Anatomy of Microscopic Structure of Ethaline Deep Eutectic Solvent

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Anatomy of Microscopic Structure of Ethaline Deep Eutectic Solvent Decoded Through Molecular Dynamics Simulations Supreet Kaur, Akshay Malik, and Hemant K. Kashyap J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b06624 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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The Journal of Physical Chemistry

Anatomy of Microscopic Structure of Ethaline Deep Eutectic Solvent Decoded Through Molecular Dynamics Simulations Supreet Kaur, Akshay Malik, and Hemant K. Kashyap∗ Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India E-mail: [email protected] Phone: +91-(0)11-26591518. Fax: +91-(0)11-26581102

Abstract Atomistic molecular dynamics simulations have been performed to investigate the microscopic structure of ethaline deep eutectic solvent (DES), a mixture of choline chloride ([Ch][Cl]) and ethylene glycol (EG) in molar ratio of 1:2, respectively. As much as the structure of a DES is derived by the composition of the species present in it, the arrangement of hydrogen bond donor species also plays a crucial role in laying down the microscopic structure of DESs. By virtue of its inherent chemical structure, EG renders both intra- and inter-molecular hydrogen bonds. Therefore, the molecular level structural landscape of DESs containing EG as hydrogen bond donor is reckoned to be a bit complex. In the present study, we aim to understand the structural morphology of ethaline using optimum force-field parameters for EG recently proposed by our group. After initial assessment of the refined force-field parameters for ethaline DES, we have presented an in-depth analysis of the arrangement and ordering of its components at molecular level. Simulated X-ray scattering structure function and its partial components reveal the presence of short as well as long range interactions in ethaline. The role of hydrogen bonding interactions between all the three species [Ch]+ , [Cl]− and EG were predominantly observed through radial and radial-angular distribution functions and substantiated by spatial distribution functions. The observation of competitive nature of [Ch]+ and EG to form hydrogen bond with the anion is one of the major outcomes of the present study. Also, weaker intra- and inter-molecular hydrogen bonding interactions between EG molecules were seen along with their simultaneous involvement with the ammonium group of choline cation.

1

Introduction

field of synthesis, separation, extraction, catalysis, electrochemical processes, ionothermal synthesis and bio-transformations. 17–20 Most commonly used hydrogen bond donor (HBD) species in combination with [Ch][Cl] salt are urea, ethylene glycol (EG), malonic acid and oxalic acid. Thus, depending on these choices, the properties of the DESs are accordingly altered. A combination of [Ch][Cl] and EG in 1:2 molar ratio forms an eutectic mixture,

Choline chloride ([Ch][Cl]) based deep eutectic solvents (DESs) have become an appealing class of designer solvents owing to their peculiar properties and benign nature. 1–16 Overcoming many drawbacks faced while using conventional solvents, such as high cost, handling difficulties, toxicity and non-benign nature, DESs have emerged as promising solvent media in the

ACS Paragon Plus Environment

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commonly called as ethaline. Ethaline serves as better alternative of conventional solvents in multiple fields, such as electrodeposition of metals, metallurgical processing, electropolishing, enhancement of drug solubility and gel electrolytes. 21–23 Ethaline is found to exhibit high current efficiency and corrosiveness, so it also acts as better alternative for electropolishing of stainless steel. In addition to this, better results for copper as well tin electrodeposition were observed when ethaline DES was employed. Following this backdrop, the interaction of ethaline with glassy carbon electrode at varying potentials has been explored recently and it was observed that choline cation gets accumulated on the electrode with increasing negative potential. 19 Experimental investigations of ethaline DES include precisely the physico-chemical properties, such as density, viscosity and self-diffusion coefficient at various temperatures, 24,25 however a very little attention is paid towards exploring the molecular level structural arrangement of ethaline. Understanding the nature of interactions and the factors that affect them, can help us gain an insight on the molecular level arrangement of the DES and thus establish its structure-property relationship. In this spirit researchers have come forward to understand the microscopic structure of ethaline by using molecular dynamics simulations, 26–30 however an accurate picture of this DES’s structure is yet to be conceived. Among only few computational studies performed in the past, ab initio MD simulations 26,27 have reported the comparison of different types of interactions present in various choline chloride based DESs. These studies mainly emphasize on the specific role played by hydrogen bonding interactions between the DES’s components and the effect of change of HBD species on the DES structure. Recently, OPLS based force-field was employed by Acevedo et al. to perform classical MD simulations of choline chloride-based DESs and their physical properties at different temperatures were predicted. 29 Another attempt to improve the model for ethaline has been put forward by Ferreira and coworkers 28 where they have performed all-atom MD simulations using

