we suggest that explicitly polarizable force fields are essential for accurate computer simulations of organic electrolytes, especially those composed of low-dielectric solvents. Fortunately, new algorithms, software development, and computer hardware now enable routine application of polarizable force

22


Page 22 of 36

Page 23 of 36 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


fields in molecular dynamics simulations. For example, extended Lagrangian schemes employing thermal Drude oscillators greatly enhance simulation efficiency compared to a selfconsistent treatment of polarization, 61,74,75 with added efficiency from graphics processing unit (GPU) code acceleration as implemented in high-performance MD software such as OpenMM, 64,65 GROMACS, 76,77 NAMD, 78,79 and LAMMPS. 80,81 Because of these important and powerful recent advancements, we believe that the development of highly-accurate polarizable force fields for organic electrolyte mixtures is an important research endeavor, examples being the APPLE&P force field of Borodin and coworkers, 34–40 the extension of the AMOEBA force field to ionic liquids and related systems, 82–86 continued development of SAPT-FF by us and others, 33,54–56,71 and other additional efforts. 32,87–97

5

Acknowledgments

This research was supported in part through research cyberinfrastructure resources and services provided by the Partnership for an Advanced Computing Environment (PACE) at the Georgia Institute of Technology, Atlanta, Georgia, USA.

6

Supporting Information

Benchmark of SAPT-FF predicted properties for acetone, acetonitrile, and 1,2-dichloroethane bulk solvents; description of SAPT-FF functional form and all new parameters developed in this work; ab initio energy benchmarks for SAPT-FF description of acetonitrile dimer, dichloroethane dimer, water dimer, acetone/BF4 dimer, and acetone/EMIM dimer interactions; torsional PES of 1,2-dichloroethane.

23



References (1) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Gostick, J. T.; Qinghua, L. Redox Flow Batteries: A Review. J. Appl. Electrochem. 2011, (2) Dmello, R.; Milshtein, J. D.; Brushett, F. R.; Smith, K. C. Cost-Driven Materials Selection Criteria for Redox Flow Battery Electrolytes. J. Power Sources 2016, 330, 261–272. (3) Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nanocomposite Polymer Electrolytes for Lithium Batteries. Nature 1998, 394, 456–458, 10.1038/28818. (4) Ong, S. P.; Chevrier, V. L.; Hautier, G.; Jain, A.; Moore, C.; Kim, S.; Ma, X.; Ceder, G. Voltage, Stability and Diffusion Barrier Differences between Sodium-Ion and LithiumIon Intercalation Materials. Energy Environ. Sci. 2011, 4, 3680–3688. (5) Choi, B. G.; Chang, S.-J.; Kang, H.-W.; Park, C. P.; Kim, H. J.; Hong, W. H.; Lee, S.; Huh, Y. S. High Performance of a Solid-State Flexible Asymmetric Supercapacitor Based on Graphene Films. Nanoscale 2012, 4, 4983–4988. (6) Varley, J. B.; Kweon, K.; Mehta, P.; Shea, P.; Heo, T. W.; Udovic, T. J.; Stavila, V.; Wood, B. C. Understanding Ionic Conductivity Trends in Polyborane Solid Electrolytes from Ab Initio Molecular Dynamics. ACS Energy Lett. 2017, 2, 250–255. (7) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621–629, 10.1038/nmat2448. (8) Maginn, E. J. Molecular Simulation of Ionic Liquids: Current Status and Future Opportunities. J. Phys. Condens. Matter 2009, 21, 373101. (9) Castner, E. W.; Wishart, J. F. Spotlight on Ionic Liquids. J. Chem. Phys. 2010, 132, 120901. 24


