pubs.acs.org/jced
Article
Understanding the Molecular-Level Structure and Dynamics of
Sodium Ions in Water in Ionic Liquid Electrolytes by Molecular
Dynamics Simulations
Shrayansh Gupta and Praveenkumar Sappidi*
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sı Supporting Information
*
ABSTRACT: In this article, we perform all-atom molecular dynamics
simulations to investigate the structure and intermolecular interactions
of Na+ ions in water-in-ionic liquids. We have considered two ionic
liquid systems with the same cation and different anions as (1) 1benzyl-3-methyl imidazolium [ZMIM+] tetrafluoroborate [BF−4] and
(2) [ZMIM+] hexafluorophosphate [PF6−]. We have analyzed the
inter-molecular structure and dynamics of Na+ ions in the ionic liquid
electrolyte with an increase in water (H2O) mole fraction (x) ranging
from 0.33 to 0.71. The neat ionic liquid [ZMIM+][BF4−] and
[ZMIM+][PF6−] coordination structure shows a decrease, with an
increase in x. We have observed that Na+ ions show significant
interaction with ionic liquid [ZMIM+][PF6−] when compared to
[ZMIM+][BF4−] over the entire range of x considered. The
coordination structure of Na+−[PF6−] is more significant when compared to the coordination structure between Na+ and
[ZMIM+]. The Na+ ions coordinated more with H2O and showed a higher intensities in the presence of [ZMIM+][PF6−] when
compared to [ZMIM+][BF4−]. The diffusion of Na+ ions showed an increase in both the electrolyte solutions with the rise in x. The
faster diffusion of Na+ is seen in the presence of [ZMIM+] [BF4−]. The ionic conductivity of ionic liquids is higher for
[ZMIM+][BF4−] when compared to [ZMIM+][PF6−]. Overall, this article provides detailed molecular-level insights into Na+ ions in
the presence of water in ionic liquids.
V.1,6 However, the reaction mechanism and operation remain
the same for both Na+ ion and Li+ batteries.7−9 The Na+ ion
batteries cannot achieve the high redox potential when
compared to the few limitations such as the weight of Na+,
which is three-fold higher than Li+, and size of Na+ is 39%,
which is higher than Li+.10 Despite these limitations, Na+ is
being considered for large-scale grid storage applications due
to its natural abundance and low cost.11
Different factors affect the Na+ ion battery performance,
such as electrode and electrolyte.1,12 The electrolytes show an
important role in any battery design due to their ion
conductivity nature to metal ions. Various different electrolytes
have been proposed such as organic mixtures,13 polymers,14
and ionic liquids (ILs).15 The ILs show an interesting
physiochemical properties such as non-flammability, nonvolatility, negligible vapor pressure, less toxic, and good
1. INTRODUCTION
The development of sustainable energy storage devices plays a
crucial role in battery technology. The design and development
of sustainable battery materials is a challenging task. Metal-ion
batteries are important energy storage devices that store the
energy generated from renewable and non-renewable sources.
The current Li-ion battery technologies are optimized to its
maximum theoretical capacities as well as highest electrochemical potential.1,2 However, there are potential challenges
with the Li-ion batteries such as limited presence of lithium ion
in earth’s crust and costly raw materials. The Li-ion batteries
could cause potential fire hazards when operated at higher
temperatures,3 which is a severe problem specifically for the
countries suffering from high average temperatures. On the
other hand, improper disposal of used Li-ion batteries may also
lead to environmental pollution.4 Due to the continuous
increase in greenhouse gas emissions, there is significant
interest in developing alternate metal-ion batteries that can
sustain at higher temperatures.
Various metal-ion batteries have been proposed in the
literature.5 Na+-ion batteries show a better alternative in place
of Li-ion batteries. The overall Na+ ion redox potential is
−2.71 V, whereas Li+-ion possesses a redox potential of −3.07
© 2022 American Chemical Society
Received: August 7, 2022
Accepted: November 28, 2022
Published: December 12, 2022
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Figure 1. Chemical structure of (a) [ZMIM+], (b) [BF4−], and (c) [PF6−] and molecular models of (d) [ZMIM+], (e) [BF4−], and (f) [PF6−].
Color codes: Green�carbons, blue�nitrogen, white�hydrogens, gray-boron, cyan�fluorine, and red�phosphorous.
Table 1. Chemical Compounds and Their Models Used in the Simulations
abbreviation
chemical name
chemical formula
CAS number
model
[ZMIM ][BF4−]
[ZMIM+][PF6−]
1-benzyl-3-methyl imidazolium tetrafluoroborate
1-benzyl-3-methyl imidazolium hexafluorophosphate
water
sodium ion
chlorine ion
C11H13BF4N2
C11H13F6N2P
H2O
Na+
Cl−
500996-04-3
433337-11-2
7732-18-5
7440-23-5
16887-00-6
OPLS-AA
OPLS-AA
TIP3p
OPLS-AA
OPLS-AA
+
H2O
Na+
Cl−
chemical and electrochemical stability.16 The ILs have been
considered for the metal ion batteries in place of organic
electrolytes.17 Chagas et al.18 considered various pyrolodiniumbased ILs for Na+-ion batteries, which show a superior
electrochemical window as 4 V to the traditional organic
electrolytes. Recently, Lourenço et al. 19 performed a
theoretical investigation on various combinations of Imidazolium and alkyl ammonium-based ILs. It is observed the an
increase in the Na+ ion concentration leads to a decrease in ion
diffusion.19 Suo et al.20 have proposed that water-in-salt (WIS)
electrolytes show the advantages such as high ionic
conductivity, non-flammability, and higher electrochemical
potential. Sennu et al.21 developed anion mixed water based on
salt electrolytes for the Na+-ion batteries and showed the good
electrochemical window as 3.1 V.