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different sets of force-field parameters and compared the simulated properties with available experimental data. 28 Despite these attempts, the prediction of complete microscopic structural arrangement in ethaline is still a hunt because of complex set of possible interactions among its components including electrostatic and hydrogen bonding interactions.

(a)

(b)

Figure 1: Chemical structures of (a) choline chloride ([Ch][Cl]) and (b) ethylene glycol (EG) components of ethaline.

In a recent study from our group, 31 we checked the reliability of CHARMM force-field parameters for atomistic simulations of choline chloride and urea based DES. An excellent agreement of experimental X-ray/neutron scattering data with the simulation results was found in addition to the coincidence of other properties for the DES, which proved the accuracy of the CHARMM force-field for such novel systems. 31 Hence, in the present work, we have employed CHARMM FF 32–35 parameters for the choline cation and chloride anion in combination with a refined FF proposed for the hydrogen bond donor species, ethylene glycol 36 to perform the molecular dynamics investigation of microscopic structural details of bulk ethaline DES. The chemical structures of the components involved in ethaline DES are shown in Fig. 1. We have explored the total and partial X-ray scattering structure of the system at 303 K. Further radial distribution functions (RDFs) and spatial distribution functions (SDFs) have been used to understand the electrostatic as well as hydrogen bond interactions present in ethaline. The competitive hydrogen bonding interactions existing between the species of ethaline are described using radial angular distribution functions (RADFs). Overall, a complex package of various balancing in-


2

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teractions and unique structural arrangement of the three species present in the eutectic mixture is described.

Table 1. We observe a maximum deviation of 1.8% between the simulated and experimental densities. 25 1.175

2

Ethaline

Computational Details

Experimental Simulated

1.15

All atom MD simulations were carried out using the GPU version of GROMACS-5.1.1 (singleprecision). 37–40 A cubic simulation cell consisting of 1000 ion-pairs of choline chloride and 2000 molecules of ethylene glycol, summing to a total of 42000 atoms, was initially generated by using PACKMOL. 41 This system was subject to equilibration in an isothermalisobaric (NPT) ensemble for 20 ns at 303 K temperature and 1 bar pressure. The temperature and pressure of the simulation cell were maintained using Nos´e-Hoover 42,43 thermostat and Parrinello-Rahman 44 barostat, respectively. The equations of motion for each atom were solved by using the leap-frog algorithm with 1 fs time step. Electrostatic interactions were evaluated using particle-mesh-Ewald (PME) 45,46 summation technique. A cutoff radius for the short-range interactions was set to 1.2 nm along with a switching function used from 1.0 to 1.2 nm. For computation of all the properties, last 10 ns trajectory was saved at every 100 fs time step. Force-field parameters from CHARMM 32–35 were adapted for choline cation and chloride anion. The refined FF parameters for ethylene glycol proposed in a previous study reported by our group were used for the hydrogen bond donor species. 36 Note that these refined parameters are validated completely in order to use them to perform classical simulations for pure EG as well as its mixtures. Equilibrium simulation cell snapshots rendered for ethaline as well as for its individual species [Ch]+ , [Cl]− and EG at 303 K are shown in Fig. S1 of the Supporting Information. Temperature dependent study was also performed for the same DES in order to perform initial validation of the new force field parameters for the DES at 303, 323, 333 and 343 K using the above mentioned methodology. A comparison of simulated bulk densities with the experimental data is shown in Fig. 2 and provided in

-3

ρ (g cm )

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


1.125

1.1

∆ρ=1.8%

1.075 300

310

320

330

340

350

T (K)

Figure 2: Comparison of experimental and simulated densities, ρ, of ethaline for temperature range 303–343 K.

