Solvation Mechanism of Task-Specific Ionic Liquids in Water: A

Subscriber access provided by UNIV MASSACHUSETTS BOSTON

Article

Solvation Mechanism of Task-specific Ionic Liquids in Water: a Combined Investigation Using Classical Molecular Dynamics and Density Functional Theory Surya Velappa Jayaraman Yuvaraj, Ravil K Zhdanov, Rodion V. Belosludov, Vladimir R. Belosludov, Oleg S. Subbotin, Kiyoshi Kanie, Kenji Funaki, Atsushi Muramatsu, Takashi Nakamura, and Yoshiyuki Kawazoe J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b05945 • Publication Date (Web): 14 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015

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 free 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 accessible to all readers and 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.

The Journal of Physical Chemistry B 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 29

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

Solvation mechanism of task-specific ionic liquids in water: A combined investigation using classical molecular dynamics and density functional theory Surya V.J. Yuvaraj*,1,4, Ravil K. Zhdanov2,4,5, Rodion V. Belosludov3, Vladimir R. Belosludov2,4,5, Oleg S. Subbotin2,4,5, Kiyoshi Kanie4, Kenji Funaki4, Atsushi Muramatsu4, Takashi Nakamura4, and Yoshiyuki Kawazoe1,5 1

New Industry Creation Hatchery Center, Tohoku University, 6-6-4 Aoba, Aramaki, Sendai – 980-

8579, Japan. 2

Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk, 630090, Russia.

3

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai – 980-8577,

Japan. 4

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira,

Aoba-ku, Sendai – 980-8577, Japan. 5

Institute of Thermophysics, SB RAS, Novosibirsk, 630090, Russia.

Corresponding Author * Surya V.J. Yuvaraj, New Industry Creation Hatchery Center (NICHe), Tohoku University, 6-6-4 Aoba, Aramaki, Sendai – 980-8579, Japan. E-mail address: [email protected], Phone: +81-(0)22-795-3670

1 ACS Paragon Plus Environment


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 2 of 29

Abstract. Solvation behavior of task-specific ionic liquids (TSILs) containing a common, L-histidine derived

imidazolium

cation

[C20H28N3O3]+

and

different

anions,

bromide-[Br]-

and

bis(trifluoromethylsulfonyl)amide- [NTF2]- in water is examined, computationally. These amino acid functionalized ionic liquids (ILs) are taken into account because of their ability to react with rare earth metal salts. It has been noted that the TSIL with [Br]- is more soluble than its counterpart TSIL with [NTF2]-, experimentally. In this theoretical work, the combined classical molecular dynamics (CMD) and density functional theory (DFT) calculations are performed to study the behavior of bulk phase of these two TSILs in the vicinity of water (H2O) molecules with different concentration. Initially, all the constructed systems are equilibrated using CMD method. The final structures of the equilibrated systems are extracted for DFT calculations. Under CMD operation, the radial distribution function (RDF) plots and viscosity of TSILs are analyzed to understand the effect of water on TSILs. In DFT regime, binding energy per H2O, charge transfer, charge density mapping and electronic density of states (EDOS) analyses are done. The CMD results along with the DFT results are consolidated to support the hydrophilic and hydrophobic nature of the TSILs. Interestingly, we have found a strong correlation between the viscosity and the EDOS results that leads to understand the hydration properties of the TSILs. Keywords: imidazolium ionic liquids, water, classical molecular dynamics, density functional theory, solvation shells, viscosity


Page 3 of 29

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. Introduction. Ionic liquids are categorized as organic salts composed of cations and anions that exist as liquids in ambient temperature and pressure conditions. Several types of ILs can be designed by combining different cations and anions. It has been demonstrated that about one million (106) simple ILs can be easily prepared in the laboratory [1]. Basically, the cations are imidazolium, pyrrolidinium, pyridinium, ammonium or phosphonium groups and the anions may be like inorganic or organic tetrafluroborate or halides [2]. The ILs possess unique properties like low vapor pressure, thermal stability, high ionic conductivity, non-flammability that make them suitable for industrial applications ranging from catalysis [3-5], organic [6] and inorganic syntheses [7-9] chemical separation [10-12] and extraction [13-16], biomass processing [17-19], as electrolytes in fuel cells and batteries [20] and more recently in electrolyte-gated transistor fabrication [21]. The non-volatility, stability and tunable solubility of the ILs project them as suitable solvents for metal extraction instead of conventional volatile organic solvents [22]. Lot of researchers have done extractions of metal ions using the ILs which includes alkali, alkaline earth, rare earth and radioactive chemical elements and it has been highlighted in the ref.[22]. Since metal extraction remains a hot topic among chemists, metallurgists and chemical engineers, they recommend two immiscible phases (biphasic systems) for energy efficient extraction technology [22]. Hence, less hydrophilic ionic liquids (ILs) are suitable for biphasic metal extraction process. In general, ILs are hygroscopic and hence it can absorb significant amount of water (H2O) from the atmosphere. The absorption of water changes their physical and chemical properties such as density, viscosity, conductivity, polarity, melting point, hydrophobicity and hydrogen bonding capability. Even a small amount of water can drastically change the reaction rates and selectivity of chemical reactions. The hydrophobicity of IL depends mainly on type of anions [23, 46] and the side chains of the cations [24, 25]. Though several ILs are utilized for extraction purpose, some of the conventional ILs cannot fulfill the required scientific tasks. To increase and to tune the applicability of such ILs, they are modified by attaching different functional groups. Such ILs are termed as task-specific ionic liquids (TSILs) or designer solvents [26,27]. They are tailored for specific applications where conventional (pure) ILs fail to succeed.