Page 24 of 36

Page 25 of 36 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


(10) Fedorov, M. V.; Kornyshev, A. A. Ionic Liquids at Electrified Interfaces. Chem. Rev. 2014, 114, 2978–3036. (11) Nishida, T.; Tashiro, Y.; Yamamoto, M. Physical and Electrochemical Properties of 1Alkyl-3-methylimidazolium Tetrafluoroborate for Electrolyte. J. Fluorine Chem. 2003, 120, 135–141. (12) Wang, J.; Tian, Y.; Zhao, Y.; Zhuo, K. A Volumetric and Viscosity Study for the Mixtures of 1-n-butyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid with Acetonitrile, Dichloromethane, 2-Butanone and N, N-dimethylformamide. Green Chem. 2003, 5, 618–622. (13) Li, W.; Zhang, Z.; Han, B.; Hu, S.; Xie, Y.; Yang, G. Effect of Water and Organic Solvents on the Ionic Dissociation of Ionic Liquids. J. Phys. Chem. B 2007, 111, 6452– 6456. (14) Shiflett, M. B.;

Yokozeki, A. Liquid-Liquid Equilibria in Binary Mixtures

Containing Fluorinated Benzenes and Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide. J. Chem. Eng. Data 2008, 53, 2683–2691. (15) Shiflett, M. B.; Niehaus, A. M. S.; Yokozeki, A. Liquid-Liquid Equilibria in Binary Mixtures Containing Chlorobenzene, Bromobenzene, and Iodobenzene with Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide. J. Chem. Eng. Data 2009, 54, 2090–2094. (16) Wang, H.; Wang, J.; Zhang, S.; Pei, Y.; Zhuo, K. Ionic Association of the Ionic Liquids [C4mim][BF4], [C4mim][PF6], and [Cnmim]Br in Molecular Solvents. ChemPhysChem 2009, 10, 2516–2523. (17) Canongia Lopes, J. N.; Costa Gomes, M. F.; Husson, P.; Padua, A. A. H.; Rebelo, L. P. N.; Sarraute, S.; Tariq, M. Polarity, Viscosity, and Ionic Conductivity of Liquid

25



Mixtures Containing [C4C1im][Ntf2] and a Molecular Component. J. Phys. Chem. B 2011, 115, 6088–6099. (18) Bhagour, S.; Solanki, S.; Hooda, N.; Sharma, D.; Sharma, V. K. Thermodynamic Properties of Binary Mixtures of the Ionic Liquid [emim][BF4] with Acetone and Dimethylsulphoxide. J. Chem. Thermodyn. 2013, 60, 76–86. (19) Zhang, S. S. A Review on Electrolyte Additives for Lithium-Ion Batteries. J. Power Sources 2006, 162, 1379–1394. (20) Moon, J. S.; Kim, H.; Lee, D.-C.; Lee, J. T.; Yushin, G. Increasing Capacitance of Zeolite-Templated Carbons in Electric Double Layer Capacitors. J. Electrochem. Soc. 2015, 162, A5070–A5076. (21) Van Aken, K. L.; Beidaghi, M.; Gogotsi, Y. Formulation of Ionic-Liquid Electrolyte To Expand the Voltage Window of Supercapacitors. Angew. Chem. Int. Ed. 2015, 54, 4806–4809. (22) Bozym, D. J.; Uralcan, B.; Limmer, D. T.; Pope, M. A.; Szamreta, N. J.; Debenedetti, P. G.; Aksay, I. A. Anomalous Capacitance Maximum of the Glassy Carbon-Ionic Liquid Interface through Dilution with Organic Solvents. J. Phys. Chem. Lett. 2015, 6, 2644–2648. (23) Park, S.; Kim, K. Tetramethylammonium Tetrafluoroborate: The Smallest Quaternary Ammonium Tetrafluoroborate Salt for use in Electrochemical Double Layer Capacitors. J. Power Sources 2017, 338, 129–135. (24) Cheng, L.; Assary, R. S.; Qu, X.; Jain, A.; Ong, S. P.; Rajput, N. N.; Persson, K.; Curtiss, L. A. Accelerating Electrolyte Discovery for Energy Storage with High-Throughput Screening. J. Phys. Chem. Lett. 2015, 6, 283–291.