Table 1: Comparison of experimental and simulated densities, ρ, of ethaline for temperature range 303–343 K. The experimental data has been taken from Ref. 25. T (K) 303 323 333 343

ρ (g cm−3 ) Experimentala Simulated 1.111 1.131 ±0.0016 1.100 1.119 ±0.0017 1.094 1.112 ±0.0018 1.089 1.106 ±0.0018

a The temperature for experimental densities ranges from 303.15 to 343.15 K.

The X-ray scattering structure function, S(q), explored in this work has been computed using 47–49

ρo S(q) =

n P n P

xi xj fi (q)fj (q)

i=1 j=1

L/2 R 0

[

n P i=1

xi fi (q)][

4πr2 [gij (r) − 1] sinqrqr ω(r)dr n P

, xj fj (q)]

j=1

(1)

where radial distribution function, gij (r), for the atoms of type i and j were calculated with minimum image convention applied. Here the computed gij (r)s include both intra- and intermolecular terms. In the above equation, xi is


3


the mole-fraction of atom of type i and fi (q) is its respective X-ray atomic form factor. 50 ρo = Natom / < V > is the total number density and L is the box length. ω(r) is a Lorch window function, ω(r) = sin(2πr/L)/(2πr/L), 51,52 which could be used to reduce the effects of finite truncation error. The species-wise partial components of total X-ray scattering structure function were computed for this system to understand the origin of total S(q) features. The scheme used to split the total S(q) into it’s cationic, anionic and HBD components is as 53–60

+

S(q) = S [Ch] +

+ 2S [Ch]

−[Ch]+

−[Cl]−

(q) + S [Cl]

(q) + 2S [Ch]

+

−

−EG

−[Cl]−

ent self and cross species-wise correlations in the lower q region, precisely marked as region I and II in the figure. Taking a closer glance of these features, we see peaks for [Ch]+ -[Ch]+ , [Cl]− -[Cl]− and EG-[Cl]− and antipeaks corresponding to [Ch]+ -[Cl]− and EG-EG in the q range 0.67-1.25 ˚ A−1 (region II). Another interesting observation that lies in this region is the exhibition of pseudo charge ordering peaks and antipeaks for the ions (0.90 ≤ q/˚ A−1 ≤ 1.0 ) which is similar to the behavior shown by ionic liquids and their mixtures. 53,63,64 Further moving towards lower q range i.e. region I, we again observe peaks for [Ch]+ -[Ch]+ , EG-EG and [Ch]+ -[Cl]− along with antipeaks for EG[Cl]− and [Ch]+ -EG. These peaks and antipeaks tend to signify the presence of unique long range structural ordering present in the system. Note that this kind of ordering is also observed previously in the same class of DES containing choline chloride. 60 However, the overall cancellation effect results in no such feature in the total S(q). Herein, the lack of availability of experimental scattering data for ethaline DES refrains us to directly compare the simulated X-ray S(q) with its experimental counterpart.

(q) + S EG−EG (q) −

(q) + 2S [Cl]

−EG

(q),

(2)

where [Ch]+ denotes choline cation and EG is ethylene glycol species. Further details regarding the computation of partial S(q)s can be found in the literature. 53–62

3 3.1

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Results and Discussion Simulated X-ray Scattering Structure Function and its Partial Components

