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Understanding Microscopic Behavior of The Mixture of Ionic Liquid/Ethylene Glycol/Lithium Salt Through Time-Resolved Fluorescence, Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) Studies Ranu Satish Dhale, Prabhat Kumar Sahu, and Moloy Sarkar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b04585 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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The Journal of Physical Chemistry

Understanding Microscopic Behavior of The Mixture of Ionic Liquid/Ethylene Glycol/Lithium Salt Through Timeresolved Fluorescence, Nuclear Magnetic Resonance(NMR) and Electron Paramagnetic Resonance (EPR) Studies Ranu Satish Dhale, Prabhat Kumar Sahu, MoloySarkar* School of Chemical Sciences, National Institute of Science Education and Research, HBNI, School of Chemical Sciences, Bhubaneswar, Khurda-752050, India,, India. E-mail:

[email protected]; Fax: : +91-674-2494004; Tel: +91-674-2494190

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ABSTRACT The present study has been undertaken with an aim to find out the suitability of a binary solvent system,

comprising

anionic

liquid,

1-(2-Hydroxyethyl)-3-methylimidazolium

bis

(trifluoromethylsulfonyl)imide ([OHEMIM][NTf2]) and ethylene glycol (EG), towards lithium ion battery applications. For this purpose, the behavior in terms of structure, intermolecular interaction and dynamics of several solvent systems, [OHEMIM][NTf2], [OHEMIM][NTf2]LiNTf2(lithium

bis(trifluoromethylsulfonyl)imide),[OHEMIM][NTf2]-EG

and

[OHEMIM][NTf2]-EG-LiNTf2are investigated by carrying out steady state and time-resolved fluorescence, nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) measurements. Both steady-state fluorescence and EPR studies have pointed out that the micropolarities of [OHEMIM][NTf2]-EG-LiNTf2 is close to that of neat RTIL. However, studies on rotational dynamics have revealed that the structural organization of [OHEMIM][NTf2]LiNTf2 is significantly influenced upon addition of EG. Interestingly, the average solvation timeis observed to be relatively faster in [OHEMIM][NTf2]-EG-LiNTf2 than those in other solvent systems. Since average solvation time and conductivity are inversely related to each other, the present observation indicates that the introduction of EG is helpful in increasing the electrical conductivity of[OHEMIM][NTf2]-EG-LiNTf2. Translational diffusion coefficient measurements in [OHEMIM][NTf2]- EG-LiNTf2and [OHEMIM][NTf2]-LiNTf2 through NMR spectroscopy have also indicatedthe suitability of [OHEMIM][NTf2]-EG-LiNTf2 as a potential electrolytic medium for battery applications.


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1. INTRODUCTION During the past decade, room temperature ionic liquids (RTILs) have been extensively used in a wide range of applications in industries and technology owing to their fascinating physicochemical properties such as low vapor pressure, high viscosity, high thermal stability, large electrochemical window, ability to dissolve a wide range of organic and inorganic compounds etc.1-2In recent times, RTIL-based solvent systems have been shown to have significant potential to be used in various electrochemical applications including lithium ion batteries.3-4It has also been depicted that RTILs are advantageous over conventional organic electrolytes with regard to large electrochemical window, non-flammability and extremely low vapourpressure.4 All these help to reduce the extra risk usually associated with volatile organic electrolytes used in lithium ion batteries.5 In view of these, understanding of the microscopic behavior of lithium ion containing RTIL-based solvent systems in terms of intermolecular interaction, structure and dynamics has been a topic of interest in very recent time.6-7 However, studies on ionic liquid-lithium salt mixtures are limited, and the kinship among structure and dynamics in such solvent systems is yet to be understood completely. Many experimental techniques such as small-angle X-ray scattering (SAXS)8, smallangle neutron scattering (SANS)9, dielectric spectroscopy10,11, NMR12,13, fluorescence,14,15 optical Kerr effect spectroscopy,16,10, and also molecular dynamics (MD) simulation studies17 have been exploited to understand the structure and dynamics of RTILs. Among these, studies based on steady state and time-resolved fluorescence has been proved to be quite useful in probing the structural organisation of RTILs18-20. For example,studies on solute21,22 and solvent dynamics

7,12,23,24

have provided valuable information with regard to the local environment of a

probe molecule and intermolecular interactions in ILs.23,25-27While investigating the rotational 3 ACS Paragon Plus Environment


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dynamics

of

nonpolar

perylenein

1-

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butyl-3-methylimidazolium

bis(trifluoromethyl)sulfonylimide with varying amounts of lithium bis(trifluoromethyl)sulfonylimide through fluorescence anisotropy, Lawler and Fayer have observed a decrease in friction coefficient with increasing lithium salt concentration.18The authors have attributed such behavior towards the structural change in the alkyl region of the RTIL due to lithium ions which are located in the ionic region of the RTIL. Recently, Dutt and coworkers have studied the influence of the length of alkyl chain of ionic liquids and the concentration of the electrolyte i.e. lithium bis(trifluoromethyl)sulfonylimide on the rotational diffusion of a nonpolar solute, 9phenylanthracene.28 The authors have demonstrated that the influence of lithium salt on the structural organization of RTILs with shorter and longer alkyl chains are considerably different.28 In a separate work the same authors have also investigated the influence of lithium salt in several other ILs through rotational relaxation dynamics of nonpolar and ionic solutes.29,30. In addition to these, studies on dynamics of solvation of lithium salts in ILs through molecular dynamics (MD) simulation have also been carried out.31-33 Apart from the above mentioned reports, Pulsed-field gradient spin echo (PGSE) nuclear magnetic resonance (NMR) technique34-39 have been carried out in IL-cosolvent mixtures and IL-Lithium salt mixtures to obtain information about rotational and translational diffusion. Even though most of the above studies have provided some useful insights into the microscopic structurally organization of neat IL-Li salt systems, they have also pointed out that the viscosities of lithium salts-ILs mixtures increase significantly compared to those of neat ionic liquids,18,40,41 which is undesirable in battery applications. One possible way to overcome this problem would be to use a suitable cosolvent along with neat IL in the solvent mixture. The addition of a cosolvent to ILs is shown to have a significant impact on the physicochemical 4 ACS Paragon Plus Environment

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properties of the IL-cosolvent mixtures which in turn can enhance their applicability.4245

Nonetheless, a deep knowledge of the influence of lithium ions on dynamics of either pure ILs

or mixed with other solvents is of great importance for scientific community in order to use this material to its fullest potential.46 To the best of our knowledge, there are no reports on solute and solvation dynamics in IL-cosolvent systems in presence of an electrolyte. Keeping the above facts in mind, we have exploited time-resolved fluorescence, NMR and EPR techniques to get an idea about the intermolecular interaction, structure and dynamics of a binary solvent system comprising a low viscous hydroxyimidazolium cation based RTIL,1(2-Hydroxyethyl)-3-methylimidazolium bis (trifluoromethylsulfonyl)imide ([OHEMIM][NTf2]) and a viscous cosolvent, ethylene glycol (EG), in absence and presence of lithium salt (LiNTf2). The spectroscopic data are also compared with neat IL and its mixture with EG. Essentially the influence of EG on IL-lithium salt mixture is probed at microscopic level. Specifically, solute and solvent dynamics of the solvent systems are carried out by using time-resolved fluorescence measurements to obtain idea about solvent reorganization time scale, microscopic structural organization and intermolecular interactions of the media. EPR spectroscopy has also been employed to determine the polarity of the concerned mixtures. Translational diffusion measurements are done by employing NMR technique. The choice of the ionic liquid is primarily govern by the fact that this type of ionic liquids are known to have lower viscosities, good electrochemical stability and high thermal stability which are of prime importance for battery applications.47-49 In particular,hydroxyimidazolium-based ionic liquids are known to possess superior thermophysical properties in terms of higher thermal stability, higher ionic conductivity and higher electrochemical stabilitythan the other commonly used alkylimidazolium ILs.50,51 Ethylene glycol has been chosen as a cosolvent for its low vapour pressure (0.06 mmHg 5 ACS Paragon Plus Environment


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at 293 K) and broad electrochemical window which are also favorable for battery applications.52 The outcome of the proposed study is expected to provide finer details on the microscopic behavior of the[OHEMIM][NTf2]-EG-LiNTf2 which would be helpful for this solvent system to be used in battery applications. The chemical structure of RTIL isshown in Figure 1 below.