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

Our experimental group has synthesized two different L-histidine derived imidazolium based TSILs in accordance with the ref. [28] due to their complexation ability with metal complexes of rare earth metals (REM) like palladium and rhodium. The experimental details are reported in our previous paper [29]. The two TSILs have common cation, Methyl 1-(N-benzoyl amino)-2-(1,3-di(npropyl)imidazolium)-propionate -[C20H28N3O3]+, and two different anions, bromide-[Br]- and bis(trifluoromethylsulfonyl)amide-[NTF2]-. The cation is typically abbreviated as [Bz-His(n-propyl)2OMe] [28,29] and it has i. a main imidazole ring, ii. propyl groups attached to the nitrogen atoms of the imidazole ring, iii. ester functional group, and iv. amide functional group as shown in the Fig. S1 of supporting information. Experimentally, the solubility test on these two TSILs have revealed that the TSIL with [NTF2]- is less soluble relatively to [C20H28N3O3]+ [Br]- in water. It is essentially due to the fact that [NTF2]- anion is hydrophobic than [Br]-. This fact has been proved by our recent gas phase calculations with single molecule of the TSIL and limited number of H2O molecules [30]. However, the gas phase calculations provide limited space such that several properties of molecules cannot be brought out. Large scale simulations on bulk phase of ILs not only ensure realistic environment but also allows one to do complete diagnosis of the systems according to the necessity. In the present study, we have considered the bulk phase of the above mentioned two TSILs to scan their solvation behavior in water. Recently, several theoretical investigations have been carried out along with the experiments to examine the effect of water on the ILs [30-49]. We have highlighted some of the facts on effect of water on ILs and the hydrophobicity of ILs. The molecular dynamics (MD) simulations on a series of imidazolium ionic liquids and water mixtures have showed that the increment in the length of the alkyl chain of the cation and the replacement of anion [BF4]- by [Cl]- alters the water distribution and slows the diffusion in the mixtures [43]. Likewise, the improvement in hydrophobicity of ILs by increasing the alkyl chain from ethyl to butyl has been confirmed both experimentally and theoretically [44]. In another work, it has been reported that the original property of 1-butyl-3methylimidazolium (BMIM) based ILs changes beyond 70% of water concentration [19]. Through ab-initio calculations it has been shown that the nature of the anion influences the solubility of solutes


Page 4 of 29

Page 5 of 29

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


like H2O, SO2, CO2, 2-propanol, ethyl acetate, n-hexane and toluene in methylimidazolium based ILs, rather than the cation [45]. Experimentally, it has been reported that halogen free chelated orthoborate-phosphonium ILs (hf-BILs) are hydrophobic and hydrolytically stable at room temperature [46]. In 2014, the same research group has developed force field for orthoborate phosphonium ionic liquids [47]. The predicted densities of neat ILs and hydrated trihexyltetradecyl phosphonium bis(oxalate)borate ([P6,6,6,14] [BOB])

IL are in agreement with the experimental data. A series of hydrophobic

phosphonium based ILs have been designed with the combination of phosphonate anion for the polar unit and the phosphonium cation for controlling affinity with water [48]. It has been reported that the interaction of water and ionic liquids is strongly influenced by the hydrogen bonding of anions with water [49]. Moreover, the aromatization and fluorination of anions are shown to reduce their interaction with water [49]. Recently, the orientations of alkylimidazolium based ionic liquids at the interface with vacuum and water have been investigated by MD simulations [50]. The effect of water concentration on the free volume of amino acid ionic liquids (AAILs) has been studied by MD simulations [51]. The authors have shown that the water content has influence on the AAILs such that their hydrophobicity and hydrophilicity vary in their free volume and fractional volume calculations. The above mentioned studies have shown that the modification of cation and the anion influences the water affinity of the ILs. In addition, most of the studies have highlighted the change of properties of ILs in the presence of water molecules. In this study, we have done computational investigation to evaluate the physical, chemical and electronic properties of TSILs in the presence of water molecules. We have done our investigations with bulk phase of these two ILs for different concentration of water molecules. The calculations combine both classical molecular dynamics (CMD) for initial equilibration of all the constructed systems followed by the DFT calculations. This computational paper mainly addresses on mechanism of solvation of the TSILs in water through investigation of (i). hydration effect on nature of cations and anions, (ii). integrity between cation and anions before and after hydration, (iii). type of interaction between cation-cation, anion-anion, cation-anion, cation-water, anion-water and water-



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

water molecules through CMD and DFT results. The CMD includes the results of radial distribution function (RDF) and viscosity analyses. On the other hand, the DFT calculations include the results of binding energy (EB), charge density distribution, charge density, and electronic density of states (EDOS) analyses. Previously, several theoretical methods like ab-initio MD, CMD, COSMO-RS and other quantum mechanical calculations have been employed to investigate the solvation properties and interaction of ILs [30-51]. Our study employs both CMD and DFT calculations together on bulk phases of pure and hydrated TSILs and we believe that this study is the first report to correlate both CMD and DFT results to understand the solvation behavior of bulk TSILs in water.

2. Computational details. a. Classical Molecular Dynamics simulations. LAMMPS code is used to perform CMD simulations [49]. The system [C20H28N3O3]+ [Br]- is denoted as IBr and [C20H28N3O3]+[NTF2]- is denoted as ITF2. The experimental densities of IBr and ITF2 are 1.22 g/cm3 and 1.4 g/cm3, respectively. We have taken ten molecules of each TSIL for the calculations to simulate the bulk phase of TSIL equal to the experimental densities. H2O molecules are included in the ratio such that each IL molecule is associated with 2, 4, 6, 8 and 10 of them, or 20, 40, 60, 80, and 100 H2O molecules per unit cell. The TSIL pairs and water molecules are inserted randomly in the unit cells using Monte Carlo method by minimizing close contacts between atoms. All the simulations are carried out at 350 K and atmospheric pressure for 30 ns with a timestep of 1 fs under NPT to obtain the final equilibrated structures. After equilibration, the densities of both the TSILs (1.24 g/cm3 and 1.39 g/cm3 for IBr and ITF2) have remained near the experimental values (1.22 g/cm3 and 1.4 g/cm3 for IBr and ITF2). The final equilibrated structures have been used for further DFT calculations and massive bulk phase construction for viscosity calculation. We have used the following parameters for CMD. A cutoff distance of 13Å is used to compute non-bonding interactions. Particle-particle particle-mesh (PPPM) solver is used to include the long–range electrostatic interactions with accuracy of 10-5[50]. Temperature and pressure are