3.2

In this section, we have examined the structure of ethaline DES through simulated X-ray scattering structure function and its partial components shown in Figs. 3(a) and 3(b), respectively. From Fig. 3(a) we observe majorly one principal peak at around q= 1.54 ˚ A−1 that corresponds to characteristic distance of 4.08 ˚ A in real space. The species-wise partial S(q)s, computed using the scheme given in eq. 2, display four major regions marked as I-IV in Fig. 3(b). In region III we observe major positive contributions from EG-EG and [Ch]+ -EG and minor participation from [Ch]+ -[Cl]− to the principal peak. This indicates a notable participation of hydrogen bond donor species in the short range interactions present in ethaline DES. Apart from this, another information displayed through the partial S(q)s shown here is the presence of peaks and antipeaks in differ-

3.2.1

Intra- and Inter-species Pair Correlations Center-of-mass RDFs

The real space pair correlations in ethaline DES at 303 K have been investigated through radial distribution functions to further help understand the molecular level organization. Fig. 4 displays the center-of-mass (COM) RDFs for all the pairs existing between different species of ethaline. We see that the interaction between [Cl]− -EG is most profound followed by the [Ch]+ -[Cl]− counter-ion correlation and [Ch]+ EG correlation being the weakest among all the cross-species correlations. Comparing the nearest neighbor peak positions in the aforementioned correlations, we observe strong association of EG and chloride anion. Also we observe a profound doublet in the [Cl]− -EG RDF, having peak positions at 0.33 nm and 0.37 nm, followed by a less intense peak at around 0.44


4

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2 1

Simulated X-ray S(q)

Ⅱ

Ⅳ

Ⅲ

0 S(q)

0 S(q)

Region Ⅰ

1

0.5

-0.5 1 0.5 0 -0.5 -1 -1.5 -2

-1 -1.5 -2 0

1

0.5

1.5

-

-1 -2 -3

0

4

8

12

2 2.5 -1 q (Å )

20

16

3

24

4

3.5

-4

-

Cl -Cl + + Ch -Ch + Ch -Cl + Ch -EG EG-Cl EG-EG 0.5

(a)

1

1.5 -1 q (Å )

2

2.5

3

(b)

Figure 3: (a) Simulated X-ray scattering structure function, S(q) for bulk ethaline at 303 K. (b) Species-wise partial components of S(q) of ethaline. The corresponding q range for the regions I-IV marked here are 0–0.67, 0.67–1.25, 1.25–1.75 and 1.75–3.0 ˚ A−1 , respectively.

+

3

+

-

[Ch] -[Ch]

[Cl] -[Cl]

-

+

[Ch] -[Cl]

-

nm. We also observe such doublet in [Ch]+ [Cl]− correlation, the first peak being at 0.42 nm and second one at 0.50 nm. These features indicate multiple close interaction sites between these species. The self-correlations between [Ch]+ -[Ch]+ and [Cl]− -[Cl]− having nearest neighbor peak positions at 0.65 and 0.75 nm, respectively, are found to be comparatively less pronounced (See Fig.4(a)). The nearest neighbor peak at 0.52 nm in the EG-EG RDF shown in Fig. 4(b) (left panel ) corresponds to close range self-interactions between EG molecules. Overall, these observations reveal appreciable intervention of hydrogen bond donor species in strong ion-ion correlations. Further we have elaborately discussed about these interactions through atomic RDFs in the upcoming section.

2.5

g(r)

2 1.5 1 0.5 0 0

0.5

1

1.5 0

0

0.5 1 1.5 r (nm)

1

0.5

1.5

(a) 3.5

+

-

[Ch] -EG

EG-EG

[Cl] -EG

3 2.5 g(r)

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


2 1.5 1 0.5 0 0

0.5

1

1.5

0

0.5 1 1.5 r (nm)

0

0.5

1

1.5

3.2.2

(b)

Ion Pairing and Hydrogen Bonding

In Fig. 5, we have shown the atomic RDFs for the key interactions present in the DES. The corresponding nearest neighbor shell minimum and coordination number are also provided in Table 2. The RDFs shown in Fig. 5(a) reveal the presence of strong hydrogen bonding interactions between hydroxyl hydrogen of [Ch]+ and [Cl]− (HO[Ch]+ -[Cl]− ) as well as hy-

Figure 4: Center-of-mass radial distribution functions for (a) [Ch]+ -[Ch]+ , [Cl]− -[Cl]− and [Ch]+ -[Cl]− and (b) EG-EG, [Ch]+ -EG and [Cl]− -EG pairs in ethaline at 303 K.