1-(2-Hydroxyethyl)-3-methylimidazolium bis (trifluoro-methylsulfonyl) imide Figure 1. Molecular formula ofRTIL

2. EXPERIMENTAL SECTION 2.1. Materials Coumarin 153 (C153) (laser grade, Exciton), 1-(2-Hydroxyethyl)-3-methylimidazolium bis (trifluoro-methylsulfonyl) imide obtained from Merck and Bis(trifluoromethane) sulfonamide lithium salt, Ethylene glycol and2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) were obtained from Sigma-Aldrich and were used without any further purification. Requisite amounts of different probes were added to prepare the solution. The cuvettes were properly sealed with septum and parafilm to maintain dry conditions. 1:1 mole fraction of [OHEMIM][NTf2]-EG mixture was prepared and 0.5 molal solution of LiNtf2 was added to each of neat RTIL, [OHEMIM][NTf2] and [OHEMIM][NTf2]-EG mixture.

2.2. Instrumentation and methods 6 ACS Paragon Plus Environment

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The absorption and fluorescence spectra of the samples wererecorded using a Beckman Coulter DU 720 spectrophotometer and EdinburghFS5 spectrofluorometer, respectively. Time-resolved fluorescence measurementswere measured using a time-correlated single-photon counting (TCSPC) spectrometer (Edinburgh, OB920). The excitation source was 405 nm picoseconds diode laser (EPL). The analysis of the decay curves were carried out by nonlinear least-squares iteration methods with F900 decay analysis program. The time resolved anisotropy decay measurements of C153in all mixtures were carried out over the temperature range 298−333 Kusing the same TCSPC setup.Analysis of the time-resolved decayprofiles and methodologies adopted to estimateparameters of dynamics of solvation and rotational relaxationhave been mentioned elsewhere.53The NMR studies have been carried out on a 9.4 T BrukerAvance NMR spectrometer with 400.1 MHz Larmor frequencies for 1H. The translational diffusion coefficients, D, were determined by applying Stimulated echo bipolar pulse-gradient pulse (stebpgp) sequence within the temperature range (298−333 K). The echo heights were acquired by variation of gradient pulse strength ( 2% - 95% ) of the maximum gradient pulse strength (50 G/cm) at 16 equal intervals.

The echo heights were fit to the equation S(g) = S(0)exp[−Dγ2δ2g2(Δ − δ/3)]

(1)

where S(g) and S(0) are the echo height at the gradient strength g and 0, respectively. γ and δ are the gyromagnetic ratio of the proton and gradient pulse length respectively, whileΔ is the duration between the two gradient pulses. Experimental for our measurement is ±10%. The hydrodynamic radius for cation of neat IL is calculated using the diffusion coefficient values obtained from PFGSE-NMR measurements. The viscosities of the samples were measured using a LVDV-III Ultra Brookfield Cone and Plate viscometer (experimental error: ±2%). EPR spectra were recorded in Bruker Super QCW-EPR bridge (model A300) X band spectrometer. The 7 ACS Paragon Plus Environment


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sample solutions were transferred into borosilicate glass capillary tubes with internal diameter of 2 mm, which was positioned centrally in the EPR cavity. Simulation of the EPR spectra were carried out through WinEPRSymfonia software (version 1.26 beta).

3.RESULTS AND DISCUSSION 3.1. Steady state spectral measurements The absorption spectra along with the emission spectra of a dipolar solute coumarin 153(C153)in different solvent mixtures are displayed in Figure 2. The absorption and emission maxima of C153

in

neat

[OHEMIM][NTf2],

[OHEMIM][NTf2]-LiNTf2,[OHEMIM][NTf2]-EG

and

[OHEMIM][NTf2]-EG-LiNTf2mixtures are shown in Table 1. As can be seen from Table 1, there is no substantial change in the absorption and emission maximum of C153 upon addition of 1:1 molefraction of EG as co-solvent to ionic liquid. It is also clear for Figure 2 that there is no apparent change in the absorption and emission profile with addition of 0.5 molal of LiNTf2to the RTIL. The very similar emission maximum for C153 in all the mixtures indicates that the polarities of the different mixtures are very similar and the polarity of neat RTIL is not significantly affected by the addition of LiNTf2 as well as EG to it.


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Figure 2. Combined absorption and emission spectra of C153 in [OHEMIM][NTf2] (black), [OHEMIM][NTf2]-EG (blue), [OHEMIM][NTf2]-EG-LiNTf2 (red) and [OHEMIM][NTf2]LiNTf2 (green). The spectra are normalized with respect to the corresponding absorption maxima. Emission profiles shown are recorded by exciting the samples at their corresponding absorption maxima. Table 1: Absorption and emission maxima of C153 in different systems. λmax(abs)[nm]

System

λmax(em)[nm]

[OHEMIM][NTf2]

427

537

[OHEMIM][NTf2]-LiNTf2

429

535

[OHEMIM][NTf2]-EG

428

538

[OHEMIM][NTf2]-EG-

431

540

LiNTf2

The polarities of different solvent systems have also been determined from EPR spectroscopy by analyzing the EPR spectral data of EPR active free radical probe 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO) in the concerned solvent systems.It has been demonstrated that the hyperfine coupling constant (aN) of the EPR signal is closely related to the charge transfer (CT) nature of the free radical probe in a manner that polar solvent stabilizes the CT structure which enhances 9 ACS Paragon Plus Environment


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the spin density on the nitrogen atom of the probe which in turn increases the aN value in polar solvents.54 The EPR spectra of TEMPO in various solvent systems under investigation (along with the corresponding simulated spectra) have been shown in Figure 3. It can be clearly seen from Figure 3 that addition of EG or Li salt or both to the neat IL introduces significant line broadening to the EPR line shapes. The solvent dependent change in the lineshape of EPR spectra clearly indicates the effect of solvent perturbation on the EPR spectra. It is pertinent to mention in this context that recently, Mladenova et. al.55,56 and Akdogan et. al.57 have also observed similar line broadening in the EPR spectra of spin probes (TEMPO and TEMPO derivatives) in various RTILs. The authors have attributed this behavior to the inhomogeneous nature of the medium. Therefore, the present solvent dependent line shape change of the EPR spectral lines also indicate the inhomogeneous nature of the concerned medium. In this scenario, determining aN from peak-to-peak distance of the EPR spectra may be erroneous. Hence, in the present calculations, the aN values have been estimated from the distance between the centers of the adjacent hyperfine lines.


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Figure 3. EPR spectra of TEMPO (red dots) in (a)[OHEMIM][NTf2], (b) [OHEMIM][NTf2]EG, (c) [OHEMIM][NTf2]-LiNTf2and (d) [OHEMIM][NTf2]-EG-LiNTf2. The black dotted lines represent the simulated spectra.

Figure 4 represents the plot of aN vs. ET(30) (empirical polarity parameter) for [OHEMIM][NTf2]-EG-LiNTf2 along with molecular solvents. The ET(30) values of various molecular solvents are taken from literature reports.54,58 From the calibration curve, the unknown ET(30) value for [OHEMIM][NTf2]-EG-LiNTf2 is estimated. The aN values in Gauss and the ET(30) values are summarized in Table 2. Please note that the ET(30) value of neat IL (62.2 kcal/mol) estimated from EPR data is observed to be very close to the reported value (60.8) which is obtained from fluorescence studies.58 In this context it may also be noted here that



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recently Mladenova et. al.55 have demonstrated that micropolarity of a given medium, estimated from EPR data, also depends on nature of spin probes. Another interesting observation from Table 2 is that the ET(30) value of [OHEMIM][NTf2]-LiNTf2 has been found to be higher than that of neat RTIL and its mixture with EG. It has been recently demonstrated that upon addition of lithium salts to ILs, structural organization of ILs considerably changes with the formation of ionic clusters.59 We believe that because of this reason the ET(30) value of [OHEMIM][NTf2]LiNTf2 is observed to be higher than those of the other three RTIL-based systems.