Page 6 of 29

Page 7 of 29

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


maintained using Nose-Hoover thermostat and Nose-Hoover barostat with coupling constants of 100fs-1 and 500fs-1, respectively. VMD is used to visualize the simulated structures [51]. The coordinates of the systems are saved for every 10ps. All optimization runs are performed under NPT ensemble, while production runs for e.g. viscosity calculation are performed under NVT ensemble. The cations and anions of both TSILs are modeled using OPLS-AA force-field parameters picked up from the work of Canongia Lopes et al.[55] and complemented with [56]. Bonds and angle constraints are represented by SHAKE algorithm [57]. For H2O molecules description TIP4P/2005 potential has been used [58]. We have selected TIP4P/2005 potential because it suits better to viscosity calculation of pure water system[59]. The van der Waals (vdW), bond stretching, angle bending and dihedral potential parameters are given in the supporting information. b. Viscosity calculations. The viscosity calculations have been done based on periodic Poiseuille flow method [60]. This method does not require additional specific simulation conditions, like unphysical momentum transfer for Muller-Plathe algorithm [61] and has very good noise to signal ratio [60]. The main idea behind this method is applying body force to each particle in the system (such as gravitational force) which yields a parabolic velocity profile for the liquid movement between two plates in linear regime. Measurement of profile parameters permits to calculate the viscosity of a liquid, directly. The boundary condition for such flow can be emulated by dividing modeling cell into two subdomains with the forces applied in the opposite directions on the two subdomains. MD for viscosity calculation has been performed using the simulation cell constructed by multiplying 162 (3x3x18) times the optimized unit cell used for DFT calculations, which makes length at the longest dimension about 350 Å. Such large system allows reducing fluctuations caused by complexity of the ionic liquid particles. Despite that the initial unit cell has been optimized for DFT calculation, for viscosity calculation equilibration procedure has been repeated but for the multiplied cell. Therefore, each structure has been equilibrated during 1 ns run in NPT ensemble before the application of force. Then, the velocity profile is obtained from the last 0.5 ns of 0.8 ns NVT run with the application of force. The external force is selected such that the resulting liquid



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

flows should be linear for all systems. It should be noted that the massive bulk phase constructed for viscosity calculation after 1 ns equilibration gives almost the same density as the experimental ones. c. DFT calculations. We have used VASP code [62] with projector augmented wave (PAW) [63] potentials under LDA. The systems after CMD optimization process are optimized again to their ground state with single gamma point due to the large size of the simulation cells (Table 1). The convergence criterion for energy is set as 0.1×10-3 eV. The structures are relaxed until the forces on the atoms become less than 1×10-2 eV/Å. The Gaussian smearing is carried out throughout the calculations. EDOS calculations are carried out using 3x3x3 and 2x2x2 k-points meshes to investigate the electronic distribution of all atoms in cation, anion and water molecules in 10IBr-nH2O and 10ITF2-nH2O (n=0, 20, 40, 60, 80 and 100) systems, respectively. Bader charge analysis is done to calculate the charge transfer [64]. The binding energy of H2O molecule is calculated using the formula,

E B (H 2 O)=

E T (IL+nH 2 O)-E T (IL)-nE T (H 2 O) n

3. Results and Discussion a. Effect of water on TSILs - RDF analysis. The effect of water on the local structural arrangements of the ions has been analyzed through atomic RDF plots of both TSILs before and after hydration. Carbon (C) atom attached with the most acidic hydrogen (H) is referred as C2 in general [19, 34 and 45], and nitrogen (N) atoms in the imidazolium ring are taken to represent the cations (Fig. S1). The NTF2 anion is represented by the N atom in it. The N atoms associated with cation and anion is referred as NC and NA for convenience (Fig. S1). Both H and oxygen (O) atoms are considered to represent the water molecules for understanding the molecular orientational correlations between the ions and water. The RDF profiles of both systems are compared as shown in Figs.1-3 and S2-S5, respectively. Cations-cations and anions-anions. The spatial distribution between cations-cations, anions-anions, and cations-anions in both 10IBr and 10ITF2 systems have been discussed here. The broader peaks


Page 8 of 29

Page 9 of 29

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


corresponding to cations-cations and anions-anions denote the weak interactions between the ions of same polarity (Fig. 1). The cations-cations peaks in both 10IBr and 10ITF2, do not follow a monotonic behavior (neither increases nor decreases). It may be due to the large size of cation and its different preferential orientations towards the water molecules. In Fig. 1b, we can observe a single peak which refers to the long range interaction between the [Br]- ions in pure 10IBr. This peak is split into two after hydration indicating the formation of solvation shells around [Br]-. However, the first peak seems to increase with increasing hydration level. This will be due to the clustering of water molecules which may leave some of the [Br]- anions lie in closer vicinity. In 10ITF2 case, the anionsanions peaks decreases as the hydration level increases referring to movement of anions away from each other in the presence of water molecules. However, there is no peak splitting as observed in 10IBr. So, it can be understood that the [NTF2]- anions are less affected than [Br]- anions by the water molecules. Cations-anions. The first sharp peaks appear at 3.86 Å and 3.40 Å in pure 10IBr (Fig. 2a) and 10ITF2 (Fig. 2c). Therefore, we can suppose that the electrostatic interaction between cation and [NTF2]- is stronger than between the cation and [Br]-. These sharp peaks demonstrate that the anions prefer to interact with cations through C2 rather than NC in the imidazolium ring. This is because the first sharp peaks related to the anions with the NA appear at 4.42 Å and 4.01 Å in 10IBr and 10ITF2 and the peaks are broader implying the weaker interaction between the oppositely charged ions as shown in Figs. 2b and 2d. The secondary peaks denote the presence of the long range spatial correlation between the cations and anions. When the hydration level increases, the peaks of the first coordination shells shift towards larger distances in both Br-C2 and Br-NC. This signifies the decreasing cation-anion interaction due to the increasing water content which leads to the complete isolation of [Br]- from the cation. The peaks show gradual decrease upon increase in water concentration in both Br-C2 and Br-NC (Fig. 2a and 2b).