In the electrolyte formed using WIS, water forms a good
coordination structure with the metal ions. This leads to
reduction of water mobility and enhanced cation−anion
interaction via interconnected network formation, which
could favor the electrolyte working cycle.22 Zhang et al.23
proposed that the water-in-ionic liquid (WIIL) electrolytes for
the Li+ ion batteries showed a great electrochemical window
ranging from 3 to 4.4 V as well as good ionic conductivity,
which show a propitious behavior. Tatlisu et al.24 used water in
Imidazolium chloride electrolytes for the high aqueous voltage
as well as low-temperature supercapacitors.
However, the presence of aromatic groups such as one
benzyl on the ILs shows lower viscosities when compared to
the other alkyl-substituted structures at room temperature.25 It
is also noted from the recent studies that the IL mixture of
cations 1-benzyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium shows an enhanced Li+ ion mobility behavior.26
The presence of aromatic groups on the imidazolium-based IL
electrolyte shows a strong steric hindrance effect, thereby
increasing the ion diffusion on the solid electrolyte
interphase.27 Recently, Sappidi et al.28 performed molecular
dynamics simulations on mixing two different cations 1-butyl3-methylimidazolium [BMIM+] and 1-benzyl-3-methylimidazolium [ZMIM+] and two different anions bistriflimide
[Tf2N−] and tetrafluoroborate [BF4−]. It was observed that
the mixing of two cations showed heterogeneous behavior.
Based on the recent studies,23,24,27 water in ionic liquids may
show promise for the Na+ ion batteries, which could enhance
the electrochemical potential. However, exploring WIILs is still
in the early stage of development for battery applications.
There are fundamental gaps in understanding these WIILs,
such as molecular-level interactions with an increase in water
concentration. How do different anions play a role when ILs
interact with water? What are the factors that affect the Na+
structure and dynamics in WIILs?
In this article, we aim to address the abovementioned issues
by performing all-atom molecular dynamics simulations on
Na+ ions in two ILs (1) 1-benzyl-3-methylimidazolium
[ZMIM +] tetrafluoroborate [BF4 −] and (2) [ZMIM +]
Hexafluorophosphate [PF6−] with an increase in concentration. We investigate various intermolecular structures,
dynamics, and solvation thermodynamics.
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Table 2. Simulated System Details
number of molecules
H2O mole fraction (x)
[ZMIM+]
0 (neat [ZMIM+][BF4−])
0 (neat [ZMIM+][PF6−])
0.77 (H2O in [ZMIM+][BF4−])
0.77 (H2O in [ZMIM+][PF6−])
300
300
300
300
0.33
0.50
0.60
0.66
0.71
300
300
300
300
300
0.33
0.50
0.60
0.66
0.71
300
300
300
300
300
[BF4−]
300
0
300
0
In [ZMIM+][BF4−]
300
300
300
300
300
In [ZMIM+][PF6−]
0
0
0
0
0
2. MODELS AND SIMULATION METHODOLOGY
The molecular structures of [ZMIM+] [BF4−] and [ZMIM+]
[PF6−] are shown in Figure 1. The chemical compounds and
their models used in the simulations along with the chemical
formula and CAS number are provided in Table 1. The OPLS
force field is used for the calculation of all the bonded and nonbonded interactions of all molecules.29 The force field
parameters of [ZMIM+] [BF4−] are adopted from our previous
work.28 The force field parameters of [PF6−] are considered
from the literature.30 The TIP3p water model is used to model
the water molecules. Electrostatic interactions are calculated
using the particle mesh Ewald method31 with the 1.4 nm cutoff radius. The van der Waals interactions are calculated using
the 6−12 potential form with a 1.4 nm cut-off radius. The
LINCS method32 is used to constrain all the bonds.
Temperature is maintained for all MD simulations at 298 K
using the Nose−Hoover thermostat,33,34 with the time
constant of 0.5 ps. Pressure is held at 1 bar using the
Parrinello-Rahman barostat,35 with the time constant of 5 ps.
The energy minimization is performed using the steepest
decent method. The leap-Frog method is used to solve the
equation of motion with the time step of 2 fs. The neighboring
list is updated every 10 steps for the MD trajectory. All MD
simulations are performed using Gromacs version 2020.1.36
The system initially contains 300 molecules of ILs, which are
filled in the box with dimensions 6.85 × 6.85 × 6.85 nm3.
Then, five simulation systems were prepared by the increasing
water molecules from 200 to 1000. Then, 100 Na+ ions were
added to this simulation system. The corresponding number of
Cl− counter-ions was added to the system to maintain the
overall system charge-neutral. The different simulations
considered in this work are shown in Table 2. Then, these
entire simulation systems were subjected to NPT-MD
simulation of 100 ns, which includes 90 ns equilibration and
10 ns for the analysis and calculation of the system averages.