5


HO[Ch]+-[Cl]

15

-

HOEG-[Cl]

-

g(r)

g(r)

N[Ch]+-OEG

3

6

2

3

1 0.4

0.8

1.2 0 r (nm)

0.4

0.8

0 0

1.2

0.4

0.8

(a)

1.2 0 r (nm)

0.4

0.8

1.2

(b)

HO[Ch]+-OEG

2

HOEG-O[Ch]+

HOEG-OEG

3

OEG-OEG

2.5

1.5 g(r)

2

1

1.5 1

0.5 0 0

-

4

9

0 0

N[Ch]+-[Cl]

5

12

g(r)

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 6 of 16

0.5 0.4

0.8

1.2 0 r (nm)

0.4

0.8

0 0

1.2

0.4

(c)

0.8

1.2 0 r (nm)

0.4

0.8

1.2

(d)

Figure 5: Radial distribution functions corresponding to (a) HO[Ch]+ -[Cl]− and HOEG -[Cl]− (b) N[Ch]+ -[Cl]− and N[Ch]+ -OEG (c) HO[Ch]+ -OEG and HOEG -O[Ch]+ and (d) HOEG -OEG and OEG -OEG atomic pairs in ethaline at 303 K. Here HO[Ch]+ , O[Ch]+ and N[Ch]+ are the hydroxyl hydrogen, oxygen, and nitrogen atoms of choline cation, respectively. HOEG and OEG are hydroxyl hydrogen and oxygen atoms of ethylene glycol, respectively.

droxyl hydrogen of EG and [Cl]− (HOEG -[Cl]− ). From here we clearly see the role of [Cl]− as hydrogen bond acceptor. Also one can observe a competition between the [Ch]+ cation and EG to form hydrogen bond with [Cl]− . Concomitantly, we also observe strong correlation between nitrogen of [Ch]+ and [Cl]− having peak at 0.43 nm in the corresponding RDF shown in Fig. 5(b) (left panel ). We can say that the primary electrostatic interaction between the counter-ions is intact. Overall the [Cl]− ion is involved with both hydroxyl group of cation as well as ammonium group leading to two type of correlations between cation and anion. This observation supports the presence of doublet in the previously discussed COM RDF between

[Ch]+ and [Cl]− . Other than this the association of N[Ch]+ with EG is also seen in Fig. 5(b) which indicates the interaction of ammonium group of choline cation with hydroxyl group of EG. Further looking into the hydrogen bonding interactions manifested by [Ch]+ and EG from Fig. 5(c), we see that both of these species act as hydrogen bond donor as well as acceptor group for each other. However, the corresponding RDFs reveal very low probability of H-bonding between the two as compared to that with [Cl]− . To visualize the molecular level arrangement depicted by these observations, we have rendered an equilibrium snapshot showing representative organization of these species in bulk phase of ethaline in Fig. 6.


6

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Figure 6: Molecular representation of frequently observed arrangement of [Ch]+ , [Cl]− and EG molecules in ethaline at 303 K. Here nitrogen, oxygen, carbon, and hydrogen atoms of [Ch]+ and EG are shown as blue, red, cyan and white spheres, respectively. [Cl]− anions are rendered as green spheres. The red-dashed lines represent prominent interactions present in the system.