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

In contrary to the 10IBr system, the peaks in NA-C2 (Fig. 2c) and NA-NC (Fig. 2d) decrease gradually from 0 to 40H2O. The reduction is drastic when it reaches 60H2O followed by gradual increase upto 100H2O. This is due to the fact that hydration effect in 60H2O is more such that interaction between cation-anion decreases suddenly. Noteworthy, the presence of a slightly higher secondary peak makes to realize the presence of weak long range interaction between the ion pairs in 10ITF2-60H2O as shown in the Fig. 2c. Eventually, the contact with each other is regained when hydration level increases (10ITF2-80H2O and 10ITF2-100H2O). This is due to the pronounced interaction between the water molecules than their interaction with the cations and anions. As a result, the counter ions come closer as reflected in Figs. 2c and 2d. In addition to the above facts, we can also observe the peak shift to larger distances in both 10IBr and 10ITF2 which projects the strong influence of water molecules on altering the structure of TSILs. cations-H2O and anions-H2O. Here, we discuss about the influence of water on cations and anions. Since, the anions play a major role in determining the solvation property of the ILs, we have presented the interaction between anions and water elaborately as shown below. The g(r) peaks in Figs. S2 and 3 denotes the formation of well defined solvation shells around the ions by the water molecules. In both 10IBr and 10ITF2 systems, the peaks corresponding to C2-H(H2O) appear at larger distance than C2O(H2O) (Fig. S2). Therefore, one can understand that oxygen in H2O prefers to orient towards C2-H in the five membered, imidazolium ring. Also, the interaction of O(H2O) with C2 is larger than the NC as the peaks are found at larger distance in the latter case (Fig. S3). In Br-H(H2O) (Fig. 3a) and Br-O(H2O) (Fig. 3b) one can see prominent peaks which refer to the well-defined solvation shells of H2O around [Br]-. The dominant first peak appears at 2.4Å in BrH(H2O). This provides a strong evidence for the presence of hydrogen bonding between the anions and H atoms of the water molecules. On the other hand, the first peak between [Br]- and O of water occurs at larger distance (3.4 Å). This value is in accordance with the experimental and theoretical values reported in ref [65]. It is obvious that the H in H2O prefers to orient towards [Br]- rather than O. The secondary peak refers to the second hydrogen of H2O.


Page 10 of 29

Page 11 of 29

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


In NA-H(H2O) (Fig. 3c) and NA-O(H2O) (Fig. 3d), there are several secondary peaks along with the first dominant peak. The secondary peaks are not well defined. It can be due to the complex hydration of the anion. In NA-O(H2O), one can observe the first peak at larger distance (2.78 Å) than NA-(H)H2O (1.91Å). This signifies that H2O prefers to orient itself with H facing the atoms in the anion as noted in its counterpart TSIL, 10IBr. The presence of first peak at 1.91 Å signifies the hydrogen bonding interaction between the anions and H of water molecules like in 10IBr-nH2O systems. H2O-H2O. The RDF plots representing the water-water interactions are shown in Figs. S4 and S5. The first peaks that appear at a distance of about 1.81 Å in Figs. S4, represent the presence of hydrogen bonds between the water molecules. On the other hand, the first peaks are observed at larger distances (greater than 2 Å) in Fig. S5. This denotes that the interaction between H(H2O) and O(H2O) is higher than H(H2O)-H(H2O) and O(H2O)-O(H2O) cases. The g(r) of 10ITF2-nH2O systems is higher than 10IBr-nH2O systems which imply the enhanced clustering of water molecules in the former case. This observation supports the fact that the pronounced clustering of water molecules reduces the interaction of water molecules with ITF2, as mentioned in the previous discussions. Overall, in this section we have shown that the presence of water disrupts the association between the ion pairs in a higher degree in 10IBr than 10ITF2. b. Viscosity profiles. The proposed model for viscosity is tested with pure water system. The calculated viscosity is 0.83 mPa.s and it is in line with the experimental data [66]. The calculated viscosity data for 10IBr and 10ITF2 lie within a range of viscosity of similar ILs [67]. The viscosity of pure and hydrated 10IBr and 10ITF2 are shown in the Fig. 4. The plots show the viscosity dependence of the TSILs on water content. The viscosity of 10ITF2 is higher than 10IBr in both unhydrated and hydrated systems. In both cases, the viscosity decreases with increase in the water content. c. Structural analysis. In all the systems, the intramolecular and intermolecular interactions are essentially through hydrogen bonding between the donor and acceptor atoms in water and TSIL molecules (Figs. 5 and 6). For instance, we have highlighted the interaction between anion and water



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

molecules in Figs. 5b-5f and 6b-6f. At earlier hydration levels, we can observe some isolated water molecules along with small water clusters. As the water concentration increases, the size of the water clusters also increases in both TSIL systems. In addition, the shape of the water clusters also changes. Most of the water molecules are distributed as linear chains in the supercells from 10IBr-20H2O to 10IBr-60H2O systems. The distribution is such a way that water interacts largely with the ions. In 10IBr-80H2O and 10IBr-100H2O, we can observe the formation of cage like water clusters percolating in between the ions. In 10ITF2-nH2O systems, the formation of cage like water clusters has started from 10ITF2-60H2O onwards. We insist that the water clustering is relatively less pronounced in 10ITF2-nH2O systems than in 10IBr-nH2O systems which indicate the less interaction of ITF2 with H2O molecules. The possible mechanism for clustering of water molecules can be described as follows. Water is a polar solvent. The water molecules interact with each other via hydrogen bonds. When the concentration of water is very low, the probability of interaction between the neighboring water molecules is low. We can notice isolated water molecules as mentioned above. However, increasing the water concentration enhances the probability of interaction between the neighbors. Gradually, the water molecules connect with each other and start clustering to finite size. The cluster size and patterns change with the amount of water and the nature of TSILs. The percentage of water molecules interacting with TSILs decreases as the water concentration increases as shown in Fig. S6. In both 10IBr-nH2O and 10ITF2-nH2O systems the respective anions interact with 1 to 6 H2O molecules. As an average, each anion is capable of interacting with 4H2O molecules. This is confirmed from 10IBr-100H2O system in which 37/100 H2O interacts with Br ~ 40H2O. Similarly, in 10ITF2-100H2O, anion reacts with 41/100 water molecules ~ 40 H2O. In-depth, one can understand that Br is surrounded by the water molecules in a way that leads to its complete hydration. Since, [NTF2]- is a large anion, it is not fully enveloped by water molecules and hence the partial hydration. Hence, we can conclude that the full hydration of [Br]- leads to its complete solvation of IBr in water whereas the partial hydration of [NTF2]- makes ITF2 partially soluble in water. d. Binding energy and charge analyses. The binding energy values of H2O in all hydrated 10IBr systems are higher (Fig. 7a) than that of ITF2 systems and the results are in line with previous


