Unusual Temperature Dependence of Nanoscale Structural

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Unusual Temperature Dependence of Nanoscale Structural Organization in Deep Eutectic Solvents Supreet Kaur, and Hemant K. Kashyap J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02378 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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The Journal of Physical Chemistry

Unusual Temperature Dependence of Nanoscale Structural Organization in Deep Eutectic Solvents Supreet Kaur 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

1

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

Abstract In a recent work we reported the existence of nanoscale spatial organization in alkylamide+Li+ /ClO4− based deep eutectic solvents (DESs). We also described that the nanoscale organization in these systems was primarily due to ion-pair self-segregation. This segregation also accompanied prominent interaction between the ionic species of the electrolyte and alkylamide polar groups. In the present study, we show that for the DESs studied the intensity of prepeak, the so-called marker of the nanoscale heterogeneity, in the X-ray and neutron scattering structure functions increases upon increasing temperature. Herein, we show that the increase in the heterogeneity is because of the enhanced correlations between the ionic species at higher temperature. We also show that the rate of enhancement in the ionic correlations with temperature is more than the rate of diminution in the electrolyte-alkylamide cross-correlations. The alkylamide-alkylamide correlations are largely unaffected by any change in the temperature.

Introduction Recently, a popular class of ionic liquid(IL) analogues, called deep eutectic solvents(DESs) have emerged as promising solvents in various applications such as synthesis, extraction, separation, bio-transformation etc. 1–16 Despite of having similar characteristics to that of ILs, 17–19 cost wise DESs are more economic and benign. The popularity of DESs as emerging solvent is quiet obvious, but still the study of structural and dynamic aspects of these promising solvents is lagging behind. Studies on structural characteristics of DESs through X-ray/neutron scattering experiments as well as simulations have emerged only recently. 20–24 Experimental and theoretical investigation on the electrolyte+alkylamide based DESs have been carried out by Biswas and coworkers. 25,26 Their work also points towards the presence of various type of aggregates, which depends also on the nature of anion present in the mixture present in the DESs and temperature. 2


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As the physico-chemical properties and the stability of DESs depend on its constituents, the use of hydrogen bond donors such as glycerols, alkylamides and urea has been seeking more attention. 27,28 The extensive hydrogen bonding in these components not only causes large depression in the melting point of the resulting mixture but also shields the charges of the ionic components by complexing with them. 29 Amongst one of the interesting properties of DESs, the study of DESs-water mixture has produced very appreciating results as these solvents are non-reactive with water. 21,30,31 In a recent work reported by Dai et al. 30 it is clearly evident that the addition of water diminishes the interactions between the components of the mixture, leading to almost negligible interactions at more than 50% water content and also causes decrease in the viscosity of the mixture. As the electrolyte-based systems are known to be a growing interest in field of electrochemistry, recently a path breaking work done by Wang et al. 32 on Li+ salt-based electrolyte system has revealed many interesting properties of such systems. One of which is their use in Li+ batteries with an enhancement in the electrochemical window from 4-5 V approximately without the intervening obstacle of dissolution of metal ion at high voltage. In particular, the authors have investigated lithium bis(fluorosulfonyl)amide (Li+ /FSA− ) plus carbonate ester electrolyte system at various salt concentrations and found that at super-high concentrations there is formation of a peculiar three-dimensional structural network, which in turn is responsible for this excellent performance of such electrolytes at higher voltages. These observations were further confirmed by the molecular dynamics (MD) simulation studies, wherein it was shown that the aggregated clusters of the electrolyte predominate over individual ion-pairs at higher salt concentrations. Similar study on Li+ based electrolyte system has been reported by Qian and coworkers using lithium bis(fluorosulfonyl)imide (Li+ /FSI− ) plus dimethoxy ethane (DME), 33 in which the observed the charaterstic ability of such systems to provide excellent high rate cycling stability of Li metal anode at high concentration of Li+ /FSI− -DME electrolyte solution in batteries. In one of our recent studies, 22 we also observed similar aggregation of Li+ /ClO4− in alkylamide systems which is similar to what

3



is observed in the above mentioned studies. 32,33 +

-

Acetamide + Li /ClO4 (Sim.) +

1.3

-

Acetamide + Li /ClO4 (Expt.) +

-

+

-

Propionamide + Li /ClO4 (Sim.) Propionamide + Li /ClO4 (Expt.)

1.25 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

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1.2

1.15

1.1 300

310

320

330

340

350

Temp. (K)

Figure 1: Comparison of simulated (open circles) and experimental (solid circles) densities, ρ at different temperatures. Please refer to reference number 26 for experimental density data in the main article.