Table 2: Positions of first minimum in different RDF pairs and their corresponding coordination number in ethaline at 303 K. RDF Pair HO[Ch]+ -[Cl]− HOEG -[Cl]− N[Ch]+ -[Cl]− N[Ch]+ -OEG HO[Ch]+ -OEG HOEG -O[Ch]+ HOEG -OEG OEG -OEG

ethylene glycol molecule. The first peak seen at around 0.2 nm in HOEG -OEG pair has contribution from both inter- and intra-molecular H-bond interactions present in between EG molecules and the next peak at around 0.24 nm is only due to intra-molecular H-bond interactions exhibited by its gauche conformers. Further, the peak at ∼0.37 nm in the same RDF corresponds to the correlation of hydroxyl hydrogen and oxygen of trans conformers of ethylene glycol. The peak at 0.52 nm is only due to the inter-molecular interactions between the hydroxyl hydrogen and oxygen atoms. Similarly, as shown in right panel of Fig. 5(d), we observe first peak in the OEG -OEG RDF at around 0.28 nm, which corresponds to the intra- and inter-molecular interactions between the oxygens of gauche conformers. Followed by this feature, is a peak at ∼0.37 nm, which is due to intra-molecular interactions between the oxygen atoms of trans isomers. The third peak at around 0.46 nm originates only due to the inter-molecular interactions between the oxygen atoms of EG of both the isomers.

rmin (nm) ncoord 0.33 0.80 0.33 0.74 0.56 2.73 0.55 6.86 0.25 0.17 0.25 0.02 0.22 0.23 0.33 0.99

As discussed earlier, the EG-EG self interactions are also observed in the DES, therefore in Fig. 5(d) we have shown the RDFs for interand intra-molecular correlations between the hydroxyl hydrogen and oxygen of EG as well as self correlation for oxygen-oxygen. The involvement of this HBD species with other species of ethaline leads to depreciation in the interand intra-molecular hydrogen bonding interactions present in pure ethylene glycol. 36 It is important to note here that the multiple peaks observed in Fig. 5(d) (left panel ) are due to the interactions among different conformers of


7

The Journal of Physical Chemistry RADF

RADF

20 10

0.4 0.2

0

0.1

0.2

0.8 0.7

20

0.8 0.6

0.6 0.5 0.4

10

0.3 0.2 0.1

0

0

0.3

0.9

0.1

rHO[Ch] −[Cl]− (nm) +

(a) RADF 0.06

0.3

0

0.03

10

g(r, θ)

0.04

0.02 0.01

0.2 rHO[Ch] −OEG (nm)

0

0.3

20

0.02 0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0

g(r, θ)

20

∠ HO−OEG−O[Ch]+ (°)

30

0.05

0.1

0.2 rHOEG−[Cl]− (nm)

(b) RADF

30

0

g(r, θ)

1

∠ HO−OEG−[Cl]− (°)

30

1.2

g(r, θ)

∠ HO−O[Ch]+−[Cl]− (°)

30

∠ HO−O[Ch]+−OEG (°)

10 0

0.1

+

0.2 rHOEG−O[Ch]

+

(c)

0.3 (nm)

(d)

80 70 60 50 40 30 20 10 0

0.1

0.2 rHOEG−OEG (nm)

0.3

0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0

g(r, θ)

RADF ∠ HO−OEG−OEG (°)

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 8 of 16

(e)

Figure 7: Radial angular distribution functions (RADFs) for all the key hydrogen bonding interactions present in ethaline at 303 K.

3.3

Radial Angular Distribution Functions and Hydrogen Bonding Interactions

Fig. 7(a)-7(b), the probability of H-bonding interaction of hydroxyl groups of [Ch]+ and EG with [Cl]− acceptor is maximum amongst all possible H-bonding interactions. This signifies the crucial role played by such strong hydrogen bonding interactions in deciding the structural framework of ethaline DES. Here we observe relatively stronger H-bonding interactions between the hydroxyl group of [Ch]+ and [Cl]− as compared to that between EG and [Cl]− . This observation slightly differs from the findings of previous studies carried through different force-

In order to affirm the observations for hydrogen bonding interactions discussed previously, in Fig. 7 we have shown the radial angular distribution functions, g(r,θ), for all possible Hbonding interactions present in ethaline. Note that the angle used in all the RADFs is 6 H-DA and the distance, r, is between the hydrogen and the acceptor atom. As can be seen from