26


Page 26 of 36

Page 27 of 36 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


(25) Heintz, A.; Kulikov, D. V.; Verevkin, S. P. Thermodynamic Properties of Mixtures Containing Ionic Liquids. 1. Activity Coefficients at Infinite Dilution of Alkanes, Alkenes, and Alkylbenzenes in 4-Methyl-n-butylpyridinium Tetrafluoroborate Using Gas-Liquid Chromatography. J. Chem. Eng. Data 2001, 46, 1526–1529. (26) Marczak, W.; Verevkin, S. P.; Heintz, A. Enthalpies of Solution of Organic Solutes in the Ionic Liquid 1-Methyl-3-ethyl-imidazolium Bis-(trifluoromethyl-sulfonyl) Amide. J. Solution Chem. 2003, 32, 519–526. (27) Lopes, P. E. M.; Huang, J.; Shim, J.; Luo, Y.; Li, H.; Roux, B.; MacKerell, A. D. Polarizable Force Field for Peptides and Proteins Based on the Classical Drude Oscillator. J. Chem. Theory Comput. 2013, 9, 5430–5449. (28) Duan, Y.; Wu, C.; Chowdhury, S.; Lee, M. C.; Xiong, G.; Zhang, W.; Yang, R.; Cieplak, P.; Luo, R.; Lee, T. et al. A Point-Charge Force Field for Molecular Mechanics Simulations of Proteins Based on Condensed-Phase Quantum Mechanical Calculations. J. Comput. Chem. 2003, 24, 1999–2012. (29) MacKerell, A. D.; Bashford, D.; Bellott,; Dunbrack, R. L.; Evanseck, J. D.; Field, M. J.; Fischer, S.; Gao, J.; Guo, H.; Ha, S. et al. All-Atom Empirical Potential for Molecular Modeling and Dynamics Studies of Proteins. J. Phys. Chem. B 1998, 102, 3586–3616. (30) Oostenbrink, C.; Villa, A.; Mark, A. E.; Van Gunsteren, W. F. A Biomolecular Force Field Based on the Free Enthalpy of Hydration and Solvation: The GROMOS ForceField Parameter Sets 53A5 and 53A6. J. Comput. Chem. 2004, 25, 1656–1676. (31) Dommert, F.; Wendler, K.; Berger, R.; Delle Site, L.; Holm, C. Force Fields for Studying the Structure and Dynamics of Ionic Liquids: A Critical Review of Recent Developments. ChemPhysChem 2012, 13, 1625–1637. (32) Salanne, M. Simulations of Room Temperature Ionic Liquids: from Polarizable to Coarse-Grained Force Fields. Phys. Chem. Chem. Phys. 2015, 17, 14270–14279. 27



(33) McDaniel, J. G.; Choi, E.; Son, C. Y.; Schmidt, J. R.; Yethiraj, A. Ab Initio Force Fields for Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2016, 120, 7024–7036. (34) Borodin, O.; Smith, G. D. Development of Many-Body Polarizable Force Fields for Li-Battery Components: 1. Ether, Alkane, and Carbonate-Based Solvents. J. Phys. Chem. B 2006, 110, 6279–6292. (35) Borodin, O.; Smith, G. D. Development of Many-Body Polarizable Force Fields for LiBattery Applications: 2. LiTFSI-Doped Oligoether, Polyether, and Carbonate-Based Electrolytes. J. Phys. Chem. B 2006, 110, 6293–6299. (36) Borodin, O.; Smith, G. D. LiTFSI Structure and Transport in Ethylene Carbonate from Molecular Dynamics Simulations. J. Phys. Chem. B 2006, 110, 4971–4977. (37) Borodin, O.; Smith, G. D. Structure and Dynamics of N-Methyl-N-propylpyrrolidinium Bis(trifluoromethanesulfonyl)imide Ionic Liquid from Molecular Dynamics Simulations. J. Phys. Chem. B 2006, 110, 11481–11490. (38) Borodin, O. Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids. J. Phys. Chem. B 2009, 113, 11463–11478. (39) Bedrov, D.; Borodin, O.; Li, Z.; Smith, G. D. Influence of Polarization on Structural, Thermodynamic, and Dynamic Properties of Ionic Liquids Obtained from Molecular Dynamics Simulations. J. Phys. Chem. B 2010, 114, 4984–4997. (40) Jenel, V.; Mihaela, V.; Oleg, B.; Dmitry, B. A Comparative Study of Room Temperature Ionic Liquids and their Organic Solvent Mixtures near Charged Electrodes. J. Phys. Condens. Matter 2016, 28, 464002. (41) Son, C. Y.; McDaniel, J. G.; Schmidt, J. R.; Cui, Q.; Yethiraj, A. First Principles United Atom Force Field for the Ionic Liquid [BMIM][BF4] : An Alternative to Charge Scaling. J. Phys. Chem. B 2016, 120, 3560–3568. 28