Figure 4. A plot of aN vs ET(30) values. Data for black squires are for molecular solvents (Table 2). Black circles represent values for various molecular solvents from Ref 54.


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Table 2: Properties of TEMPO in [OHEMIM][NTf2]-EG-LiNTf2 mixtures, RTILs and molecular solvents determined by EPR and estimated ET(30) values Solvents

aN /G c

ET (30) / kcal mol-1

[OHEMIM][NTf2]

16.56

62.2

[OHEMIM][NTf2]-EG

16.40

59.8

[OHEMIM][NTf2]LiNTf2

17.01

67.6

[OHEMIM][NTf2]-EGLiNTf2

15.95

53.8

[BIMIM][PF6]

15.99

54.8a

[BIMIM][BF4]

15.90

53.9a

Benzene

14.48

34.5a

Ethylene glycol

16.22

56.3a

1,4 dioxane

14.62

36.0b

Chloroform

14.84

39.1b

Acetone

15.08

42.2b

Ethyl alcohol

15.80

51.9a,b

a

ref. 58, b ref. 60, cvalues determined by EPR

Since many of the physical attributes of RTILs can be linked to the microheterogeneous behavior of the medium, we have tried to understand the microheterogeneous behavior of different solvent systems that are employed in this study by carrying out excitation wavelength dependent fluorescence measurements.27,61,62A steady bathochromic shift in the fluorescence maximum of C153 has been observed in [OHEMIM][NTf2]-EG-LiNTf2 compared to both neat [OHEMIM][NTf2]and [OHEMIM][NTf2]-LiNTf2 (Figure 5), which indicates that these mixtures 13 ACS Paragon Plus Environment


are more structurally heterogeneous than the neat IL. However, it has been observed that the difference in the magnitude of the total shift of the fluorescence maxima of these mixtures is quite significant. For example, with a change in λexc from 380 nm to 440 nm, no significant shift in the fluorescence maximum of the probe has been observed in [OHEMIM][NTf2] and [OHEMIM][NTf2]-LiNTf2 (Figure 5). However, in case of [OHEMIM][NTf2]-EG-LiNTf2 the total shift in the fluorescence maxima is observed to be 8 nm (Figure 5). Since the total shift in the fluorescence maxima (with a change in the excitation wavelengths) can provide qualitative idea about the microheterogeneous medium,

27,61,62

the present data indicates that presence of

EG causes significant changes in the structural organization of [OHEMIM][NTf2]-LiNTf2.. This fact is also evident when we monitor the full width at half maximum (fwhm) of the fluorescence spectrum at different excitation wavelength. For example, in case of in case of [OHEMIM][NTf2]-EG-LiNTf2 the change in fwhm is estimated to be ~500 cm-1with a change in excitation wavelength from 395 nm to 435 nm, whereas no excitation wavelength dependent change in fwhm of fluorescence spectra is observed for other solvent systems.

564

[OHEMIM][NTf2]-LiNTf2 [OHEMIM][NTf2]-EG-LiNTf2

560

[OHEMIM][NTf2]

556

em (nm)

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552

8 nm

548 544 540

1 nm

2 nm

536 380

400

420

440

460

exc (nm) 14 ACS Paragon Plus Environment

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Figure 5. Plots of λem(nm) at [OHEMIM][NTf2],[OHEMIM][NTf2]-LiNTf2 Experimental error: ± 2 nm.

corresponding exc(nm) of C153 in and [OHEMIM][NTf2]-EG-LiNTf2 at298 K.

3.2. Dynamics of Solvation Since, reactivity of chemical ingredients of a chemical reaction (including electron transfer) is intricately related to dynamics of solvation,62,63 it is important to understand the dynamics of solvation in neat RTIL along with its mixtures with EG and LiNTf2. Particularly we would like to know the dynamical aspects of the solvent systems when EG is added to [OHEMIM][NTf2]LiNTf2 mixture. The dynamics of solvation can be monitored by following time-dependent Stokes shift of the emission spectra of a probe solute.63 For this purpose time-resolved emission spectra (TRES) for each of the samples have beenconstructed from the collected decays by following a standard procedure.63 One representative TRES of C153 in [OHEMIM][NTf2]LiNTf2 at different time intervals is shown in Figure 6. The progressive red shift of the emission maxima with time carries the typical signature of solvation process. Table 3 lists the solvation relaxation parameters ofC153 for the various systems. The log-normal fitting of the time resolved emission spectra (TRES) gives the emission maxima (frequencies) which is used to calculate solvent correlation function (C(t))given as

C(t) = where

(2) ,

and

are the emission maxima (frequencies) at times infinity

, zero

(t = 0) and t respectively. The plot of C(t) against t (time) was fitted by a biexponential function as given below. C(t) = a1 e-t/1 + a2 e-t/2

(3)



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where 1 and 2 are the solvent relaxation time and a1 and a2 are normalized preexponential factors. The average solvation time of the observable dynamics was determined by the following relation as = a11 + a22

(4)

Figure 7 shows the plots of the spectral shift correlation function, C(t), versus time for the studied systems.

Figure 6. TRES of C153 in [OHEMIM][NTf2]-LiNTf2 at different times at 298 K (λexc=405nm). All spectra are normalized at their corresponding peak maxima.


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Table 3. Solvent relaxation parameters of C153 in [OHEMIM][NTf2]–EG– LiNTf2 298 K(exc. = 405nm). Systems Viscosity Biexponential Fit (cP)a a1 a2 [ns] [ns] [OHEMIM][NTf2] 57.4 0.61 0.19 0.39 1.09 [OHEMIM][NTf2]-EG 33.2 0.52 0.11 0.48 0.65 [OHEMIM][NTf2]213.4 0.33 2.82 0.67 0.55 LiNTf2 [OHEMIM][NTf2]-EG63.2 0.77 0.23 0.23 1.39 LiNTf2 a Experimental error: ±2%, bExperimental error: ±5%

mixtures at

[ns]b 0.54 0.37 1.31 0.50

Figure 7. Decay of the spectral shift correlation function, C(t) of C153 in different mixtures of RTIL at 298K (λexc. = 405 nm).

From Table 3, it is noticeable that with addition of LiNTf2, the solvation time corresponding to the solvent systems increases. This observation can be attributed to the increase in viscosity of the medium with addition of Li+ salt. It may be highlighted here that the anion of the ILs can surround the Li+ ion which leads to enhancement of the viscosity of the medium.41,63 Similar effect have also been observed in other IL-Li+ systems.28,29,40 Interestingly, we have observed that solvation becomes significantly faster when EG is added to [OHEMIM][NTf2]-LiNTf2 (Table 3). For example, τs has been observed to be ~2.6 times faster in [OHEMIM][NTf2]-EG-LiNTf2 as 17 ACS Paragon Plus Environment


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compared to that in [OHEMIM][NTf2]-LiNTf2. We have also seen that addition of EG has also made solvation dynamics in [OHEMIM][NTf2]-EG mixture faster (~1.5 times) than in neat IL. This lowering of average solvation time upon addition of EG is consistent with the decrease in bulk viscosity of the medium upon addition of EG (Table 3). The observation can be rationalized by considering the fact that the electrostatic attraction between the constituent ions decrease with the addition of co-solvent which reduces the cohesive forces and thereby decrease the bulk viscosity of the medium.This in turn leads to a faster solvation process.14 By looking at the viscosity values and average solvation times in Table 3, one can also get an idea about the trend in conductivities of the different solvent systems under investigation. We know from Nernst-Einstein equation that conductivity (σ) is directly proportional to diffusion coefficient (D) of a mobile species and follow the relation ,