8

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fields. 28 On the other hand, Figs. 7(c)-7(d) depict comparatively weaker H-bonding interaction between the choline cation and ethylene glycol species which is similar to that observed in the RDFs for the same pairs (See Fig. 5(c)). Comparing the two RADFs in Figs. 7(c)-7(d), we also see that the probability of EG to act as H-bond acceptor is more than as donor with respect to choline cation. Again the EG-EG hydrogen bonding is complex due to the presence of inter- and intra-molecular hydrogen bonding interactions (Fig. 7(e)). In the past, the existence of significant EG-EG H-bonding has been discussed by Ferreira and coworkers, 28 though the understanding of the distinct role played by inter- and intra-molecular EG-EG hydrogen bonding interactions would provide much clearer picture. Here the maximum probability observed at rHA ≤0.25 nm and 6 H-D-A≤30◦ corresponds to the intermolecular interactions and that for 6 H-D-A≤60◦ corresponds to both inter- and intra-molecular H-bonding interactions exhibited by ethylene glycol species. We can see the intermolecular interactions between EG molecules are less profound than the intramolecular because of the association of ethylene glycol with other DES species. However, the EG-EG H-bonding interactions are still more prominent than that between EG-[Ch]+ .

multaneously. Hence, the intervention of HBD species is clearly depicted through this SDF plot.

3.4

3.4.2

3.4.1

Figure 8: Spatial distribution functions around central [Ch]+ in ethaline at 303 K. Here solid green and transparent pink isosurfaces represent [Cl]− and oxygen atoms of EG respectively. The isovalues (in nm−3 ) chosen for rendering these surfaces around central cation for oxygen of EG and [Cl]− are 6.2 and 4.6, respectively.

Spatial Isodensity Surfaces Three-dimensional isodensity surfaces around choline cation

Three-dimensional isodensity surfaces around ethylene glycol

As known, EG exists broadly into ∼80% gauche and ∼20% trans conformations in the neat liquid. 36,65,66 However, when used as a component in an eutectic mixture, this ratio changes (see Fig. S2 in the Supporting Information). The trans conformers become more probable in the DES as compared to that in pure ethylene glycol. In virtue of that, here in this section we have analysed crucial atomic SDFs around central EG with either of its conformations. Fig. 9(a) shows the isodensity surface of [Cl]− as well as oxygen and nitrogen atoms of [Ch]+ around gauche EG. It is quite evident that the [Cl]− likes to be concentrated near the hydroxyl hydrogens of ethylene glycol species thus confirming the formation of hydro-

The atomic spatial distribution functions of EG oxygen and [Cl]− around the central [Ch]+ cation are shown in Fig. 8. The isodensity surface corresponding to [Cl]− is specifically observed around the hydroxyl and ammonium groups of [Ch]+ cation. This observation is in good corroboration with that obtained through RDFs (Fig. 5(b) and Fig. 5(c)), indicating the presence of strong hydrogen bonding as well as electrostatic interactions between the two species. In addition to this, the first solvation shell around [Ch]+ cation also consists of oxygen of EG which shows the association of oxygen of EG with cation as well as anion si-


9


(a)

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(b)

Figure 9: Spatial distribution functions around central EG having (a) gauche and (b) trans conformations in ethaline at 303 K. Here solid green represent [Cl]− . Solid red and transparent blue isosurfaces represent oxygen and nitrogen atoms of [Ch]+ , respectively. Isovalues (in nm−3 ) chosen for rendering these isosurfaces for chloride ion, oxygen and nitrogen atoms of cation around gauche EG are 7.6, 3.15 and 5.26 and that around trans EG are 6.0, 3.02 and 4.05, respectively.

gen bond between them. Along with that, we observe the isodensity surface of oxygen and nitrogen of [Ch]+ in the first solvation shell around EG which depicts the association of the respective hydroxyl and ammonium group of [Ch]+ with EG. The atomic spatial isodensity functions around the central trans EG isomer are shown in Fig. 9(b). The isodensity surface of [Cl]− distributed around the hydroxyl groups of EG confirms strong hydrogen bonding interactions between the two species irrespective of the conformation of EG. This [Cl]− isodensity surface is followed by the isosurface of nitrogen and oxygen of [Ch]+ . Hence, we can infer an arrangement of [Cl]− sandwiched between the EG and [Ch]+ from here.