The block sum averaging method is used to evaluate the
statistical uncertainties. We first monitor the total system
density as well as energy to monitor the system equilibrium.
Also, we have also performed several independent repeated
simulations to eliminate the unnecessary artifacts from the
initialization of simulations.
[PF6−]
H2O
Na+
Cl−
0
300
0
300
0
0
1000
1000
0
0
0
0
0
0
0
0
0
0
0
0
0
200
400
600
800
1000
100
100
100
100
100
100
100
100
100
100
300
300
300
300
300
200
400
600
800
1000
100
100
100
100
100
100
100
100
100
100
3. RESULTS AND DISCUSSION
3.1. Density Behavior. We have simulated 14 different
systems in total. The total density (ρ) is presented in Table 3.
Table 3. Density Values of Different Simulated Systems at
298 K. Standard Deviation Values are Provided in the
Parenthesis
density (g/cm3)
H2O mole fraction (x)
[ZMIM ][BF4−]
0 (Neat [ZMIM+][BF4−])
0 (Neat [ZMIM+][PF6−])
0.77 (H2O in [ZMIM+][BF4−])
0.77 (H2O in [ZMIM+][PF6−])
0.33
0.50
0.60
0.66
0.71
1.217 (0.007)
+
[ZMIM+][PF6−]
1.488 (0.028)
1.171 (0.0002)
1.215 (0.025)
1.215 (0.011)
1.209 (0.008)
1.202 (0.008)
1.194 (0.007)
1.418 (0.0003)
1.373 (0.083)
1.374 (0.047)
1.375 (0.072)
1.378 (0.061)
1.379 (0.052)
The simulated density values of [ZMIM+][BF4−] are in good
agreement with the previous literature [1.214 (g/cm3)].28
However, the density value of [ZMIM+] [PF6−] is not available
in the literature. We have observed lower ρ values when water
is immersed in ILs. With the increase in water concentration,
the ρ value shows a decrease for [ZMIM+] [BF4−] and increase
for [ZMIM+] [PF6−]. We have seen a 1.7% decrease in ρ
values with an increase in H2O mole fraction (x) from 0.33 to
0.71 for the simulated system mixed with [ZMIM+] [BF4−].
On the other hand, we have not seen any significant change in
the ρ values when the system mixed with [ZMIM+] [PF6−] and
water.
A similar shift in density values is observed when increasing
water concentration in the pyridinium-based ILs of 1butylpyridinium + tetrafluoroborate.37
The equilibrated simulation snapshots are shown in Figure
2. With an increase in x, two distinct structural observations
are being made. The Na+ ions show clustering with Cl− ions in
the presence of [ZMIM+][BF4−]. On the other hand, opposite
behavior is observed in the presence of [ZMIM+][PF6−]. The
clustering shows an increase with the increase in x ranging
from 0.33 to 0.71. On the other hand, we observe a more
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Figure 2. Simulation snapshots of water in [ZMIM+][BF4−] at different x values (a) 0.33, (b) 0.50, (c) 0.60, (d) 0.66, and (e) 0.71. Similarly, water
in [ZMIM+][PF6−] is shown at different x values (f) 0.33, (g) 0.50, (h) 0.60, (i) 0.66, and (j) 0.71. Color codes: green�[ZMIM+],
purple�[BF4−], orange�[PF6−], cyan�water, red�Na+ ion, and gray�Cl− ion.
Figure 3. RDF plot of (a) [ZMIM+]−[BF4−] and (b) [ZMIM+]−[PF6−].
plot the RDF between the [ZMIM+]−[BF4−] and [ZMIM+]−
[PF6−] neat IL systems and IL water mixed system without
NaCl shown in Figure S1. The peak positions do not change in
neat and water-mixed IL systems. However, we see lower peak
heights in the current work due to the presence of water
molecules. The neat IL systems show a significant coordination
structure when compared to the water in IL systems. The
coordination numbers are calculated by integrating the RDF
curves up to the cut off distance at minima followed by the first
peak of the curve. The corresponding coordination number
(Ncr) values are presented in Tables 4 and 5. The Ncr value
shows a decrease of ∼16% for [ZMIM+]−[BF4−], while the Ncr
value of the [ZMIM+]−[PF6−] pair shows a ∼19% decrease
with the rise in x.
The RDF plots of [ZMIM+]−H2O presented in Figure S2
show a coordination structure between the cation and H2O
molecules in the presence of [ZMIM+]−[BF4−] with a peak
located at 0.46 nm. Similarly, we observe a coordinated peak
for the pair [ZMIM+]−H2O at 0.45 nm in the presence of
[ZMIM+]−[PF6−]. We observe a decrease in peak intensity
with an increase in x in the presence of [ZMIM+]−[PF6−] and
there is no change in peak intensities observed in the presence
of [ZMIM+]−[BF4−]. The corresponding Ncr for the pair of
[ZMIM+]−H2O shows an increase with an increase in x in the
presence of both ILs.
coordinated structure of water molecules around Na+ and Cl−
ions in the presence of [ZMIM+] [PF6−] when compared to
the presence of [ZMIM+] [BF4−]. However, there is no
respective experimental evidence available till date for
understanding the solubility of these ILs. Finally, in the case
of H2O in [ZMIM+] [PF6−], we do not see a significant density
variation due to strong intermolecular interactions of Na+, Cl−,
and water molecules around the ILs. These structural
interactions between the ILs, water, and Na+ ions will be
discussed in the next section.