Herein, we are interested in exploring the structures of DESs and examine their molecular level structural arrangement towards the thermodynamic alterations, such as temperature. Leron et al. experimentally investigated a few DESs and their aqueous mixtures within a temperature range of 298.15-333.15 K and studied the effects of temperature on their densities and refractive indices which in turn showed a linearly decreasing trend with increase in temperature. 34 Similar effects of temperature has been observed on the vapour pressure of DESs. 35 Our current work focuses on the temperature dependence of two different alkylamides (RCONH2 ) plus Li+ /ClO4− based DESs, viz., acetamide+Li+ /ClO4− and propionamide + Li+ /ClO4− in molar ratio 0.81:0.19. Notice that the composition of the DESs studied here is very similar to super-concentrated electrolyte solutions used in batteries. 32 The effects of temperature on the densities, structure functions and radial distribution function for both the systems have been explored and compared with experimental data wherever possible. These properties have been computed at four different temperatures: 303, 323, 333 and 343 K. As we are already aware about the tendency of Li+ /ClO4− ions to segregate in the alkylamide, in the present analysis we have discussed about the temperature dependence of this segregation. Since the prepeak in the X-ray scattering structure function is a marker 4


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of this nanoscale segregation, how temperature change affects the nanoscale segregation is primary goal of this study. We observe that the Li+ /ClO4− electrolyte based DESs show different behavior in this aspect as compared with several pure solvents and ionic liquids. 36–40

Simulation Details All the simulations were performed following the protocol used in our previous work. 22 In brief, the simulation boxes of acetamide+Li+ /ClO4− and propionamide+ Li+ /ClO4− consisted of 4750 ionic-couples and 20250 alkylamide molecules. This is equivalent to 0.19 and 0.81 mole-fractions of Li+ /ClO4− and alkylamide, respectively. The MD simulations were performed by using the GPU version of GROMACS-5.1.1 (single-precision) package. 41,42 Periodic boundary conditions and minimum image convention were applied. The Li+ ion and both alkylamide molecules were modeled using CHARMM 43 force field parameters as reported in the previous work on similar systems from our group. 22 The parameters for O and Cl atoms of perchlorate anion were adapted from those given by Maginn et al. excluding the Lennard-Jones parameters for Cl which were taken as σ =0.4417 nm and =0.4928 kJ mol−1 . 44–48 Further for the sake of completeness we have also provided all the bonded and non-bonded parameters in tables S2 through S4 of the supplementary information. The equations of motion were solved by using the leap-frog algorithm with 1 fs time step. PACKMOL 49 package was used to generate the initial configuration for each system. The simulation boxes were equilibrated for at least 28 ns at each target temperature and 1 bar pressure. The temperature and pressure of each system was kept constant by using NoseHoover thermostat 50–52 and Parrinello-Rahman 53 barostat, respectively. A production run of 5 ns for each system was carried out for analyzing the DES’s properties. The cutoff radius for the short-range interactions were set to 1.2 nm with a switching function used from 1.0 to 1.2 nm. Electrostatic interactions were evaluated using Particle Mesh Ewald (PME) 54,55 summation technique. The equilibrium values of the simulated bulk densities at all the four

5



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temperatures for both the systems were in excellent agreement with experimental data. 26 The comparison is provided in Table S1 and Fig. 1. The maximum deviation in the simulated and experimental density is about 2% for the acetamide system and 0.6% for the propionamide DES. The X-ray/neutron scattering static structure function, S(q) and its sub-components were computed using the general methodology proposed in the literature. 56 We first compute the radial distribution function (RDF), gij (r), for the atoms of type i and j. The computed RDFs include both intra- and intermolecular terms. By using the gij (r)s the X-ray/neutron scattering static structure function, S(q) can be calculated as,

ρo S(q) =

n P n P

xi xj yi (q)yj (q)

i=1 j=1

L/2 R 0

[

n P

xi yi (q)][

i=1

4πr2 [gij (r) − 1] sinqrqr ω(r)dr n P

.

(1)

xj yj (q)]

j=1

In Eq. 1, xi is the mole-fraction of atom of type i. We take yi (q) as the X-ray atomic form factors (fi (q)) for the calculation of X-ray scattering structure function. 57 For neutron scattering structure function, we take yi (q) as bi , the coherent part of the neutron scattering length for ith type atom. 58 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), 59,60 which could be used to reduce the effects of finite truncation error. In this study we split the total S(q) into its cationic, anion and alkylamide components as 22,36,61–66

S(q) = S Li

+

−Li+

−

(q)+S ClO4

− −ClO4

+

(q)+2S Li

− −ClO4

(q)+2S Li

+

−RCON H2

−

(q)+2S ClO4

−RCON H2

(q)+S RCON H2 −RCON H2 (q). (2)

In the above expression, it is advantageous to collate the electrolyte components together such that 22

S(q) = S Li

+

− − /ClO4 −Li+ /ClO4

(q) + 2S Li

+

− /ClO4 −RCON H2

6

(q) + S RCON H2 −RCON H2 (q).