Page 12 of 29

Page 13 of 29

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


calculations [30]. This shows relatively stronger interaction of H2O with IBr. The partial charge on H2O in hydrated 10IBr is higher than 10ITF2 (Fig. 7b). The trend is found to be increasing on addition of H2O in 10IBr-nH2O systems. However, in 10ITF2-nH2O systems there is no gradual behavior. In 10IBr-nH2O systems, the partial charge on cation is increasing while in anion it is decreasing (Fig. 7c). Hence, the cation becomes more positive and anion becomes less negative by losing its electron charge. Hence, there is a net repulsive interaction between the cation and anion in the presence of water molecules such that they drag away the anion from cations. The charge distribution plot showing water molecules surrounding [Br] is shown in Fig. 8a. Upon hydration in ITF2, while the charge on the cation is decreasing, the charge on anion is also decreasing (Fig. 7d). This implies that the cation is becoming less positive, while the anion is becoming less negative. Thus, there is a net attractive interaction in the presence of water molecules. The interaction between water molecules and the [NTF2]- anion is shown in charge distribution profile (Fig. 8b). As a result, both the cations and anions remain intact even in the presence of water molecules. The aromatic rings close to H2O is strongly perturbed. Also, we have noted the charge localization on the oxygen and nitrogen of the functional group in the TSIL. Same as the gas phase calculations, we can see the charge perturbation is not so high in 10ITF2 than 10IBr. e. Density of states profiles. In the projected EDOS of 10IBr-nH2O systems (Fig. 9), the anion (Br) and the water peaks decreases and increases, respectively in the presence of H2O molecules. Eventually, the peaks of water overlap with the anion peaks implying the complete hydration of anion. In 10ITF2-nH2O systems, the wide anion peaks show non-uniform change on increase in hydration (Fig. 10). However, there is no drastic change in anion peaks. In contrast to 10IBr-nH2O systems, there is no complete overlap of anion ([NTF2]-) peaks by water peaks. This is essentially due to partial hydration of [NTF2]- anion. The gap between valence band maximum and conduction band minimum in EDOS is calculated and it is shown in the Fig. 11. In 10IBr-nH2O systems (Fig. 11a), the gap has large difference when compared to pure 10IBr. On the other hand in 10ITF2-nH2O (Fig. 11b), the gap of all hydrated systems is relatively closer to the unhydrated 10ITF2.



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

To know the individual contribution of atoms, we have plotted the partial DOS of C2-H and the atoms present in anions in both 10IBr-20H2O and 10ITF2-20H2O systems and matched with the total EDOS (TDOS). Valence and conduction band edges are ruled by C2-H in both 10IBr-20H2O and 10ITF2-20H2O (Figs. S7(a) and S7(c)). As noted from the projected EDOS, the valence band edge is governed by both the [Br]- and water (Figs. S6(b) and S6(d)). In10ITF2-20H2O, the valence band edge is predominantly occupied by N, O and C (less contribution) atoms in NTF2 (Fig. S8). The bands between -4 eV to 8 eV belong to fluorine (F) and sulpur (S) atoms in [NTF2]- (Fig. S8). These peaks are not overlapped by water and this denotes less interaction of water with F and S. This observation supports the fact reported in the ref.[49] stating that the fluorination of anions reduces their interaction with water. f. Viscosity and EDOS comparison. As a most striking feature, we are able to correlate the viscosity and EDOS results that are obtained from CMD and DFT calculations, respectively. In IBr, there is a sudden decrease of viscosity from IBr-20H2O to IBr-40H2O. After that the reduction is gradual upto IBr-100H2O (Fig. 4) system. In IBr-20H2O, the partial hydration of [Br]- anion can be observed from the partial overlapping of anion peaks by water peaks (Fig. 9b). Thereafter, the water peaks starts to overlap the anion peaks completely from IBr-40H2O to IBr-100H2O systems (Figs. 9c-9f). This transition is in accordance with the viscosity behavior of hydrated IBr. In 10ITF2-nH2O systems, the sudden transition in viscosity can be noted from 10ITF2-40H2O to 10ITF2-60H2O system (Fig. 4) followed by gradual decrease up to 10ITF2-100H2O system. Similar to 10IBr-20H2O and 10IBr-40H2O, we can note the partial overlapping in 10ITF2-20H2O and 10ITF240H2O (at the valence band edge in Fig. 10b and 10c). However, some of the peaks show complete overlapping with water peaks from 10ITF2-60H2O to 10ITF2-100H2O systems (Fig. 10d-10f). In reference to these observations, we fix the critical limit for maximum hydration level in 10IBr and 10ITF2. Therefore, 40H2O and 60H2O serve as critical limit for maximum hydration in 10IBr and 10ITF2, respectively. Similar to the above mentioned results, here too it is proved that the effect of hydration in IBr is higher than ITF2.


Page 14 of 29

Page 15 of 29

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


4. Summary and Conclusions In summary, the synergic CMD and DFT studies reveal the solvation mechanism of the TSILs, IBr and ITF2 through the physical, chemical and electronic properties. The electrostatic interaction between the cations and anions is stronger in ITF2 than IBr. The resolved atomic RDF plots have shown that the cation-anion coordination is highly perturbed by the water molecules in hydrated IBr than hydrated ITF2. In addition, the binding energy results have confirmed this fact. The interaction between the ions and water molecules are mainly through the hydrogen bonds. An average of 4H2O is associated with each anion in IBr and ITF2. The [Br]- anion is enclosed within solvation shell completely that leads to full hydration of IBr. In NTF2, only some of the atoms like N, O and C react with water molecules and hence it is partially hydrated. These results are supported by the charge distribution and EDOS analyses. Interestingly, the viscosity results are correlated with EDOS results and it is found that the 40H2O and 60H2O serve as critical limit for maximum hydration in IBr and ITF2. In accordance with literature, our study has proved that the solubility of IL is mainly governed by the anions. Also, we conclude that anions composed of S and F along with different cations are helpful for designing hydrophobic ILs suitable for specific tasks. Associated content Supporting information. optimized structures of cation and [NTF2] anion, RDFs’ of cations-water and water-water, percentage of water molecules interacting with anions plot, partial DOS plots, List of potential parameters with a graphical representation are included. Notes. The authors declare no competing financial interest. Acknowledgments This project is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan for “High Efficiency Rare Elements Extraction Technology Area, Tohoku Innovative Materials Technology Initiatives for Reconstruction”. We would like to express our sincere thanks to the crew of Centre for Computational Materials Science of the Institute for Materials Research, Tohoku University for their continuous support and help in using the SR16000 supercomputing



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

facilities. VJYS and YK like to thank HPCI, Hokkaido University and Tohoku University computing centers for providing supercomputer for computations (grant ID hp150076). RKZ, VRB, OSS and YK also thank the Russian Megagrant Project (No.14.B25.31.0030) “New energy technologies and energy carriers”.