(5)

where n is the number density of charge carriers, e is the electronic charge and k is Boltzmann’s constant. We also know that D is related to viscosity of the medium (η) through Stokes-Einstein equation, (6) Comparing equation 5 and 6 we can say that decrease in viscosity can lead to increase in conductivity of the medium. This conjecture has been further supported by the results of the translational diffusion coefficient (D) measurementsof the cation of the RTILthrough NMR investigations (vide infra). Recently, Maroncelli and coworkers’64 through dielectric continuum model, have shown that the average solvation time is inversely proportional to electrical conductivity of a medium. Considering this results in mind, the fastest solvation time in [OHEMIM][NTf2]-EG-LiNTf2 mixture (Table 3) in the present case certainly indicates the 18 ACS Paragon Plus Environment

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highest electrical conductivity of this mixture among other systems under investigation. Therefore, the outcomes of the above discussions clearly point out that [OHEMIM][NTf2]-EGLiNTf2 mixture can serve as a better electrolytic medium as compared to [OHEMIM][NTf2]LiNTf2 for applications in Li ion batteries. 3.3. Time-resolved fluorescence anisotropy study Time-resolved fluorescence anisotropy technique has been exploited in order to obtain a clearer pictureof the microscopicenvironment surrounding the probe (C153) molecule in different solvent mixtures. A representative time-resolved fluorescence anisotropy decay profile of C153 in [OHEMIM][NTf2]-EG-LiNTf2 is shown in Figure 8. Rotational relaxation times (τr) for C153 in different solvent systems obtained from analysis of the time-resolved fluorescence anisotropy decay profiles of C153, are listed in Table 4. As can be seen from Table 4, τr decreases with increase in temperature for all the systems which is due to the decrease in the bulk viscosity of the medium upon increase in temperature. Table 4: Rotational relaxation parameters of C153 in different mixtures at different temperatures (λexc=405nm). Temperature (K)

Rotational relaxation time (ns)a / Viscosity (cP) [OHEMIM][NTf2]

298 4.18/(57.4) 308 2.75/(36.1) 318 1.79/(24.6) 328 1.29/(18.2) 333 1.11/(15.9) a Experimental error: ± 5%

[OHEMIM][NTf2]- [OHEMIM] LiNTf2 [NTf2]-EG 7.92/(213.4) 5.49/(113.6) 3.66/(65.8) 2.47/(41.5) 2.08/(34.6)

2.30/(33.2) 1.51/(21.8) 1.09/(15) 0.77/(11) 0.67/(9.65)


[OHEMIM] [NTf2]-EGLiNTf2 3.72/(63.2) 2.40/(38.6) 1.67/(30.8) 1.18/(20.4) 1.00/(14.7)


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Figure 8. Time resolved fluorescence anisotropy decay for C153 in [OHEMIM][NTf2]-EGLiNTf2 mixture at 298 K. The solid line in the figure represents the single-exponential fit to the data points. It is also evident from Table 4that with the addition of lithium salt to IL, rotational relaxation becomes slower which can be ascribed to the increased viscosity of the medium in presence of lithium salt.18,22,29 Interestingly, when EG is added to the same mixture, a significant drop in viscosity is noticed. Consequently a faster rotational relaxation is observed in[OHEMIM][NTf2]EG-LiNTf2 compared to that in [OHEMIM][NTf2]-LiNTf2. For example, at 298 K, τr becomes ~ 2 times faster with addition of EG to [OHEMIM][NTf2]-LiNTf2 (Table 4). However, one can also see from Table 4 that while bulk viscosity decreases ~3.4 times upon going from [OHEMIM][NTf2]-LiNTf2to [OHEMIM][NTf2]-EG-LiNTf2, τrincrease ~2 times. Such a disproportionate change in τr values with bulk viscosity values of the media can be ascribed to the differencein microviscosity experienced by the probe in these media. It has been reported that Strong hydrogen bond donors like glycol derivatives are known to bring about significant changes in the magnitude of hydrogen-bonding interactions through formation of new hydrogen bonds or breaking/weakening of existing hydrogen bonds in the pure IL.

30


This change in intermolecular

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interactions is expected to induce change in the structural organization of the media. In light of these facts, we can say that addition of EG to IL+Li mixture will bring about changes in the structural organization and consequently the microenvironment of the medium.

To get a better understanding on the solute-solvent interaction, the results of rotational relaxation dynamics of C153 in different media have been analyzed through Stokes-EinsteinDebye (SED) hydrodynamic theory.65 According to the SED theory, the rotational time constant (τr) of a solute rotating along the axis of the ellipsoid is given by

 r SED 

Vh k BT

(7)

whereVh is the hydrodynamic volume of the solute molecule. Vh = VfC, where Vand f are the vander Waals volume that accounts for the size of the solute (243 Å3) and f is the shape factor (1.5) that takes into consideration its nonspherical nature, respectively. C is the boundary condition parameter (rotational coupling constant) which determines the extent of coupling between the solute and solvent. k, T and ƞ are Boltzmann constant, absolute temperature and viscosity of the medium, respectively. The coupling constant C lies between two limiting boundary conditions: stick and slip. 65 For stick boundary condition C becomes unity and is applicable when the solute molecules are larger in size than the solvent molecules. Value of C less than unity, represent the slip boundary condition when size of the solute molecule is smaller or comparable to that of the solvent molecule. For C153, Cslip = 0.18, V=243, and f=1.5.26 It may be noted that analysis of rotational relaxation data with the help of hydrodynamic theories can provide information on structural organization of a medium at microscopic level.66-69



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Figure 9. The log-log plots of vs. (η/T) showing the stick and slip boundary conditions with black and red line respectively for (A) [OHEMIM][NTf2], (B) [OHEMIM][NTf2]-EG, (C) [OHEMIM][NTf2]-LiNTf2and (D) [OHEMIM][NTf2]-EG-LiNTf2. Experimental error: ±5%.

The analysis of the rotational relaxation data of different solvent systems is demonstrated by loglog plot of rotational relaxation time (r) against temperature reduced viscosity (Figure 9). As is evident from Figure 9, rotational motion of C153 lie close to stick boundary in both neat IL and [OHEMIM][NTf2]-EG. Figure 9 also reveals that addition of Li salt to neat IL brings the rotational motion of C153 within the boundary condition as described by SED theory. Interestingly, Crot values are observed to be significantly lower in [OHEMIM][NTf2]-Li compared to those observed in neat IL (Figure 10). For example, Crot decreases from 0.82 to 0.42 at 298K upon addition of 0.5molal Li salt to neat IL. Significant lowering in friction coefficients (Crot) in the present study perhaps indicate that the structural arrangement in the ionic region is


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significantly perturbeddue to the presence of LiNTf2,and because of this the solute molecule located in these domain experiences lower friction which eventually leads to its faster rotation. A similar explanation has been provided by Lawler and Fayer for the faster rotational diffusion of nonpolar perylene in [BMIM][Tf2N]−LiTf2N mixtures.18 Interestingly, upon addition EG to [OHEMIM][NTf2]-LiTf2N,Crotvalues are observed to increase (Figure 10). As discussed earlier, change in the hydrogen bonding interaction upon addition of EG by may be attributed for the observed behavior. Further, the variation of Crotvalues against temperature is observed to be different for different solvents. This behavior is also indicative of the different microstructural arrangement for different solvents.