Page 28 of 36

Page 29 of 36 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


(42) Shiflett, M. B.; Yokozeki, A. Hydrogen Substitution Effect on the Solubility of Perhalogenated Compounds in Ionic Liquid [bmim][PF6]. Fluid Ph. Equilibria 2007, 259, 210–217. (43) Dong, S.; Heyda, J.; Yuan, J.; Schalley, C. A. Lower Critical Solution Temperature (LCST) Phase Behaviour of an Ionic Liquid and its Control by Supramolecular Hostguest Interactions. ChemComm 2016, 52, 7970–7973. (44) Najdanovic-Visak, V.; Esperanca, J. M. S. S.; Rebelo, L. P. N.; Nunes da Ponte, M.; Guedes, H. J. R.; Seddon, K. R.; Szydlowski, J. Phase Behaviour of Room Temperature Ionic Liquid Solutions: An Unusually Large Co-solvent Effect in (Water + Ethanol). Phys. Chem. Chem. Phys. 2002, 4, 1701–1703. (45) Rollet, A.-L.; Porion, P.; Vaultier, M.; Billard, I.; Deschamps, M.; Bessada, C.; Jouvensal, L. Anomalous Diffusion of Water in [BMIM][TFSI] Room-Temperature Ionic Liquid. J. Phys. Chem. B 2007, 111, 11888–11891. (46) Stassen, H. K.; Ludwig, R.; Wulf, A.; Dupont, J. Imidazolium Salt Ion Pairs in Solution. Chem. Eur. J. 2015, 21, 8324–8335. (47) Merlet, C.; Rotenberg, B.; Madden, P. A.; Salanne, M. Computer Simulations of Ionic Liquids at Electrochemical Interfaces. Phys. Chem. Chem. Phys. 2013, 15, 15781– 15792. (48) Uralcan, B.; Aksay, I. A.; Debenedetti, P. G.; Limmer, D. T. Concentration Fluctuations and Capacitive Response in Dense Ionic Solutions. J. Phys. Chem. Lett. 2016, 7, 2333–2338. (49) Sieffert, N.; Wipff, G. The [BMI][Tf2N] Ionic Liquid/Water Binary System: A Molecular Dynamics Study of Phase Separation and of the Liquid-Liquid Interface. J. Phys. Chem. B 2006, 110, 13076–13085.