References: 1. K. R. Seddon, "Ionic liquids: Designer solvents?" in The International George Papatheodorou Symposium: Proceedings, Eds. S. Boghosian, V. Dracopoulos, C.G. Kontoyannis and G.A. Voyiatzis (Institute of Chemical Engineering and High Temperature Chemical Processes, Patras, 1999), 131-135. 2. K. Binnemans, Lanthanides and Actinides in ionic liquids, Chem. Rev. 2007, 107, 2592-2614. 3. T. Welton, Ionic liquids in Catalysis. Coord. Chem. Rev. 2004, 248, 2459-2477. 4. Z. C. Zhang, Catalysis in ionic liquids. Adv. Catal. 2006, 49, 155-237. 5. P. J. Dyson and T.J. Geldbach, Applications of Ionic liquids in synthesis and catalysis, The Electrochemical Society Interface 2007, 50-53. 6. K. Richter, P. S. Campbell, T. Baecker, A. Schimitzek, D. Yaprak and A-V. Mudring, Ionic liquids for the synthesis of metal nanoparticles, Phys. Status Solidi B 2013, 250, 1152–1164. 7. C. M. Gordon, J. D. Holbrey, A.R. Kennedy, K.R. Seddon, Ionic liquid crystals: hexafluorophosphate salts, J. Mat. Chem. 1998, 8, 2627-2636. 8. J. D. Holbrey and K.R. Seddon, The phase behaviour of 1-alkyl-3-methylimidazolium tetrafluoroborates; ionic liquids and ionic liquid crystals, J. Chem. Soc., Dalton Trans. 1999, 2133-2140. 9. R. Hagiwara and Y. J. Ito, Room temperature ionic liquids of alkylimidazolium cations and fluoroanions, Fluorine Chem. 2000, 105, 221-227.


Page 16 of 29

Page 17 of 29

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. E.G. Yanes, S.R. Gratz, M.J.Baldwin, S.E. Robinson and A.M. Stalcup, Capillary Electrophoretic Application of 1-Alkyl-3-methylimidazolium-Based Ionic Liquids, Anal. Chem. 2001, 73, 3838-3844. 11. M. Vaher, M. Koel and M.Kaljurand, Ionic liquids as electrolytes for nonaqueous capillary electrophoresis, Electrophoresis 2002, 23, 426-30. 12. M. L. Dietz and J. A. Dzielawa, Ion-exchange as a mode of cation transfer into roomtemperature ionic liquids containing crown ethers: Implications for the ‘greenness’ of ionic liquids as diluents in liquid-liquid extraction, Chem. Commun. 2001, 2124-2125. 13. A. E. Visser, R. P. Swatloski, W. M. Reichert, R. Mayton, S. Sheff, A. Wierzbicki, J. H. Davis Jr, and R. D. Rogers, Task-specific ionic liquids for the extraction of metal ions from aqueous solutions, Chem. Commun. 2001, 135-136. 14. A.P. Abbott, G. Frisch, J. Hartley, Karl S. Ryder, Processing ofmetals and metal oxides using ionic liquids, Green Chem. 2011, 13, 471–481. 15. T. J. Bell and Y. Ikeda, Efficient extraction of Rh(III) from nitric acid medium using hydrophobic ionic liquid, Dalton Trans. 2012, 41, 4303-4305. 16. D. Villemin and M. A. Didi, Extraction of rare earth and heavy metals, using ionic solvents as extraction medium, Orient. J. Chem. 2013, 29, 1267-1284. 17. J. Kahlen, K. Masuch, K. Leonhard, Modelling cellulose solubilities in ionic liquids using COSMO-RS, Green Chem. 2010, 12, 2172-2181. 18. J. Viell, W. Marquardt, Disintegration and dissolution kinetics of wood chips in ionic liquids, Holtzforschung 2011, 65, 519-525. 19. A. A. Niazi, B. D. Rabideu and A. E. Ismail, Effects of water concentration on the structural and diffusion properties of imidazolium based ionic liquid-water mixtures, J. Phys. Chem. B 2013, 117, 1378-1388. 20. J. F. Wishart, Energy applications of Ionic liquids, Energy Environ. Sci 2009, 2, 956-961.



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

21. J Sayago, F Soavi, Y Sivalingam, F Cicoira, C Santato, Low Voltage Electrolyte-Gated Organic Transistors Making Use of High Surface Area Activated Carbon Gate Electrodes, J. Mat. Chem. C 2014, 2, 5690–5694. 22. H. Zhao, S. Xia, P. Ma, Use of ionic liquids as ‘green’ solvents for extractions, J. Chem. Technol. Biotechnol. 2005, 80, 1089–1096. 23. J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker and R. D. Rogers, Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation. Green Chem. 2001, 3, 156–164. 24. A. E. Visser, R. P. Swatloski, W. M. Reichert, S. T. Griffin and R. D. Roger, Traditional extractants in nontraditional solvents: groups 1 and 2 extraction by crown ethers in roomtemperature ionic liquids. Industrial and Engineering Chemistry Research 2000, 39, 3596–3604. 25. S. Chun, S. V. Dzyuba and R.A. Bartsch, Influence of structural variation in roomtemperature ionic liquids on the selectivity and efficiency of competitive alkali metal salt extraction by a crown ether. Ana. Chem. 2001, 73, 3737–3741. 26. J.H. Davis Jr., Task-specific ionic liquids, Chem. Lett. 2004, 33, 1072–1077. 27. T.V. Hoogerstraete, B. Onghena, K. Binnemans, Homogeneous liquid–liquid extraction of metal ions with a functionalized Ionic liquid, J. Phys. Chem. Lett. 2013, 4, 1659–1663. 28. F. Hannig, G. Kehr, R. Frohlich, G. Erker, Formation of chiral ionic liquids and imidazol-2ylidene metal complexes from the proteinogenic aminoacid L-histidine, J. Organomet. Chem. 2005, 690, 5959–5972. 29. A. Kumar, G. Gupta and S. Srivastava, Functional ionic liquid mediated synthesis (FILMS) of dihydrothiophenes and tacrine derivatives, Green Chem., 2011, 13, 2459–2463. 30. S.V.J. Yuvaraj, O. S. Subbotin, R. V. Belosludov, V.R. Belosludov, K. Kanie, K. Funaki, A. Muramatsu, T. Nakamura, H. Mizuseki, and Y. Kawazoe, J. Mol. Liq. 2014, 200, 232-237.