3.2. Radial Distribution Functions. To understand the
density behavior and intermolecular interactions, we calculate
the radial distribution functions (RDFs) of different pairs in
the simulation system. We first calculate the RDF between the
center of mass of cation and anion pairs. Figure 3 presents the
RDF between the pair of [ZMIM+]−[BF4−] and [ZMIM+]−
[PF6−]. In calculating RDF of the [ZMIM+]−[BF4−] pair,
[ZMIM+] acts as the central atom and [BF4−] is considered as
the nearest neighbor; this definition of the RDF pair is
maintained throughout the article. The first peak is observed at
0.48 nm for [ZMIM+]−[BF4−] and 0.52 nm for [ZMIM+]−
[PF6−]. The intensity of the RDF peak for the pair [ZMIM+]−
[BF4−] and [ZMIM+]−[PF6−] shows a decrease with an
increase in x. Compared to our previous work, the peak
positions are in good agreement with the [ZMIM+]−[BF4−]
RDF pair.28 In order to compare the peak positions, we also
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peak is located at 0.36 nm for [BF4−]−H2O. Similarly, the first
solvation peak is located at 0.34 nm for [PF6−]−H2O. As
expected, the anion participates in the strong intermolecular
interaction with the H2O molecules in the first solvation shell,
and this is in agreement with the literature.38 With the increase
in x, the coordination number shows an increase from 1 to
3.63 for the pair of [BF4−]−H2O and 1.26 to 5.07 for the pair
of [PF6−]−H2O.
The center of mass of RDF between Na+ and cation
structures is presented in Figure S3. We observe a very small
peak located at 0.62 nm for the RDF pair of [ZMIM+]−Na+ in
the presence of [ZMIM+]−[BF4−]. As opposed to that,
[ZMIM+]−Na+ shows a coordinating structure with the peak
located at 0.58 nm in the presence of [ZMIM+]−[PF6−].
However, the peak intensity shows a decrease with an increase
in x. The corresponding Ncr values are presented in Tables 4
and 5. The Ncr value for the RDF pair [ZMIM+]−Na+ shows a
decrease from 0.4 to 0.1 in the presence of [ZMIM+]−[BF4−]
and from 1.28 to 0.68 in the presence of [ZMIM+]−[PF6−]
with an increase in x.
However, to understand this behavior in more detail, the
site−site RDF plots of Na+ with cations are presented in Figure
S4. The first peak is located at 0.53 nm for the atomic pair of
Na+−NIM (imidazolium nitrogens) in the presence of both IL
systems, as shown in Figure 5a,d. The intensity of the peak
shows a decrease with an increase in x. However, the intensity
of the peaks is ∼80% higher in the presence of [ZMIM+]−
[PF6−] when compared to the systems in the presence of
[ZMIM+]−[BF4−]. With the increase in x, the corresponding
Ncr for the pair of Na+−NIM shows a decrease from 4.6 to 1.22,
which accounts for 74% decrease in the presence of
[ZMIM+]−[BF4−]. On the other hand, Ncr shows a decrease
from 6.48 to 5.11, which accounts only 21% decrease in the
presence of [ZMIM+]−[PF6−] (see Tables 4 and 5). This
behavior shows a more hydrophilic interaction of cations
toward Na+ when [PF6−] anions are present in the system.
Similarly, the site−site RDF between Na + and C IM
(imidazolium carbons) is presented in Figure S4b,e. We have
observed two very small peaks located at 0.55 nm followed by
another peak at 0.66 nm, and the peak intensity shows a
decrease with an increase in x in the presence of [ZMIM+]−
[BF4−]. Two distinct peaks are observed for the RDF pair of
Na+ and CIM, one located at 0.48 nm and another at 0.66 nm
in the presence of [ZMIM+]−[PF6−]. The intensities of the
peak are higher when [PF6−] is present in the system.