29



(50) Umebayashi, Y.; Jiang, J.-C.; Shan, Y.-L.; Lin, K.-H.; Fujii, K.; Seki, S.; Ishiguro, S.-I.; Lin, S. H.; Chang, H.-C. Structural Change of Ionic Association in Ionic Liquid/Water Mixtures: A High-pressure Infrared Spectroscopic Study. J. Chem. Phys. 2009, 130, 124503. (51) Yee, P.; Shah, J. K.; Maginn, E. J. State of Hydrophobic and Hydrophilic Ionic Liquids in Aqueous Solutions: Are the Ions Fully Dissociated? J. Phys. Chem. B 2013, 117, 12556–12566. (52) Borodin, O.; Price, D. L.; Aoun, B.; Gonzalez, M. A.; Hooper, J. B.; Kofu, M.; Kohara, S.; Yamamuro, O.; Saboungi, M.-L. Effect of Water on the Structure of a Prototype Ionic Liquid. Phys. Chem. Chem. Phys. 2016, 18, 23474–23481. (53) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids. J. Am. Chem. Soc. 1996, 118, 11225–11236. (54) McDaniel, J. G.; Schmidt, J. R. Physically-Motivated Force Fields from SymmetryAdapted Perturbation Theory. J. Phys. Chem. A 2013, 117, 2053–2066. (55) McDaniel, J. G.; Schmidt, J. R. First-Principles Many-Body Force Fields from the Gas Phase to Liquid: A “Universal” Approach. J. Phys. Chem. B 2014, 118, 8042–8053. (56) Schmidt, J. R.; Yu, K.; McDaniel, J. G. Transferable Next-Generation Force Fields from Simple Liquids to Complex Materials. Acc. Chem. Res. 2015, 48, 548–556. (57) Szalewicz, K. Symmetry-adapted Perturbation Theory of Intermolecular Forces. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 254–272. (58) Parker, T. M.; Burns, L. A.; Parrish, R. M.; Ryno, A. G.; Sherrill, C. D. Levels of Symmetry Adapted Perturbation Theory (SAPT). I. Efficiency and Performance for Interaction Energies. J. Chem. Phys. 2014, 140, 94106. 30


Page 30 of 36

Page 31 of 36 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


(59) Sambasivarao, S. V.; Acevedo, O. Development of OPLS-AA Force Field Parameters for 68 Unique Ionic Liquids. J. Chem. Theory Comput. 2009, 5, 1038–1050. (60) Chaban, V. V.; Voroshylova, I. V.; Kalugin, O. N. A New Force Field Model for the Simulation of Transport Properties of Imidazolium-Based Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 7910–7920. (61) Lamoureux, G.; Harder, E.; Vorobyov, I. V.; Roux, B.; MacKerell Jr, A. D. A Polarizable Model of Water for Molecular Dynamics Simulations of Biomolecules. Chem. Phys. Lett. 2006, 418, 245–249. (62) Parrish, R. M.; Burns, L. A.; Smith, D. G. A.; Simmonett, A. C.; DePrince, A. E.; Hohenstein, E. G.; Bozkaya, U.; Sokolov, A. Y.; Di Remigio, R.; Richard, R. M. et al. Psi4 1.1: An Open-Source Electronic Structure Program Emphasizing Automation, Advanced Libraries, and Interoperability. J. Chem. Theory Comput. 2017, 13, 3185– 3197. (63) Werner, H.-J.; Knowles, P. J.; Lindh, R.; Manby, F. R.; Schutz, M.; Celani, P.; Korona, T.; Mitrushenkov, A.; Rauhut, G.; Adler, T. B. et al. MOLPRO, a Package of Ab Initio Programs, Version 2009.1; see http://www.molpro.net. (64) Eastman, P.; Friedrichs, M. S.; Chodera, J. D.; Radmer, R. J.; Bruns, C. M.; Ku, J. P.; Beauchamp, K. A.; Lane, T. J.; Wang, L.-P.; Shukla, D. et al. OpenMM 4: A Reusable, Extensible, Hardware Independent Library for High Performance Molecular Simulation. J. Chem. Theory Comput. 2013, 9, 461–469. (65) Eastman, P.; Swails, J.; Chodera, J. D.; McGibbon, R. T.; Zhao, Y.; Beauchamp, K. A.; Wang, L.-P.; Simmonett, A. C.; Harrigan, M. P.; Stern, C. D. et al. OpenMM 7: Rapid Development of High Performance Algorithms for Molecular Dynamics. PLOS Comput. Biol. 2017, 13, e1005659.