Page 18 of 29

Page 19 of 29

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


31. C. D. Tran, S. H. De Paoli Lacerda, and D. Oliveria, Absorption of water by roomtemperature ionic liquids: Effect of anions on concentration and state of water, Applied spectroscopy 2003, 57, 152-156. 32. W. Shi, K. Damodaran, H.B. Nulwala, D.R. Luebke, Theoretical and experimental studies of water interaction in acetate based ionic liquids. Phys. Chem. Chem. Phys. 2012, 14, 15897– 15908. 33. M. Kohagen, M. Brehm, Y. Lingschied, R. Giernoth, J. Sangoro, F. Kremer, S. Naumov, C. Iacob, J. Karger, R. Valiullin and B. Kirchner, How hydrogen bonds influence the mobility of imidazolium-based ionic liquids. A combined theoretical and experimental study of 1-n-butyl3-methylimidazolium bromide, J. Phys. Chem. B 2011, 115, 15280-15288. 34. Y. Danten, M.I. Cabaco and M.Besnard, Interaction of water highly diluted in 1-Alkyl-3methyl imidazolium ionic liquids with the PF6- and BF4- anions, J. Phys. Chem. A 2009, 113, 2873-2889. 35. B. L. Bhargava and M. L. Klein, Formation of micelles in aqueous solutions of a room temperature ionic liquid:

a study using coarse grained molecular dynamics, Molecular

Physics 107, 393-401, 2009. 36. H. V. Spohr, and G. N. Patey, The influence of water on the structural and transport properties of model ionic liquids, J. Chem. Phys. 2010, 132, 234510-13. 37. B. L. Bhargava, Y. Yasakaa, and M. L. Klein, Computational studies of room temperature ionic liquid–water mixtures. Chem. Commun. 2011, 47, 6228–6241. 38. T. Mendez-Morales, J. Carrete, O. Cabeza, L.J. Gallego, and L.M. Varela, Dynamics

Simulation

of

the

Structure

and

Dynamics

of

Molecular

Water—1-Alkyl-3-

methylimidazolium Ionic Liquid Mixtures, J. Phys. Chem. B 2011, 115, 6995–7008. 39. T. M. Chang, Liem X. Dang and R. Devanathan and M.Dupuis, Structure and dynamics of N, N-Diethyl-N-methylammonium triflate ionic liquid, Neat and with water, from Molecular dynamics simulations, J. Phys. Chem. A 2010, 114, 12764-12774.



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

40. W. Jiang, Y. Wang, and G. A. Voth, Molecular dynamics simulation nanostructural organization in ionic liquid/water mixtures, J. Phys. Chem. B 2007, 111, 4812-4818. 41. S. Zahn, M. Brehm, M. Brüssel, O. Hollóczki, M. Kohagen, S. Lehmann, F. Malberg, A. Sanmartin Pensado, M. Schöppke, H. Weber, and B. Kirchner, Understanding ionic liquids from theoretical methods. J. Mol. Liq. 2014, 192, 71-76. 42. M. G. Freirea, S. P. M. Venturaa, L. M.N.B.F.Santos, I. M. Marruchoa, J.A.P. Coutinhoa, Evaluation of COSMO-RS for the prediction of LLE and VLE of water and ionic liquids binary system. Fluid Phase Equilibria 2008, 268, 74–84. 43. S. Feng, and G. A. Voth, Molecular dynamics simulations of imidazolium based ionic liquid/water mixtures: Alkyl side chain length and anion effects, Fluid Phase Equilibria, 2010 294, 148-156. 44. L. E. Ficke and J. F. Brennecke, Interactions of ionic liquids and water, J. Phys. Chem B, 2010, 114, 10496-10501. 45. E. Rezabal and T. Schafer, First principles approach to solvation by methylimidazolium based ionic liquids, J. Phys. Chem. B 2013, 117, 553-562. 46. F. U. Shah, S. Glavatskih, D.R. MacFarlane, A. Somers, M. Forsyth, and O. N. Antzukin, Novel halogen-free chelates orthoborate-phosphonium ionic liquids: synthesis and tribophysical properties, Phys. Chem. Chem. Phys. 2011, 13, 12865-12873. 47. Y-L. Wang, F. U. Shah, S. Glavatskih, O. N. Antzutkin and A. Laaksonen, Atomistic insight into orthoborate-based ionic liquids: force field development and evalution, J. Phys. Chem. B 2014, 118, 8711-8723. 48. Y. Fukaya and H. Ohno, Hydrophobic and polar ionic liquids, Phys. Chem. Chem. Phys. 2013, 15, 4066. 49. I. Khan, K. A. Kurnia, F. Mutelet, S. P. Pinho and J.A.P. Coutinho, Probing the interactions between ionic liquids and water: Experimental and Quantum chemical approach, J.Phys. Chem. B 2014, 118, 1848-1860.


Page 20 of 29

Page 21 of 29

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


50. J. K. Konieczny and B. Szefczyk, Structure of Alkylimidazolium-based ionic liquids at the surface with vacuum and water-a molecular dynamics study, J. Phys. Chem. B 119, 37953807 (2015). 51. A.R. Shaikh, E. Kamio, H. Takaba and H. Matsuyama, Effect of water concentration on the free volume of amino acid ionic liquids investigated by molecular dynamics simulations, J. Phys. Chem. B 119, 263-273, 2015. 52. S. Plimpton, Fast Parallel Algorithms for Short-Range Molecular Dynamics, J. Comput. Physics 1995, 117, 1-19. 53. R. W. Hockney, and J.W. Eastwood, Computer Simulation Using Particles, McGraw-Hill, New York 1981. 54. W. Humphery, A. Dalke and K. Schulten, VMD: visual molecular dynamics, J. Mol. Graph. 1996, 14, 33-8. 55. J.N. Canongia Lopes, J. DesChamps, A.A.H Padua, Modeling Ionic Liquids Using a Systematic All-Atom Force Field, J. Phys. Chem. B 2004, 108, 2038-2047. 56. W. Jorgenson, D. Maxwell, and J. Tirado-Rives, J. Am. Chem. Soc. 1996, 118, 1122511236.; OPLS All-Atom Parameters for Organic Molecules, Ions, Peptides & Nucleic Acids, July 2008" as provided by W. L. Jorgensen, Yale University. 57. J. P. Ryckaert, G. Ciccotti, H. J. C Berendenson, Numerical integration of the Cartesian Equations of Motion of a System with Constraints: Molecular Dynamics of n-Alkanes, J. Comput. Phys. 1977, 23, 327-341. 58. J. L. F. Abascala and C. Vega, A general purpose model for the condensed phases of water: TIP4P/2005, J. Chem. Phys. 2005, 123, 234505-12. 59. A. P. Markesteijn, Remco Hartkamp, S. Luding, and J. Westerweel, A comparison of the value of viscosity for several water models using Poiseuille flow in a nano-channel, J. Chem. Phys. 2012, 136, 134104-8.