Figure 10. Variation of Crot with temperature in different solvent systems. 3.4. Translational diffusion study through NMR investigation To throw more light on the microscopic structure of the IL and IL-mixtures, pulsed field gradient(PFG)-NMR technique is also employed for the measurement of self-diffusion coefficient of the cation of the IL in different mixtures. Note that PFG-NMR is a robust 23 ACS Paragon Plus Environment


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noninvasive approach for the measurement of translational diffusion, in a given set of conditions and with the assumption that the molecule is spherical.70

Figure 11. Temperature dependent variation of the self-diffusion coefficients of [OHEMIM]+ in [OHEMIM][NTf2], [OHEMIM][NTf2]-LiNTf2, [OHEMIM][NTf2]-EG and [OHEMIM][NTf2]EG-LiNTf2.

The diffusion coefficient, D, are measured by fitting the data with the Stejskal–Tanner equation.71 The data show that diffusion coefficient values of [OHEMIM]+ in all the systems increase with increase in the temperature which can be attributed to the decrease in viscosity of the medium with increase in the temperature. Upon addition of Li salt to neat IL, D for [OHEMIM]+ has been observed to be slower in [OHEMIM][NTf2]-LiNTf2 (Figure 11). However, it has also been observed that addition of EG to [OHEMIM][NTf2]-LiNTf2 leads to increase of D of [OHEMIM]+. It is also evident from Figure 11 that the rate of increase of D with respect to temperature

is

higher

in

[OHEMIM][NTf2]-EG-LiNTf2

as

compared

to

that

in

[OHEMIM][NTf2]-LiNTf2. From these results, it can be inferred that the rate of translational diffusion of [OHEMIM]+ is increased upon mixing of EG to [OHEMIM][NTf2]-LiNTf2. Since it 24 ACS Paragon Plus Environment

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is known that D is directly proportional to conductivity of the medium, the highest value of D in [OHEMIM][NTf2]-EG-LiNTf2 indicates the higher conductivity of this system. The above observations clearly suggest that addition of EG to [OHEMIM][NTf2]-LiNTf2 increases the transport properties of ions. The present behavior of the solvent system is expected to be helpful for this system to be used in battery applications. In this context we would also like to note that addition of EG to IL+Li mixture will also bring about changes in microscopic behavior of the solvent system in terms of intermolecular interactions, structure and dynamics.

3.5.Arrhenius behavior Activation energies for rotational, translational diffusion and viscous flow of the medium are estimated for solvent system to obtain information about themicroenvironment of the diffusing species in a particular medium. Activation energies of viscous flow, rotational and translational diffusion have been calculated from Arrhenius equation. Here, we would like to note that the temperature dependence of viscosity of the IL and IL-mixtures under investigation also follow the Vogel-Tamman-Fulcher (VTF) relationship (Figure 12).72



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Figure 12. Variation of bulk viscositieswith temperature for theneat IL along with other solvent systems.

The temperature dependence of the viscous flow can be fitted by using the Arrhenius equation (eq8). expE/RT

(8)

where, Eis the activation energy for viscous flow, is the bulk viscosity of the medium, R is the gas constant, T is the absolute temperature in Kelvin, and is the preexponential factor. Similarly the temperature dependence of the diffusive flow of [OHEMIM]+ can also be fitted by using the Arrhenius equation (eq.9) D[IL]+ = D0,[IL]+expED,[IL]+/RT where,

D[IL]+is

the

(9) measured

self-diffusion

coefficientsof

[OHEMIM]+,

D0,[IL]+is

thepreexponential factor and ED,[IL]+ is the activation energy.73

Figure 13. Arrhenius plots of the viscosity for [OHEMIM][NTf2], [OHEMIM][NTf2]-LiNTf2, [OHEMIM][NTf2]-EG and [OHEMIM][NTf2]-EG-LiNTf2.


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Figure 14. Arrhenius plots of the (a) self-diffusion coefficients for [OHEMIM]+and (b)rotational relaxation of C153in [OHEMIM][NTf2]-LiNTf2 and [OHEMIM][NTf2]-EG-LiNTf2. The Arrhenius plots of the viscosity and self-diffusion coefficients of [OHEMIM]+ for [OHEMIM][NTf2]-LiNTf2 and [OHEMIM][NTf2]-EG-LiNTf2 are shown in Figure 13

and

Figure 14 respectively and the calculated values are collected in Table 5. One can see from Table 5 that there is 1.3 fold reduction in the activation energy of viscous flow in case of [OHEMIM][NTf2]-EG-LiNTf2 as compared to [OHEMIM][NTf2]-LiNTf2 which indicates that presence of EG decreases the viscous flow.However, there is very small difference in the magnitude of their activation energy in terms of diffusive flow (Table 5). It is also evident from Table 5 that Eη and ED for [OHEMIM]+differ quite significantly in [OHEMIM][NTf2]-EGLiNTf2. This observation also indicates that the microenvironment of the diffusing species, [OHEMIM]+, in [OHEMIM][NTf2]-LiNTf2and [OHEMIM][NTf2]-EG-LiNTf2, is different. Figure 13(b) shows the Arrhenius plots for the rotational relaxation time of C153 in two systems. Activation energies for rotational relaxation of C153 (Er) in [OHEMIM][NTf2]-LiNTf2and [OHEMIM][NTf2]-EG-LiNTf2 have been observed to be very close (13.85 kJ/moland 13.36 kJ/mol respectively). Moreover, close values of Eand Er in [OHEMIM][NTf2]-EG-LiNTf2 (14.0 27 ACS Paragon Plus Environment


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kJ/mol and 13.36 kJ/mol) indicate that the variation of rotational relaxation and bulk viscosity with temperature are more linearly correlated in [OHEMIM][NTf2]-EG-LiNTf2 than that in [OHEMIM][NTf2]-LiNTf2. Table 5. Activation energy of viscous flow (Eand diffusive flow of [IL]+ (ED,[IL]+ ) in various systems. The values of correlation coefficient, r2 are given in the parenthesis. Systems

E/kJ mol-1 (r2)

ED,[IL]+ /kJ mol-1 (r2)

Er/kJ mol-1 (r2) for C153

[OHEMIM][NTf2]-

18.7 (0.996)

18.4 (0.997)

13.85(0.998)

14.0 (0.972)

19.1 (0.984)

13.36(0.999)

LiNTf2 [OHEMIM][NTf2]-EGLiNTf2

3.7. EPR Studies EPR studies have also been carried out to further probe the microenvironment of the solvent systems by investigating rotational correlation coefficient (r) of an EPR active probe TEMPO in different solvent systems. We have analyzed the EPR spectral data and determined r of TEMPO in neat IL and its mixtures.r has been estimated by following the method reported by Bales et.al.74,75 by invoking correction for the inhomogeneous broadening of the EPR lines. The uncorrected r can be determined from the following equation

(10) where H(0), I0, I+1, I-1 are width of the central EPR line, intensities of central, right and left EPR line respectively. Corrected B can be calculated by following equation (equation 11). (11) 28 ACS Paragon Plus Environment

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In the above relation (12) and (13) In equation 12 and 13, the term χ is the Voigt parameter determined by four point line shape analysis method developed by Bales.76 Correctedr can then be calculated as per the relation r = 1.27 × 10-9B(corr)

(14)

From r rotational diffusion coefficient D has been estimated by Debye-Stokes-Einstein relationship as given below.65 In this equation, r =

(15)

Table 6. Rotational correlation coefficient and rotational diffusion coefficient parameters of TEMPO in [OHEMIM][NTf2]-EG-LiNTf2 mixtures at 293 K as determined from EPR measurements Solvents [OHEMIM][NTf2] [OHEMIM][NTf2] -LiNTf2 [OHEMIM][NTf2] -EG-LiNTf2

r /ns 0.22 0.49 0.30

D (ns-1) 0.76 0.34 0.55

Table 6 enlists the rotational correlation coefficient and rotational diffusion coefficient parameters as derived from EPR measurements for [OHEMIM][NTf2]-EG-LiNTf2 mixtures. The data reveal that r of TEMPO in [OHEMIM][NTf2]-LiNTf2 is much higher than that in neat IL which perhaps happen due to the increase in viscosity of the IL-Li mixture upon addition of Li salt. Interestingly, rotational diffusion has been observed to be much faster (by ~4 times) in [OHEMIM][NTf2]-EG-LiNTf2 as compared to [OHEMIM][NTf2]-LiNTf2. This observation indicates that the structural organization of [OHEMIM][NTf2]-LiNTf2 is significantly influenced 29 ACS Paragon Plus Environment