31



(66) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577–8593. (67) Kumar, S.; Rosenberg, J. M.; Bouzida, D.; Swendsen, R. H.; Kollman, P. A. The Weighted Histogram Analysis Method for Free-energy Calculations on Biomolecules. I. The Method. J. Comput. Chem. 1992, 13, 1011–1021. (68) Hess, B.; Holm, C.; van der Vegt, N. Osmotic Coefficients of Atomistic NaCl (aq) Force Fields. J. Chem. Phys. 2006, 124, 164509. (69) Timko, J.; Bucher, D.; Kuyucak, S. Dissociation of NaCl in Water from Ab Initio Molecular Dynamics Simulations. J. Chem. Phys. 2010, 132, 114510. (70) Jorgensen, W. L.; McDonald, N. A.; Selmi, M.; Rablen, P. R. Importance of Polarization for Dipolar Solutes in Low-Dielectric Media: 1,2-Dichloroethane and Water in Cyclohexane. J. Am. Chem. Soc. 1995, 117, 11809–11810. (71) Choi, E.; McDaniel, J. G.; Schmidt, J. R.; Yethiraj, A. First-Principles, Physically Motivated Force Field for the Ionic Liquid [BMIM][BF4]. J. Phys. Chem. Lett. 2014, 5, 2670–2674. (72) Jorgensen, W. L. Internal Rotation in Liquid 1,2-dichloroethane and n-butane. J. Am. Chem. Soc. 1981, 103, 677–679. (73) Jorgensen, W. L.; Binning, R. C.; Bigot, B. Structures and Properties of Organic Liquids: N-butane and 1,2-dichloroethane and their Conformation Equilibriums. J. Am. Chem. Soc. 1981, 103, 4393–4399. (74) Rick, S. W.; Stuart, S. J. Potentials and Algorithms for Incorporating Polarizability in Computer Simulations. Rev. Comput. Chem. 2002, 18, 89–146.

32


Page 32 of 36

Page 33 of 36 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


(75) Lamoureux, G.; Roux, B. Modeling Induced Polarization with Classical Drude Oscillators: Theory and Molecular Dynamics Simulation Algorithm. J. Chem. Phys. 2003, 119, 3025–3039. (76) Pronk, S.; Pall, S.; Schulz, R.; Larsson, P.; Bjelkmar, P.; Apostolov, R.; Shirts, M. R.; Smith, J. C.; Kasson, P. M.; van der Spoel, D. et al. GROMACS 4.5: A HighThroughput and Highly Parallel Open Source Molecular Simulation Toolkit. Bioinformatics 2013, 29, 845–854. (77) Abraham, M. J.; Murtola, T.; Schulz, R.; Pall, S.; Smith, J. C.; Hess, B.; Lindahl, E. GROMACS: High Performance Molecular Simulations through Multi-Level Parallelism from Laptops to Supercomputers. SoftwareX 2015, 1-2, 19–25. (78) Phillips, J. C.; Braun, R.; Wang, W.; Gumbart, J.; Tajkhorshid, E.; Villa, E.; Chipot, C.; Skeel, R. D.; Kale, L.; Schulten, K. Scalable Molecular Dynamics with NAMD. J. Comput. Chem. 2005, 26, 1781–1802. (79) Jiang, W.; Hardy, D. J.; Phillips, J. C.; MacKerell, A. D.; Schulten, K.; Roux, B. HighPerformance Scalable Molecular Dynamics Simulations of a Polarizable Force Field Based on Classical Drude Oscillators in NAMD. J. Phys. Chem. Lett. 2011, 2, 87–92. (80) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1–19. (81) Dequidt, A.; Devemy, J.; Padua, A. A. H. Thermalized Drude Oscillators with the LAMMPS Molecular Dynamics Simulator. J. Chem. Inf. Model. 2016, 56, 260–268. (82) Starovoytov, O. N.; Torabifard, H.; Cisneros, G. A. Development of AMOEBA Force Field for 1,3-Dimethylimidazolium Based Ionic Liquids. J. Phys. Chem. B 2014, 118, 7156–7166.