(3)

Page 7 of 25

In Eq. 3, we defined

+

S Li

− − /ClO4 −Li+ /ClO4

+

(q) = S Li

−Li+

−

(q) + S ClO4

− −ClO4

+

(q) + 2S Li

− −ClO4

(q)

(4)

and +

S Li

/ClO4− −RCON H2

(q) = S Li

+

−RCON H2

−

(q) + S ClO4 −RCON H2 (q).

(5)

One more informative partitioning scheme could be polarity based, which is given by

+

S(q) = S Li

/ClO4− /CON H2 −Li+ /ClO4− /CON H2

(q) + 2S Li

+

/ClO4− /CON H2 −R

(q) + S R−R (q).

(6)

Further details of the S(q)’s partitioning scheme used here can be found in the literature. 56,67 4

X-ray Scattering

4

303 K 323 K 333 K 343 K

3

303 K 323 K 333 K 343 K

S(q)

2

1 0

1 0

+

-1 -2 0

X-ray Scattering

3

2

S(q)

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

3

-2 0

Acetamide + Li /ClO4 0.5

1

1.5 -1

2

2.5

+

-

Propionamide + Li /ClO4 0.5

1

1.5 -1

q (Å )

q (Å )

(a)

(b)

2

2.5

3

Figure 2: Temperature dependence of X-ray scattering structure functions (S(q)s) for (a) acetamide+Li+ /ClO4− and (b) propionamide+Li+ /ClO4− systems. The data for 323 K were taken from Kaur et al. 22

7



45

-

40

) -1

35

0.25 0.2

30 25

310

320

330

Temp. (K)

340

25 20

0.15

15

0.1 300

10 350

35 30

0.25 0.2

20

0.15

40

qlow D

0.3

D (Å)

0.3

45

-

0.35

)

qlow D

+

Propionamide + Li /ClO4

-1

0.35

0.1 300

0.4

D (Å)

+

Acetamide + Li /ClO4

q (Å

0.4

q (Å

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 25

15

310

(a)

320

330

Temp. (K)

340

10 350

(b)

Figure 3: Temperature dependence of low q peak position (qlow ) and corresponding characteristic length (D) for (a) acetamide+Li+ /ClO4− and (b) propionamide+Li+ /ClO4− systems.

Results and Discussion X-ray and Neutron Scattering Structure Functions and Partial Components In Fig. 2(a)-2(b) one can observe that two peaks are present in the total X-ray scattering structure functions, S(q)s for both the DESs. As shown earlier, the lowest q peak, called as prepeak or first sharp diffraction peak (FSDP) resembles nanoscale structural organization and corresponds to self-segregated domains of Li+ /ClO4− . 22 The intensity of the prepeak shows an increasing trend with increase in temperature for both the systems. This behavior of DESs, in which the intensity of FSDP increases with increasing temperature, is opposite to what one usually observe in neat and simple liquids and also in several ionic liquids. 37–40 But, the current observation is similar to trihexyltetradecylphosphonium ([P666,14 ]+ ) 68,69 and ammonium 36 based ILs. In addition, it is also observed that the prepeak position shifts towards lower q value with rise in the temperature as is observed in most cases (Figs. 3(a) and 3(b)). Accordingly, as depicted in Fig. 3(b) the real space characteristic length scale increases with increasing temperature. Another interesting observation is the dependence of the prepeak

8


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position on the alkylamide chain length i.e. nanoscale segregation occurs at lower length scale in the propionamide+Li+ /ClO4− system as compared to acetamide+Li+ /ClO4− . For ˚−1 shows very minor variation with both the systems, the principal peak at around 1.5 A increasing temperatures than the prepeak, displaying almost same intensities and positions for all temperatures. As we have already discussed about nanoscale ordering in these systems in our previous work, 22 one can interpret that the probability of nanoscale organization/heterogeneity increases with increasing temperature. Hence, one can infer that both the factors - temperature and tail length of alkylamide have impact on the prepeak or nanoscale heterogeneity of these DESs. Similar low q peak dependence on temperature is observed in the neutron scattering structure functions for both the DESs (Please see Fig. S2 in SI). 303 K