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cate the presence of pseudo charge alternation behavior. However, other long range correlations dominated by EG also exhibit peaks and antipeaks in the low q region. Such important features depicting the presence of unique long range structural ordering in the DES are not observed through total S(q) due to cancellation effect. The existence of both long- and shortranged features in ethaline structure and its hidden anatomy were deciphered only through species-wise judicious partitioning of the simulated total X-ray structure function. Strong hydrogen bond network is observed through the inter- and intra-species real space correlations as well as radial angular distribution functions. Choline cation and ethylene glycol are found in a perpetual competition to form H-bond with the anionic species. Concomitantly, choline cation and ethylene glycol also form comparatively weak hydrogen bond. The H-bond donor, ethylene glycol, is found to intrude the ion pair correlation, however the primary co- and counter-ions electrostatic interactions are still significant to give rise to a unique long range structural organization on molecular scales. The inter-molecular H-bonding interactions for the EG-EG pair are not much profound as compared to the intra-molecular interactions, which indeed exist due to gauche

Conclusions

Refined force-field parameters were employed to perform atomistic simulation of ethaline DES in order to appreciate it’s bulk phase morphology. The simulated partial X-ray scattering structure function unveils a complex arrangement of the ions and HBD at short range as well as long range. Peaks and antipeaks present in the low q region of the partial S(q)s for the co-ions and counter-ion correlations indi-


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isomers of EG present in the DES. In addition to this, the real-space EG-EG pair correlations reveal the vital contributions of the two conformers of EG in inter- and intra-molecular hydrogen bonding interactions. All these balancing associations lead to a complex hybrid structural arrangement of this eutectic mixture and its overall stability. The elucidation of structure of ethaline presented in this study will definitely aid further utilization of this DES in multiple application domains.

Lunkenheimer, P.; Loidl, A.; Mamontov, E. Glycerol Hydrogen-Bonding Network Dominates Structure and Collective Dynamics in a Deep Eutectic Solvent. J. Phys. Chem. B 2018, 122, 1261–1267. (4) Singh, B.; Lobo, H.; Shankarling, G. Selective N-Alkylation of Aromatic Primary Amines Catalyzed by Bio-Catalyst or Deep Eutectic Solvent. Catal. Lett. 2010, 141, 178–182. (5) Li, C.; Li, D.; Zou, S.; Li, Z.; Yin, J.; Wang, A.; Cui, Y.; Yao, Z.; Zhao, Q. Extraction Desulfurization Process of Fuels with Ammonium-Based Deep Eutectic Solvents. Green Chem. 2013, 15, 2793– 2799.

Supporting Information Figures for equilibrium simulation boxes and comparison of dihedral angle distributions. The Supporting Information is available free of charge via the internet at http://pubs.acs.org. Notes: The authors declare no competing financial interest.

(6) Das, A.; Biswas, R. Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time-Resolved Fluorescence Measurements of Reactive and Nonreactive Dynamics in (Choline Chloride + Urea) Melts. J. Phys. Chem. B 2015, 119, 10102–10113.

Acknowledgments SK thanks UGC India and AM thanks IIT Delhi for fellowship. The authors thank Mr. H. S. Dhattarwal for helping us in creating some of the graphics. This work is financially supported by the Department of Science and Technology (DST), India, through the FIST grant awarded to Department of Chemistry, IIT Delhi, for augmentation of HPC facility at IIT Delhi.

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Anatomy of Microscopic Structure of Ethaline Deep Eutectic Solvent

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