33



(83) Tu, Y.-J.; Allen, M. J.; Cisneros, G. A. Simulations of the Water Exchange Dynamics of Lanthanide Ions in 1-Ethyl-3-methylimidazolium Ethyl Sulfate ([EMIm][EtSO4]) and Water. Phys. Chem. Chem. Phys. 2016, 18, 30323–30333. (84) Torabifard, H.; Reed, L.; Berry, M. T.; Hein, J. E.; Menke, E.; Cisneros, G. A. Computational and Experimental Characterization of a Pyrrolidinium-Based Ionic Liquid for Electrolyte Applications. J. Chem. Phys. 2017, 147, 161731. (85) Tu, Y.-J.; Lin, Z.; Allen, M. J.; Cisneros, G. A. Molecular Dynamics Investigation of Water-Exchange Reactions on Lanthanide Ions in Water/1-Ethyl-3-methylimidazolium Trifluoromethylsufate ([EMIm][OTf]). J. Chem. Phys. 2018, 148, 024503. (86) Lagardere, L.; Jolly, L.-H.; Lipparini, F.; Aviat, F.; Stamm, B.; Jing, Z. F.; Harger, M.; Torabifard, H.; Cisneros, G. A.; Schnieders, M. J. et al. Tinker-HP: A Massively Parallel Molecular Dynamics Package for Multiscale Simulations of Large Complex Systems with Advanced Point Dipole Polarizable Force Fields. Chem. Sci. 2018, 9, 956–972. (87) Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A. Molecular Dynamics Simulation of Ionic Liquids: The Effect of Electronic Polarizability. J. Phys. Chem. B 2004, 108, 11877–11881. (88) Yan, T.; Li, S.; Jiang, W.; Gao, X.; Xiang, B.; Voth, G. A. Structure of the LiquidVacuum Interface of Room-Temperature Ionic Liquids: A Molecular Dynamics Study. J. Phys. Chem. B 2006, 110, 1800–1806. (89) Yan, T.; Wang, Y.; Knox, C. On the Structure of Ionic Liquids: Comparisons between Electronically Polarizable and Nonpolarizable Models I. J. Phys. Chem. B 2010, 114, 6905–6921. (90) Yan, T.; Wang, Y.; Knox, C. On the Dynamics of Ionic Liquids: Comparisons between Electronically Polarizable and Nonpolarizable Models II. J. Phys. Chem. B 2010, 114, 6886–6904. 34


Page 34 of 36

Page 35 of 36 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


(91) Schroder, C.; Steinhauser, O. Simulating Polarizable Molecular Ionic Liquids with Drude Oscillators. J. Chem. Phys. 2010, 133, 154511. (92) Schroder, C.; Sonnleitner, T.; Buchner, R.; Steinhauser, O. The Influence of Polarizability on the Dielectric Spectrum of the Ionic Liquid 1-Ethyl-3-methylimidazolium Triflate. Phys. Chem. Chem. Phys. 2011, 13, 12240–12248. (93) Taylor, T.; Schmollngruber, M.; Schroder, C.; Steinhauser, O. The Effect of Thole Functions on the Simulation of Ionic Liquids with Point Induced Dipoles at Various Densities. J. Chem. Phys. 2013, 138, 204119. (94) Gu, Y.; Yan, T. Thole Model for Ionic Liquid Polarizability. J. Phys. Chem. A 2012, 117, 219–227. (95) Cavalcante, A. d. O.; Ribeiro, M. C. C.; Skaf, M. S. Polarizability Effects on the Structure and Dynamics of Ionic Liquids. J. Chem. Phys. 2014, 140, 144108. (96) Millot, C.; Chaumont, A.; Engler, E.; Wipff, G. Distributed Polarizability Models for Imidazolium-Based Ionic Liquids. J. Phys. Chem. A 2014, 118, 8842–8851. (97) Wu, Y.; Hu, N.; Yue, L.; Wei, L.; Guan, W. Effects of Polarizability on the Structural and Thermodynamics Properties of [Cnmim][Gly] Ionic Liquids (n = 1-4) Using EEM/MM Molecular Dynamic Simulations. J. Chem. Phys. 2015, 142, 064503.

35



Graphical TOC Entry

36


Page 36 of 36

Polarization Effects in Binary - ACS Publications - American Chemical

Recommend Documents