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

60. J. A. Backer, C. P. Lowe, H. C. J. Hoefsloot, and P. D. Iedema, Poiseuille flow to measure the viscosity of particle model fluids, J. Chem. Phys. 2005, 122, 154503-6. 61. F. Muller-Plathe, Reversing the perturbation in nonequilibrium molecular dynamics: An easy way to calculate the shear viscosity of fluids, Phys. Rev. E 1999, 59, 4894-4898. 62. G. Kresse and J. Hafner, “Ab initio molecular dynamics for liquidmetals,” Phys. Rev. B 1993, 47, 558–561, 1993.; G. Kresse and J. Furthmuller, “Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set,” Phys. Rev. B 1996, 54, 11169–11186. 63. P. E. Blöchl, Projector augmented-wave method, Phys. Rev. B 1994, 50, 17953; G. Kresse, and D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758. 64. G. Henkelman, A. Arnaldsson, and H. Jónsson, A fast and robust algorithm for Bader decomposition of charge density, Comput. Mater. Sci. 36 254-360 (2006). 65. P. D. Angelo, V. Migliorati and L. Guidoni, Hydration properties of the bromide aqua ion:interplay of first principle and classical molecular dynamics and x-ray absorption spectroscopy, Inorg. Chem. 2010, 49 4224-4231. 66. J.M. Crostwaite M.J. Muldoon, J.K. Dixon, J.L. Anderson, J.F. Brennecke, Phase transition and decomposition temperatures, heat capacities and viscosities of pyridinium ionic liquids, J. Chem. Thermodynamics 2005, 37, 559-558. 67. Dortmund Data Bank, URL: http://www.ddbst.com/ddb.html


Page 22 of 29

Page 23 of 29

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


Figures and Tables. Table 1. Simulation cell dimensions of pure and hydrated 10IBr and 10ITF2 systems. System

10IBr

10ITF2

0H2O

18.22x18.23x18.21 (550)* 19.83x19.83x19.82 (690)*

20H2O

18.74x18.73x18.73 (610)

20.05x20.05x20.05 (750)

40H2O

19.23x19.23x19.23 (670)

20.52x2052x20.51 (810)

60H2O

19.59x19.59x19.58 (730)

21.01x21.01x21.01 (870)

80H2O

20.24x20.24x20.49 (790)

21.52x21.53x21.53 (930)

100H2O

20.44x20.24x20.49 (850)

21.98x21.98x21.98 (990)

*Total number of atoms in a supercell

Figure 1. The RDF plots of, a) C2-C2 in IBr-nH2O systems, b) [Br]--[Br]- in IBr-nH2O systems, c) C2-C2 in ITF2-nH2O systems, and d) NA-NA in ITF2-nH2O systems.



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 2. The RDF plots of, a) C2-[Br]- in IBr-nH2O systems, b) NC-[Br]- in IBr-nH2O systems, c) C2-NA in ITF2-nH2O systems, and d) NC- NA in ITF2-nH2O systems.

Figure 3. The RDF plots of, a) [Br]--H(H2O) in IBr-nH2O systems, b) [Br]--O(H2O) in IBr-nH2O systems, c) NA-H(H2O) in ITF2-nH2O systems, and d) NA-O(H2O) in ITF2-nH2O systems.


Page 24 of 29

Page 25 of 29

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 4. Viscosity of pure and hydrated IBr and ITF2.

Figure 5. a). Optimized structure of IBr, and distribution of anions and water molecules in b) IBr-20H2O, c) IBr-40H2O, d) IBr-60H2O, e) IBr-80H2O, and f) IBr-100H2O, respectively. Cations are highlighted in blue color, brown balls represents [Br]- anions, white and red color sticks represent hydrogen and oxygen atoms of water molecules.



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 6. a) Optimized structure of ITF2, and distribution of anions and water molecules in b) ITF2-20H2O, c) ITF2-40H2O, d) ITF2-60H2O, e) ITF2-80H2O, and f) ITF2-100H2O, respectively. Cations are highlighted in magenta color, [NTF2]- anions are shown in green color, white and red color sticks represent hydrogen and oxygen atoms of water molecules.

Figure 7. a) Binding Energy of H2O in all hydrated systems of IBr and ITF2. b) Comparison of partial charge on H2O in all systems. c) Comparison of cat/ani ratio and H2O/anion ratio in IBrnH2O (n=0 to 100). d) Comparison of cat/ani ratio and H2O/anion ratio in ITF2-nH2O (n=0 to 100).


Page 26 of 29

Page 27 of 29

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 8. a) Water molecules surrounding [Br]- anion (Rose – positive charge and yellow – negative charge), and b) Water molecules surrounding [NTF2]- anion ((yellow – positive charge and cyan – negative charge). white, grey, blue, red, brown, green and yellow balls represent hydrogen, carbon, nitrogen, oxygen, bromine, fluorine and sulfur atoms, respectively.

Figure 9. Projected DOS of cations, anions and water molecules in a) IBr, b) IBr-20H2O, c) IBr40H2O, d) IBr-60H2O, e) IBr-80H2O, and f) IBr-100H2O, respectively.



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 10. Projected DOS of cations, anions and water molecules in a) ITF2, b) ITF2-20H2O, c) ITF2-40H2O, d) ITF2-60H2O, e) ITF2-80H2O, and f) ITF2-100H2O, respectively.

Figure 11. VBM and CBM gap in pure and hydrated a) IBr and b) ITF2 systems.


Page 28 of 29

Page 29 of 29

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


Table of Contents (TOC) Image


Solvation Mechanism of Task-Specific Ionic Liquids in Water: A

Recommend Documents