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upon addition of EG to the mixture. The above findings are in line with the resultsthat are obtained from time-resolved fluorescence anisotropy studies. Please note that that the difference in τr valuesfor the two methods is due to the difference in the size of the solute probes. The above discussion supports our claim that [OHEMIM][NTf2]-EG-LiNTf2 can serve as a better solvent system than [OHEMIM][NTf2]-LiNTf2 for battery applications where faster diffusion of ionic species is preferable. 5. CONCLUSION The present report delineates the outcome of a combined time-resolved fluorescence, NMR and EPR investigations on a binary solvent mixture comprising ofalow viscous ionic liquid, [OHEMIM][NTf2] and viscous ethylene glycol (EG) inpresence and absenceof lithium salt.The work has been carried out keeping in mind the fact that findings of the present investigations would help to throw light on the suitability of this mixture towards lithium ion battery applications. While steady state fluorescence and EPR studies reveal that polarity of several solvents systems employed in this study are similar, investigation on rotational dynamics through both time resolved fluorescence and EPR spectroscopy have revealed that the structural organization of neat[OHEMIM][NTf2],[OHEMIM][NTf2]-LiNTf2and [OHEMIM][NTf2]-EGLiNTf2are different. This indicate that the influence of EG is quite significant towards modulating the structural organization of the media.Moreover, this study also reveals that solvent system containing EG offer lower friction to the rotating solutes due to the lowering of the viscosity of the medium. Solvation dynamics data reveal the shorter average solvation time for [OHEMIM][NTf2]-EG-LiNTf2 as compared to those in other solvent systems. Since,average solvation time and conductivity is inversely related, this observation clearly points out the suitability of

[OHEMIM][NTf2]-EG-LiNTf2

system

for


battery applications.

Further

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investigations based on translational diffusion coefficient measurements also support that [OHEMIM][NTf2]-EG-LiNTf2system has the potential to be used in

battery applications.

Essentially this study reveals that instead of using only neat ionic liquid the use of a suitablebinary mixture, comprisingneat RTIL and a cosolvent whose thermophysicalproperties are similar to the ionic liquid, can be useful in overcoming the problem that arises due to the increase in viscosities upon addition of Li ionsto neatRTIL.

Acknowledgements This work was supported by the Science & Engineering Research Board (SERB), New Delhi, (award no. EMR/2016/000850). R. S. D. thanks NISER for infrastructure and P.K.S thanks CSIR for fellowship. M. S. thanks National Institute of Science Education and Research (NISER), Bhubaneswar for infrastructure facilities.

REFERENCES 1.

Freemantle, M. New frontiers for ionic liquids. Chem. Eng. News 2007, 85, 23−26.

2.

Nanda, R. Unusual linear dependency of viscosity with temperature in ionic liquid/water mixtures. Phys. Chem. Chem. Phys. 2016, 18, 25801–25805.

3.

McEwen, A. B.; Ngo, H. L.; LeCompte, K.; Goldman, J. L. Electrochemical properties of imidazolium salt electrolytes for electrochemical capacitor applications. J. Electrochem. Soc. 1999, 146, 1687–1695.

4.

Kim,

K.;

Cho,

Y.

H.;

Shin,

H.

C.

1-Ethyl-1-methyl

piperidinium

bis(trifluoromethanesulfonyl)imide as a co-solvent in Li-ion batteries. J. Power Sources 2013, 225, 113–118. 5.

Arbizzani, C.; Gabrielli, G.; Mastragostino, M. Thermal stability and flammability of 31 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 32 of 41

electrolytes for lithium-ion batteries. J. Power Sources 2011, 196, 4801–4805. 6.

Bayley, P. M.; Best, A. S.; MacFarlane, D. R.; Forsyth, M. Transport properties and phase behaviour in binary and ternary ionic liquid electrolyte systems of interest in lithium batteries. ChemPhysChem 2011, 12, 823–827.

7.

Zhang, X.; Liang, M.; Hunger, J.; Buchner, R.; Maroncelli, M. Dielectric relaxation and solvation dynamics in a prototypical ionic liquid + dipolar protic liquid mixture : 1 ‑ butyl-3-methylimidazolium tetra fluoroborate + water. J. Phys. Chem. B 2013, 117, 15356–15368.

8.

Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R.; Xiao, D.; Hines, L. G. Jr.; Bartsch, R.; Quitevis, E. L.; Pleckhova, N.; Seddon, K. R. Morphology and intermolecular dynamics of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ionic liquids: Structural and dynamic evidence of nanoscale segregation. J. Phys.: Condens. Matter 2009, 21, 424121.

9.

Atkin, R. and Warr, G. G. The smallest amphiphiles: Nanostructure in protic roomtemperature ionic liquids with short alkyl groups. J. Phys. Chem. B 2008, 112, 4164–4166.

10. Turton, D. A.; Hunger, J.; Stoppa, A.; Hefter, G.; Thoman, A.; Walther, M.; Buchner, R.; Wynne, K. Terahertz dynamics of ionic liquids from a combined dielectric relaxation, terahertz, and optical Kerr effect study: evidence for mesoscopic aggregation. J. Am. Chem. Soc. 2009, 131, 11140–11146. 11.

Ploetz, E. A.; Bentenitis, N.; Smith, P. E. Kirkwood-Buff integrals for ideal solutions. J. Chem. Phys. 2010, 132, 164501.

12.

Imanari, M.; Uchida, K.-I.; Miyano, K.; Seki, H.; Nishikawa, K. NMR study on relationships between reorientational dynamics and phase behaviour of room-temperature


Page 33 of 41

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


ionic liquids: 1-alkyl-3-methylimidazolium cations. Phys. Chem. Chem. Phys. 2010, 12, 2959–67. 13.

Chiappe, C. Nanostructural organization of ionic liquids: Theoretical and experimental evidences of the presence of well defined local structures in ionic liquids. Monatshefte fur Chemie 2007, 138, 1035–1043.

14.

Pramanik, R.; Rao, V. G.; Sarkar, S.; Ghatak, C.; Setua, P.; Sarkar, N. To probe the interaction of methanol and acetonitrile with the ionic liquid n,n,n-trimethyl-n-propyl ammonium bis- (trifluoromethanesulfonyl) imide at different temperatures by solvation dynamics study. J. Phys. Chem. B 2009, 113, 8626−8634.

15.

Chakrabarty, D., Chakraborty, A., Seth, D. & Sarkar, N. Effect of water, methanol, and acetonitrile on solvent relaxation and rotational relaxation of coumarin 153 in neat 1hexyl-3-methylimidazolium hexafluorophosphate. J. Phys. Chem. A 2005, 109, 1764– 1769.

16.

Nicolau, B. G.; Sturlaugson, A.; Fruchey, K.; Ribeiro, M. C. C.; Fayer, M. D. Room temperature ionic liquid-lithium salt mixtures: Optical kerr effect dynamical measurements. J. Phys. Chem. B 2010, 114, 8350−8356

17.

Deshpande, A.; Kariyawasam, L.; Dutta, P.; Banerjee, S. Enhancement of lithium ion mobility in ionic liquid electrolytes in presence of additives. J. Phys. Chem. C 2013, 117, 25343–25351.

18.

Lawler, C. and Fayer, M. D. The influence of lithium cations on dynamics and structure of room temperature ionic liquids. J. Phys. Chem. B 2013, 117, 9768–9774.

19.