However, we have observed two peaks because of the location
Table 4. Coordination Number Values of Different Atomic
and Molecular Pairs for the System H2O in [ZMIM+]−
[BF4−]
coordination numbers at different H2O mole
fractions (x)
RDF pair
0.33
0.50
0.60
0.66
0.71
[ZMIM+]−[BF4−]
[ZMIM+]−H2O
[BF4−]−H2O
[ZMIM+]−Na+
Na+−NIM
Na+−CIM
Na+−CBZ
[BF4−]−Na+
[ZMIM+]−Cl−
[BF4−]−Cl−
Na+−OW
Na+−HW
Cl−−H2O
Na+−Cl−
3.13
1.76
1.00
0.40
4.76
2.71
0
0.12
0.36
0.62
0.56
2.46
0.78
3.11
3.00
3.51
1.85
0.31
3.69
2.04
0
0.07
0.28
0.44
0.80
4.34
0.99
3.57
2.86
5.01
2.53
0.22
2.59
1.34
0
0.04
0.21
0.35
0.96
5.08
1.17
3.98
2.73
6.47
3.15
0.13
1.80
0.90
0
0.03
0.14
0.26
0.97
5.46
1.21
4.23
2.63
7.75
3.63
0.10
1.22
0.60
0
0.02
0.10
0.20
1.02
5.85
1.28
4.40
Table 5. Coordination Number Values of Different Atomic
and Molecular Pairs for the System, H2O in [ZMIM+]−
[PF6−]
coordination numbers at different H2O mole
fractions (x)
RDF pair
0.33
0.33
0.33
0.33
0.33
[ZMIM+]−[PF6−]
[ZMIM+]−H2O
[PF6−]−H2O
[ZMIM+]−Na+
Na+−NIM
Na+−CIM
Na+−CBZ
[PF6−]−Na+
[ZMIM+]−Cl−
[PF6−]−Cl−
Na+−OW
Na+−HW
Cl−−H2O
Na+−Cl−
4.12
1.69
1.26
1.28
6.48
5.19
8.38
0.87
0.77
0.27
0.20
2.50
0.63
0.51
3.89
3.26
2.39
0.92
6.24
5.09
8.31
0.85
0.71
0.25
0.40
4.03
1.47
0.66
3.67
4.68
3.42
0.84
6.11
4.61
7.91
0.89
0.68
0.25
0.48
5.58
2.27
0.54
3.52
6.03
4.30
0.78
5.45
4.03
6.77
0.92
0.65
0.20
0.59
6.74
3.00
0.33
3.32
7.27
5.07
0.68
5.11
3.70
6.12
0.88
0.60
0.21
0.62
7.47
3.51
0.52
Article
Figure 4 presents the center of mass RDF plots of the anion
with H2O molecules, which show a coordinating structure
between the anions and H2O molecules. The first solvation
Figure 4. RDF plot of anion−H2O in the presence of ILs. (a) [ZMIM+]−[BF4−] and (b) [ZMIM+]−[PF6−].
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Figure 5. RDF plots of (a) Na+−[BF4−] and (b) Na+−[PF6−].
Figure 6. RDF plot of (a) Na+−Ow (oxygen’s of H2O) and (b) Na+−Hw (hydrogens of H2O) in the presence of [ZMIM+]−[BF4−]. Similarly, RDF
plots of (a) Na+−Ow (oxygen’s of H2O) and (b) Na+−Hw (hydrogen’s of H2O) in the presence of [ZMIM+]−[PF6−].
of three carbons on the imidazolium ring. The Ncr of the pair
also shows a decrease with an increase in x.
Furthermore, the Na+ structure toward the benzyl ring on
the cations is investigated by plotting the site−site RDF
between para carbon (1,4) atoms of the benzyl rings (CBZ)
with Na+ shown in Figure S4c,f. We do not see any structure
between Na+ and CBZ in the presence of [ZMIM+]−[BF4−]. As
opposed to that, we see a coordinating structure between
Na+−CBZ in the presence of [ZMIM+]−[PF6−] with a peak
located at 0.73 nm. The Ncr value for the pair of Na+−CBZ
shows a decrease with an increase in x. This behavior indicates
that Na+ coordinates more with the cation in the IL comprising
[ZMIM+]−[PF6−] mixed with H2O. This structural coordination toward the cation species is also depicted in the snapshots
shown in Figure 2. We observe a less aggregated behavior of
Na+ ions with Cl− and Na+ with Na+ when H2O immersed in
IL [ZMIM+]−[PF6−]. This also indicates that [PF6−] anions
influence the strong IL−Na+ when compared to [BF4−] anions.
The RDF plots between the center of the mass distance
between Na+ and anions ([BF4−] and [PF6−]) in different
simulation systems are presented in Figure 5. The RDF
between [BF4−]−Na+ shows the first peak located at 0.30 nm
and the second peak located at 0.36 nm, and the intensity of
the peak shows a decrease with an increase in x. On the other
hand, the RDF of [PF6−]−Na+ shows the first peak located at
0.29 nm, and RDF between [PF6−]−Na+ do not show any
variation in the intensity of the peak with an increase in x.
Interestingly, the intensity of the peak for the RDF pair of
[PF6−]−Na+ is 10-fold higher than that of Na+−[BF4−]. The
Ncr values show a decrease with an increase in water
concentration Na+−[BF4−], whereas no significant change is
observed for Na+−[PF6−]. Similarly, the Ncr values in these
pairs (Na+ with anions) are fivefold higher in the presence of
[ZMIM+]−[PF6−]. Interestingly, it is observed that Na+
displays favorable coordination with the anions when
compared to the atomic sites of cations in both WIILs.
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Figure 7. RDF of the Na+−Cl− pair in the presence of (a) [ZMIM+]−[BF4−] and (b) [ZMIM+][PF6−]. Similarly, RDF of the Na+−Na+ pair in the
presence of (c) [ZMIM+]−[BF4−] and (d) [ZMIM+][PF6−].
A similar behavior is observed for the RDF plot of Cl− with
the cations and anions in the presence of both WIILs (see
Figure S5). The first peaks are observed at 0.47 nm for the pair
of [ZMIM+]−Cl− in both WIILs. On the other hand, the first
peak locations are observed at 0.49 nm for [BF4−]−Cl− and
0.42 nm for [PF6−]−Cl−. However, Ncr values (shown in
Tables 4 and 5) of Cl− ions with anions and cations show a
decrease with an increase in x in both the WIILs.
The Na+ structure with the atomic sites of H2O molecules is
presented in Figure 6. The first peak is located at 0.24 nm for
the pair of Na+−Ow (oxygen of H2O) and at 0.30 nm for the
pair of Na+−Hw (hydrogen’s of H2O) in the presence of
[ZMIM+]−[BF4−]. However, in the presence of [ZMIM+]−
[PF6−], we have observed two peaks for the RDF pair of Na+−
Ow (oxygen’s of H2O), with the first peak located at 0.30 nm
and the second peak located at 0.39 nm, whereas we only see
one peak located at 0.33 nm for the pair of Na+−Hw (hydrogen
of H2O). The observed peak positions for Na+−OW are in
good agreement with the literature.39 Interestingly, H2O
molecules show a strong interaction toward the Na+ ions in
the presence of [ZMIM+][BF4−]. On the other hand, we
observe two distinct coordination peaks, which depict more
interaction toward the Na+ ions in the presence of [ZMIM+][PF6−]. Overall, Na+ ions show more favorable water
interaction with the sodium ions when the [BF4−] anion is
present when compared to [PF6−]. On the other hand, the Cl−
structure also shows a similar behavior with H2O molecules
(see Figure S6).
Figure 7a,b presents the RDF plot of Na+−Cl− in the
presence of both WIILs. We see that a first peak is located at
0.27 nm for the Na+−Cl− pair in both WIILs. The intensity of
the peak is more than fivefold higher, and multiple
coordinating peaks are observed in the presence of
[ZMIM+]−[BF4−] when compared to the presence of
[ZMIM+][PF6−], where we see only one sharp peak. This
behavior shows a Na+−Cl− clustering in WIIL [ZMIM+]−
[BF4−]. This is also observed in the snapshots presented in
Figure 2. The corresponding Ncr values also show an increase
with the increase H2O concentration in IL [ZMIM+]−[BF4−],
while the Ncr values do not change in IL [ZMIM+]−[PF6−].
Similarly, Figure 7c,d presents the RDF plot between the Na+−
Na+ pair in the presence of both WIILs. The RDF Na+−Na+
pair shows a first peak located at 0.39 nm in IL [ZMIM+]−
[BF4−] and 0.37 nm in IL [ZMIM+]−[PF6−]. These Na+−Na+
structures also follow a similar trend as Na+−Cl−. From both
the RDF plots, it is clearly seen that Na+−Cl− and Na+−Na+
form more clustering in the presence of IL [ZMIM+]−[BF4−].
On the other hand, these all intermolecular and interatomic
interactions depict the fact that Na+ shows a dispersive
behavior when [PF6−] is used as an anion in WIILs. To
understand the thermodynamic behavior of Na+, we have
calculated the solvation enthalpy.
3.3. Solvation Enthalpy. The solvation enthalpy of (Na+)
(ΔHNa+,Solv) in WIILs is calculated using the method adapted
by Hess and van der Vegt40
HNa+,Solv = Hsolution
HIL
H H 2O
HCl
HNa
(1)
In the abovementioned equation, H = U + P*ΔV, where U is
potential energy, P is the pressure, and fluctuations of the box
volume are given as ΔV. Hsolution is the total enthalpy of the
total solution mixture comprising WIILs + NaCl, HH2O is the
enthalpy of pure H2O, HCl is the enthalpy of Cl ions, and HNa
is the enthalpy of Na+. Here, the data for HCl are taken as
−60.66 kJ/mol from the study of Grosfield et al.41 and HNa is
taken as −443.9 kJ/mol from the study of Cieplak and
Kollman.42 The observed ΔHNa+,Solv becomes more negative
with an increase in H2O concentration in both WIILs.
However, we see that ΔHNa+,Solv is more favorable for the
d
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WIIL system comprising H2O−[ZMIM+]−[PF6−]. The more
favorable behavior shows more solvation of Na+ in the system.
Figure 8 and Table 6 present ΔHNa+,Solv as a function of x. The
Figure 9. Na+ self-diffusion coefficient (D) as a function of H2O
concentration.
Na+ ion diffusion shows enhanced mobility behavior in the
presence of both the WIILs. The slightly lower values of D in
the presence of water [ZMIM+][PF6−] are observed due to the
significant intermolecular interactions of Na+ with the water,
[ZMIM+] and [PF6−] molecules as evident from the structural
analysis shown above. On the other hand, we have also
observed that the mobility of Na+ in [ZMIM+][BF4−] does not
affect by clustering Na+−Cl− as seen in the structural analysis.
Due to the strong interactions between [PF6−] anions and Na+,
we have observed lower D values. Thus, anions play a crucial
role in determining the mobility of Na+ ions.
The D values of [ZMIM+],[BF4−], [PF6−], Cl−, and H2O are
presented in Tables 7 and 8. It is observed that the D values of
all the [ZMIM+], [BF4−], Cl−, and H2O show an increase and
D values of [PF6−] show a decrease with an increase in water
mole fraction in both WIILs. Overall, it is observed that an
increase in the x show increased mobility of all the ionic
species present in the simulation system.