Das, S. K.; Sahu, P. K. and Sarkar, M. Diffusion − viscosity decoupling in solute rotation and solvent relaxation of coumarin153 in ionic liquids containing fluoroalkylphosphate (



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 34 of 41

fap ) anion : A thermophysical and photophysical study. J. Phys. Chem. B 2013, 117, 636– 647. 20.

Kumar Das, S. and Sarkar, M. Studies on the solvation dynamics of coumarin 153 in 1ethyl-3- methylimidazolium alkylsulfate ionic liquids: Dependence on alkyl chain length. ChemPhysChem 2012, 13, 2761–2768.

21.

Prabhu, S. R. and Dutt, G. B. Rotational diffusion of organic solutes in 1-methyl-3octylimidazolium tetrafluoroborate−diethylene glycol mixtures: influence of organic solvent on the organized structure of the ionic liquid. J. Phys. Chem. B 2014, 118, 5562– 5569.

22.

Prabhu, S. R. and Dutt, G. B. Rotational diffusion of nonpolar and ionic solutes in 1-alkyl3-methylimidazolium tetrafluoroborate-LiBF4 mixtures: Does the electrolyte induce the structure-making or structure-breaking effect? J. Phys. Chem. B 2015, 119, 15040–15045.

23.

Paul, A. and Samanta, A. Effect of nonpolar solvents on the solute rotation and solvation dynamics in an imidazolium ionic liquid. J. Phys. Chem. B 2008,112, 947–953.

24.

Daschakraborty, S. and Biswas, R. Stokes shift dynamics in ( ionic liquid + polar solvent ) binary mixtures : Composition dependence. J. Phys. Chem. B 2011, 115, 4011–4024.

25.

Ito, N.; Arzhantsev, S.; Heitz, M.; Maroncelli, M. Solvation dynamics and rotation of Coumarin 153 in alkylphosphonium ionic liquids. J. Phys. Chem. B 2004, 108, 5771– 5777.

26.

Ito, N.; Arzhantsev, S.; Maroncelli, M. The probe dependence of solvation dynamics and rotation in the ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate. Chem. Phys. Lett.2004, 396, 83–91.

27.

Samanta, A. Solvation dynamics in ionic liquids : What we have learned from the dynamic


Page 35 of 41

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


fluorescence stokes shift studies. J. Phys. Chem. Lett. 2010, 1, 1557–1562. 28.

Prabhu, S. R.and Dutt, G. B. How does the alkyl chain length of an ionic liquid influence solute rotation in the presence of an electrolyte? J. Phys. Chem. B 2016, 120, 13118– 13124.

29.

Prabhu, S. R.and Dutt, G. B. Does addition of an electrolyte influence the rotational diffusion of nondipolar solutes in a protic ionic liquid? J. Phys. Chem. B 2015, 119, 6311– 6316.

30.

Prabhu, S. R. and Dutt, G. B. Effect of low viscous nondipolar solvent on the rotational diffusion of structurally similar nondipolar solutes in an ionic liquid. J. Phys. Chem. B 2015, 119, 2019–2025.

31.

Varela, L. M.; Mendez-Morales, T. Carrete, .

o mez-

onzalez,

. Docampo-A

lvarez, B.; Gallego, L. J.; Cabeza, O.; Russina, O. Solvation of molecular cosolvents and inorganic salts in ionic liquids: a review of molecular dynamics simulations. J. Mol. Liq. 2015, 210, 178−188. 32.

endez- orales, T. Carrete, . Cabeza,

.

ussina,

. Triolo, A.; Gallego, L. J.;

Varela, L. M. Solvation of lithium salts in protic ionic liquids: A molecular dynamics study. J. Phys. Chem. B 2014, 118, 761−770. 33.

Russina,O.; Caminiti,R.; Méndez-Morales,T.; Carrete,J.; Cabeza,O.; Gallego,L. J.; Varela, L. M.; Triolo, A.. How does lithium nitrate dissolve in a protic ionic liquid? J. Mol. Liq.2015, 205, 16–21.

34.

Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Usami, A.; Mita, Y.; Kihira, N.; Watanabe, M.; Terada, N. Lithium secondary batteries using modified-imidazolium roomtemperature ionic liquid. J. Phys. Chem. B 2006, 110, 10228−10230.



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

35.

Page 36 of 41

Hayamizu, K.; Tsuzuki, S.; Seki, S.; Umebayashi, Y. Nuclear magnetic resonance studies on the rotational and translational motions of ionic liquids composed of 1-ethyl-3methylimidazolium

cation

and

bis(trifluoromethanesulfonyl)amide

and

bis(fluorosulfonyl)amide anions and their binary systems including lithium salts. J. Chem. Phys.2011, 135, 84505. 36.

Nanda, R. Thermal dynamics of lithium salt mixtures of ionic liquid in water by PGSE NMR spectroscopy. RSC Adv. 2016,6, 36394–36406.

37.

Castiglione, F.; Ragg, E.; Mele, A.; Appetecchi, G. B.; Montanino, M.; Passerini, S. Molecular environment and enhanced diffusivity of li+ ions in lithium-salt-doped ionic liquid electrolytes. J. Phys. Chem. Lett. 2011, 2, 153−157

38.

Marekha, B. A.; Kalugin, O. N.; Bria, M.; Buchner, R.; Idrissi, A. Translational diffusion in mixtures of imidazolium ils with polar aprotic molecular solvents. J. Phys. Chem. B 2014, 118, 5509−5517.

39.

Kumar, P., Nanda, R., Seth, S., Ghosh, A. & Sarkar, M. Nuclear magnetic resonance , fluorescence correlation spectroscopy and time-resolved fluorescence anisotropy studies of intermolecular interactions in bis ( 1-methyl-1H-imidazol-3-ium-3-yl ) dihydroborate bis ( trifluoromethylsulfonyl ) amide and its mixtures with various cosolvents. Chem. Phys. Lett. 2016, 661, 100–106.

40.

Takada, A., Imaichi, K., Kagawa, T.; Takahashi, Y. Abnormal viscosity increment observed for an ionic liquid by dissolving lithium chloride. J. Phys. Chem. B 2008, 112, 9660–9662.

41.

Monteiro, M. J., Bazito, F. F. C., Siqueira, L. J. a, Ribeiro, M. C. C. & Torresi, R. M. Transport coefficients, raman spectroscopy, and computer simulation of lithium salt


Page 37 of 41

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


solutions in an ionic liquid. J. Phys. Chem. B 2008, 112, 2102–2109. 42.

Hough, W. L. and Rogers, R. D. Ionic liquids then and now: From solvents to materials to active pharmaceutical ingredients. Bull. Chem. Soc. Jpn. 2007, 80, 2262–2269.

43.

Sun, L.; Morales-collazo, O.; Xia, H.; Brennecke, J. F. Effect of structure on transport properties (viscosity, ionic conductivity, and self-diffusion coefficient) of aprotic heterocyclic anion (AHA) room-temperature ionic liquids. 1. Variation of anionic species. J. Phys. Chem. B 2015, 119, 15030–15039.

44.

Canongia Lopes, J. N.; Gomes, M. F. C.; Husson, P.; Padua, A. A. H.; Rebelo, L. P. N.; Sarraute, S.; Tariq, M. Polarity, viscosity, and ionic conductivity of liquid mixtures containing [C4C1im][NTf2] and a molecular component. J. Phys. Chem. B 2011, 115, 6088−6099.

45.

Kalugin, O. N.; Voroshylova, I. V.; Riabchunova, A. V.; Lukinova, E. V.; Chaban, V. V. Conductometric study of binary systems based on ionic liquids and acetonitrile in a wide concentration range. Electrochim. Acta 2013, 105, 188–199.

46.

Kühnel, R.-S. and Balducci, A. Lithium ion transport and solvation in n-butyl-nmethylpyrrolidinium bis(triﬂuoromethanesulfonyl)imide − propylene carbonate mixtures. J. Phys. Chem. C2014, 118, 5742–5748.

47.