3.5. Ionic Conductivity. To understand the charge
transport behavior, we have calculated the ionic conductivity
σ using the Einstein−Helfand method.19,43 This method uses
the collective mean squared displacement of dipole moment
for calculating the σ as per the following equation
1
=
lim
qiqj([ri(t ) ri(0)]·
6VKBT t
i j(j > i)
Figure 8. Na+ solvation enthalpy as a function of H2O concentration.
Table 6. Solvation Enthalpy of Na+ Ions (ΔHNa+,Solv) in the
Presence of Both WIILsa
solvation enthalpy (ΔHNa+,Solv)(kJ/mol)
H2O mole fraction (x)
[ZMIM+][BF4−]
[ZMIM+][PF6−]
0.33
0.50
0.60
0.66
0.71
−423.55 (4.95)
−516.79 (1.12)
−606.15 (0.07)
−693.99 (0.35)
−779.81 (0.52)
−459.23 (19.61)
−609.47 (15.84)
−661.78 (24.73)
−782.38 (16.24)
−884.30 (14.05)
a
The standard deviation values are provided in the parenthesis.
observed value of ΔHNa+,Solv is in agreement with the literature
value shown as −443.9 kJ/mol.42 Overall, the solvation
enthalpy data show a consistent behavior with the
intermolecular structural transitions as seen in the RDF plots
of cation−Na+ (Figure S1) and anion−Na+ (Figure 6). We
observe that the structural peak intensities and coordination
numbers of these RDF pairs were higher in the presence of the
WIIL system of H2O−[ZMIM+]−[PF6−]. Also, Na+−Cl− and
Na+−Na+ were less coordinating themselves when the [PF6−]
anion is present in the system compared to the presence of the
[BF4−] anion. Due to structural transition, ΔHNa+,Solv shows
more favorable behavior in the WIIL system H2O−[ZMIM+]−
[PF6−].
3.4. Diffusion Behavior. The Na+ ion self-diffusion
coefficient (D) is calculated using the Einstein equation. The
variation in mean squared displacement (MSD) curves as a
function of time is shown in Figure S7. Figure 9 presents the
variation of D as a function of x. With an increase in ΔHNa+,Solv,
we observe a fourfold increase in the D value in the presence of
[ZMIM+][BF4−]. However, in the presence of [ZMIM+][PF6−], we have observed an 82% increase in the D value of
Na+. However, there are direct experimental data available for
the systems simulated in this work. However, our observed D
values are in a similar order of magnitude as reported in the
literature on the presence of imidazolium-based ILs.19
Although Lourenco et al.19 have employed the non-polarizable
charges with the charge scaling factor of 0.8 in their
simulations to improve the transport properties, it is well
known that the self-diffusion confident values estimated using
non-polarizable charges would be one order of magnitude
smaller than that of direct experimental values. It is clear that
[rj(t )
rj(0)])
(2)
In the abovementioned equation, ri, and rj are the vectors of
the center of mass positions of ionic species such as cation and
anion, respectively. qi and qj are the charges of the
corresponding ionic species. The time is represented as t, T
is the temperature, KB is the Boltzmann’s constant, and V is the
volume of the simulation box.
Table 9 presents the ionic conductivities σ as a function of x.
It is observed that with an increase in the H2O concentration,
the σ values show a 4-fold increase in the presence of
[ZMIM+] [BF4−] and a 14% decrease in the presence of
[ZMIM+][PF6−]. The higher values of σ are observed in H2O
in [ZMIM+][BF4−]. This behavior is in good accordance with
observed self-diffusion coefficient values. The significant
interactions between the Na+ with [PF6−] show a decreased
mobility of the Na+ ions; thereby, relatively lower σ values are
seen when the WIIL consisting of [ZMIM+][PF6−] is at x =
0.71. The σ values are in agreement with the results presented
in the theoretical study of Lourenco et al.19 for the
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Table 7. Self-Diffusion Coefficient of Cation, Anion, Cl−, and H2O Molecules in the Presence of [ZMIM+]−[BF4−]a
D (cm2/s) × (107)
H2O mole fraction (x)
[ZMIM ]
[BF4−]
0.33
0.50
0.60
0.66
0.71
1.33 (0.06)
1.95 (0.01)
3.53 (0.05)
5.04 (0.04)
7.67 (0.02)
1.47 (0.00)
2.36 (0.03)
4.38 (0.04)
7.19 (0.01)
11.11 (0.10)
+
Cl−
H2O
0.50 (0.01)
0.72 (0.01)
1.23 (0.04)
1.34 (0.04)
2.54 (0.06)
11.87 (1.71)
16.48 (0.50)
23.21 (0.46)
30.66 (1.30)
38.46 (0.78)
+
Na
0.46 (0.07)
0.80 (0.04)
1.08 (0.02)
1.02 (0.02)
2.58 (0.02)
a
The standard deviation values are presented in the parenthesis.
Table 8. Self-Diffusion Coefficient of Cation, Anion, Cl−, and H2O Molecules in the Presence of [ZMIM+]−[PF6−]a
D (cm2/s) × (107)
H2O mole fraction (x)
[ZMIM ]
[PF6−]
0.33
0.50
0.60
0.66
0.71
1.95 (0.19)
2.31 (0.26)
2.09 (0.05)
2.13 (0.04)
2.22 (0.01)
0.77 (0.06)
0.97 (0.01)
0.10 (0.07)
0.10 (0.01)
0.12 (0.02)
+
Cl−
H2O
1.80 (0.30)
2.00 (0.30)
2.55 (0.2)
2.86 (0.03)
2.62 (0.04)
11.40 (0.30)
18.93 (1.60)
17.76 (0.60)
19.53 (0.90)
19.38 (0.30)
+
Na
0.40 (0.04)
0.54 (0.06)
0.68 (0.06)
0.53 (0.09)
0.73 (0.01)
a
The standard deviation values are presented in the parenthesis.
[ZMIM+]−[PF6−] over the entire range of x applied in this
study. The Na+ ion self-diffusion coefficient (DNa+) and ionic
conductivity (σ) values are higher when H2O in [ZMIM+]−
[BF4−] when compared to H2O in [ZMIM+]−[PF6−]. The
significant intermolecular interactions between Na+ with the
anions and water molecules influence the dynamic behavior of
Na+. Interestingly, Na+ and Cl− clustering do not influence the
dynamics of Na+ in the presence of [ZMIM+][BF4−] when
compared to [ZMIM+][PF6−]. Therefore, based on the
abovementioned observations, H2O in [ZMIM+][BF4−]
electrolyte showed a more hydrophilic nature when compared
to H2O in [ZMIM+][PF6−] especially considering the dynamic
behavior of Na+ ions.
Table 9. Ionic Conductivity in the Presence of Both WIILsa
ionic conductivity σ (S/m)
H2O mole fraction (x)
[ZMIM+][BF4−]
0 (Neat [ZMIM ][BF4−])
0 (Neat [ZMIM+][PF6−])
0.77 (H2O in [ZMIM+][BF4−])
0.77 (H2O in [ZMIM+][PF6−])
0.13 (0.02)
+
0.33
0.50
0.60
0.66
0.71
[ZMIM+][PF6−]
0.08 (0.01)
0.65 (0.03)
0.56 (0.04)
0.36 (0.06)
0.52 (0.06)
0.79 (0.12)
2.05 (0.13)
0.26 (0.06)
0.85 (0.07)
0.83 (0.13)
0.97 (0.21)
0.35 (0.14)
0.73 (0.11)
■
a
Uncertainty Values are Provided in the Parenthesis
ASSOCIATED CONTENT
sı Supporting Information
*
imidazolium-based ILs. Interestingly, Na+ and Cl− clustering
shown in the snapshots of Figures 2a−e and 8a,c does not
influence slowing down the dynamics of Na+ in the presence of
[ZMIM+][BF4−] when compared to [ZMIM+][PF6−].
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jced.2c00521.
Additional RDF plots as a function of x and MSD plots
of [ZMIM+ ], [BF4− ], [PF6− ], Na+, Cl−, and H2O
(PDF)
4. CONCLUSIONS
In this article, we have performed all-atom MD simulations to
understand the Na+ ion structure, dynamics, and thermodynamic behavior in the presence of two water-in-ionic liquids
(WIILs): (1) 1-benzyl-3-methylimidazolium [ZMIM+] tetrafluoroborate [BF4−] and (2) [ZMIM+] hexafluorophosphate
[PF6−] with an increase in water concentration (x). The Na+
ions show clustering with Cl− ions in the presence of [ZMIM+]
[BF4−]. On the other hand, the coordinated structure of Na+ is
seen in the presence of [ZMIM+] [PF6−]. The intermolecular
interactions between Na+ show favorable coordination with the
anions when compared to the atomic sites of cations in both
WIILs. The Na+−anion pair coordination number values are
fourfold higher in the presence of H2O in [ZMIM+]−[PF6−]
when compared to H2O in [ZMIM+]−[BF4−]. On the other
hand, it is observed that Na+ ions show more favorable
interaction with H2O in the presence of [ZMIM+]−[PF6−].
These structural underpinnings also affect the thermodynamic
and dynamic behavior of Na+. The solvation enthalpy of Na+
ions shows more negative in the presence of H2O in
■
AUTHOR INFORMATION
Corresponding Author
Praveenkumar Sappidi − Department of Chemical
Engineering, Indian Institute of Technology Jodhpur, Jodhpur
342037, India; orcid.org/0000-0001-9038-608X;
Phone: (91 291)280 1712 (O), (91 944) 596 4579 (M);
Email: praveenks@iitj.ac.in
Author
Shrayansh Gupta − Department of Chemical Engineering,
Indian Institute of Technology Jodhpur, Jodhpur 342037,
India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jced.2c00521
Notes
The authors declare no competing financial interest.
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■
ACKNOWLEDGMENTS
This work is supported by the SEED grant of Indian Institute
of Technology Jodhpur, India. We acknowledge the SERBEMEQ grant no: EEQ/2021/000267. We acknowledge the
High-Performance Computing (HPC) Facility at the Indian
Institute of Technology Jodhpur, India for the computational
support.
■
Article
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