Macfarlane, D. R.; Forsyth, M.; Howlett, P. C.; Pringle, J. M.; Sun, J.; Annat, G.; Neil, W.; Izgorodina, E. I. Ionic liquids in electrochemical devices and processes: managing interfacial electrochemistry. Acc. Chem. Res. 2007, 40, 1165−1173.

48.

MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D.; Davis, J. H., Jr; Watanabe, M.; Simon, P.; Angell, C. A. Energy applications of ionic liquids. Energy Environ. Sci. 2014, 7, 232−250



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

49.

Page 38 of 41

Lewandowski, A. and Świderska-Mocek, A. Ionic liquids as electrolytes for Li-ion batteries-An overview of electrochemical studies. J. Power Sources 2009, 194, 601–609 .

50.

Yeon, S. H.; Kim, K. S.; Choi, S.; Lee, H.; Kim, H. S.; Kim, H.

Physical and

electrochemical properties of 1-(2-hydroxyethyl)-3-methyl imidazolium and N-(2hydroxyethyl)-N-methyl morpholinium ionic liquids. Electrochim. Acta 2005, 50, 5399– 5407. 51.

Allen, Jr. M. H.; Wang, Sharon; Hemp, Sean T.; Chen, Ying; Madsen, Louis A.; Winey, Karen I.; Timothy, E. Long. Hydroxyalkyl-containing imidazolium homopolymers: correlation of structure with conductivity. Macromolecules 2013, 46, 3037–3045.

52.

da Silva, F. T.; Lima, D. W.; Becker, M. R.; de Souza, R. F.; Martini, E. M. A. Transport properties of binary solutions containing the ionic liquid BMIM BF4 and ethylene glycol. J. Braz Chem Soc. 2015, 26, 2125–2129.

53.

Sahu, P. K., Das, S. K. and Sarkar, M. Fluorescence response of a dipolar organic solute in a dicationic ionic liquid ( IL ): Is the behavior of dicationic IL different from that of usual monocationic IL? Phys. Chem. Chem. Phys. 2014, 16, 12918–12928.

54. Kawai, A; Hidemori, T; Shibuya, K. Polarity of room-temperature ionic liquid as examined by epr spectroscopy. Chem. Lett. 2004, 33,1464-65. 55. Mladenova, B. Y.; Katting, D. R.; Gramp, G. Room-temperature ionic liquids discerned via nitroxyl spin probe dynamics. J. Phys. Chem. B 2011, 115, 8183–8198. 56. Mladenova, B. Y.; Chumakova,N. A.; Pergushov,V. I.; Kokorin, A. I.; Gramp, G. Katting, D. R. Rotational and translational diffusion of spin probes in room-temperature ionic liquids. J. Phys. Chem. B 2012, 116, 12295−12305. 57.

Akdogan,Y.; Heller, J.; Zimmermannb, H.; Hinderberger,D. The solvation of nitroxide


Page 39 of 41

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


radicals in ionic liquids studied by high-field EPR spectroscopy. Phys. Chem. Chem. Phys. 2010, 12, 7874–7882 58.

Dzyuba, S. V.; Bartsch, R. A. Expanding the polarity range of ionic liquids. Tetrahedron Lett. 2002, 43, 4657−4659.

59.

Le, M. L. P.; Alloin, F.; Strobe, P.; Leprêtre, J-C.; del Valle, C. P.; Judeinstein, P. Structure−properties relationships of lithium electrolytes based on ionic liquid. J. Phys. Chem. B 2010, 114, 894-903.

60. Reichardt, C. Empirical parameters of solvent polarity as linear free-energy relationships. Angew Chem. Int. Ed. Engl. 1979, 18, 98-110. 61.

Mandal, P. K., Sarkar, M. and Samanta, A. Excitation-wavelength-dependent fluorescence behavior of some dipolar molecules in room-temperature ionic liquids. J. Phys. Chem. A 2004, 108, 9048–9053.

62.

Samanta, A. Dynamic stokes shift and excitation wavelength dependent fluorescence of dipolar molecules in room temperature ionic liquids. J. Phys. Chem. B 2006, 110, 13704– 13716.

62.

Maroncelli, M. and Fleming, G. R. Picosecond solvation dynamics of coumarin 153: The importance of molecular aspects of solvation. J. Chem. Phys.1987, 86, 6221.

63.

Umebayashi, Y.; Hamano, H.; Seki, S.; Minofar, B.; Fujii, K.; Hayamizu, K.; Tsuzuki, S.; Kameda, Y.; Kohara, S.; Watanabe, M. Liquid structure of and Li + ion solvation in bis(trifluoromethanesulfonyl)amide

based

ionic

liquids

composed

of

1-ethyl-3-

methylimidazolium and n-methyl-n-propylpyrrolidinium cations. J. Phys. Chem. B 2011, 115, 12179−12191. 64.

Zhang, X., Liang, M., Ernsting, N. P. and Maroncelli, M. Conductivity and solvation



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 40 of 41

dynamics in ionic liquids. J. Phys. Chem. Lett. 2013, 4, 1205–1210. 65.

Hu, C. and Zwanzig, R. Rotational friction coefficients for spheroids with the slipping boundary condition. J. Chem. Phys. 1974, 60, 4354.

66.

Daschakraborty, S.; Biswas, R. Dielectric relaxation in ionic liquid/dipolar solvent binary mixtures: A semi-molecular theory. J. Chem. Phys. 2016, 144, 104505.

67.

Guchhait, B.; Al, H.; Gazi, R.; Kashyap, H. K.; Biswas, R. Fluorescence spectroscopic studies of ( acetamide + sodium / potassium thiocyanates ) molten mixtures : Composition and temperature dependence. J. Phys. Chem. B 2010, 114, 5066–5081.

68.

Pal, T. and Biswas, R. Heterogeneity and viscosity decoupling in ( acetamide + electrolyte ) molten mixtures : A model simulation study. Chem. Phys. Lett. 2011, 517, 180–185.

69. Das, S. K.; Sahu, P. K.; Sarkar, M. Diffusion−viscosity decoupling in solute rotation and solvent relaxation of coumarin153 in ionic liquids containing fluoroalkylphosphate (fap) anion: a thermophysical and photophysical study. J. Phys. Chem. B 2013, 117, 636−647. 70.Damodaran, K. Recent NMR Studies of Ionic Liquids, Annual Reports on NMR Spectroscopy 2016, 88, 215. 71. Stejskal, E.O.; Tanner, J.E.Spin Diffusion Measurements: Spin Echoes in the Presence of a Time‐ Dependent Field Gradient,J. Chem. Phys.1965, 42, 288–292. 72. Costa, A. . L.

speranca, .

. S. S.

arrucho, .

.

ebelo, L. P. N. Densities and

viscosities of 1-ethyl-3-methylimidazolium n-alkyl sulfates. J. Chem. Eng. Data2011, 56, 3433−3441. 73.

Evans, R. G.; Klymenko, O. V.; Price, P. D.; Davies, S. G.;Hardacre, C.; Compton, R. G. A comparative Electrochemical Study ofDiffusion in Room Temperature Ionic Liquid Solvents VersusAcetonitrile. ChemPhysChem2005, 6, 526−533.


Page 41 of 41

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


74. Bales, B. L.; Persson, K. EPR Study of an Associative Polymer in Solution :Determination of Aggregation Number and Interactions with Surfactants J. CHEM. SOC. FARADAY TRANS., 1995, 91(17), 2863-2870. 75.

Bales, B. L.; Stenland, C. Statistical Distributions and Collision Rates of Additive Molecules in Compartmentalized Liquids Studied by EPR Spectroscopy. 1. Sodium Dodecyl Sulfate Micelles, 5-Doxylstearic Acid Ester,and Cobalt (II). J. Phys. Chem. 1993, 97, 3418-3433.

76.

Bales, B. L. Correction for Inhomogeneous Line Broadening in Spin Labels, II. J. MAG. RES.1982,48, 418-430.

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