5 Total Polar-Polar Polar-Apolar Apolar-Apolar

3

Total Polar-Polar Polar-Apolar Apolar-Apolar

4 3

2

Total Polar-Polar Polar-Apolar Apolar-Apolar

6 4

2

S(q)

1 0

1 0

2 0

-1

-1 +

-


-2 -3 0

343 K

333 K

S(q)

4

S(q)

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


0.5

1

1.5 -1

q (Å )

(a)

2

2.5

-2

-2

+


-3 3

-4 0

0.5

1

1.5 -1

2

2.5

-

+

3

0

0.5

1

1.5 -1

q (Å )

(b)

-


-4

2

2.5

3

q (Å )

(c)

Figure 4: Polarity based partial X-ray scattering structure acetamide+Li+ /ClO4− ((a)-(c)) system at different temperatures.

functions

for

In order to give a more clear picture of the peaks observed in total S(q), we have also examined the partial components of the X-ray scattering structure functions at all the temperatures but results for only three are presented for clarity. Herein, we have used two distinct type of partitioning scheme to understand the origin of the peaks i.e. component based and polarity based partial X-ray scattering structure functions, which are given by Eqs 2 & 6 respectively. Polarity based partial and total X-ray structure functions of acetamide+Li+ /ClO4− system is shown in Fig. 4(a)-4(c). The figure represents that while polar-polar components contribute positively to the prepeak, the polar-apolar components 9



contribute negatively to the prepeak at all the temperatures. Moreover, polar-polar correlations has overwhelming positive contribution to the prepeak than its apolar-apolar counterpart. Thus, the enhanced interactions between the polar-polar components at higher temperature is much more than the decrease in the polar-apolar interactions, which consequently leads to enhancement in prepeak with temperature. On the other hand, as clearly observed in Fig. 4 the apolar-apolar correlations are less likely to be affected by temperature alterations. 303 K

8

6

Total + + Li /ClO4 - Li /ClO4 RCONH2 - RCONH2

-4

+

-

Li /ClO4 - RCONH2

-6 0.5

1

1.5 -1

q (Å )

(a)

2

2.5

3

6

3

S(q)

S(q)

0 -2

0 -3

Total + + Li /ClO4 - Li /ClO4 RCONH2 - RCONH2

-6 -9

+

-

Li /ClO4 - RCONH2

-12 -15 0

R= CH3

12

9

4

343 K

18

R= CH3

12

2

-8 0

333 K

15

R= CH3

6

S(q)

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 10 of 25

0.5

1

1.5 -1

2

2.5

q (Å )

(b)

3

0 Total + + Li /ClO4 - Li /ClO4 RCONH2 - RCONH2

-6 -12 -18 0

+

-

Li /ClO4 - RCONH2

0.5

1

1.5 -1

2

2.5

3

q (Å )

(c)

Figure 5: Electrolyte and alkylamide based partial X-ray scattering structure functions for acetamide+Li+ /ClO4− ((a)-(c)) system at different temperatures.

From the second partitioning scheme, i.e. the electrolyte and alkylamide based partial structure functions, as shown in Fig. 5(a)-5(c), one can observe significant positive contribution in the total S(q) from Li+ /ClO4− -Li+ /ClO4− component, which increases with increasing temperature. Conversely, the peak corresponding to Li+ /ClO4− -RCONH2 component decreases upon increasing temperature. These components predominantly contribute to the prepeak. Notice that the rate at which the electrolyte-electrolyte correlations on the length scale corresponding to the prepeak increases with temperature is higher than the rate at which the electrolyte-alkylamide cross-correlations decreases upon increasing temperature. The RCONH2 -RCONH2 partial S(q) also shows slight increment with rise in temperature. Further, these observations are confirmed through residue wise interaction energy calculations as mentioned in Table S5. From the table, we can observe that the real space part of the 10


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electrostatic (short-range) attractions between Li+ and ClO4− is maximum and it increases with rise in temperature, while the other interactions are still weaker than that of Li+ -ClO4− . Hence, the overall sum of these partial components leads to increased prepeak intensity or spatial heterogeneity at higher temperature because of increase in Li+ -ClO4− interactions. Similar effects are observed for the propionamide+Li+ /ClO4− system, see Figs. S3((a)-(c)) and S4((a)-(c)).

Real Space Correlations and Hydrogen Bonding The real space pair correlations for the two systems at four different temperatures have also been investigated via radial distribution functions to further help understand the changes in the structural landscape with temperature. The correlations between the various species and their first solvation shells have already being discussed in our previous work, so herein we focus only on exploring the effect of temperature on these correlations and interactions. The Li+ -Cl (Fig. 6(a)), Li+ -Li+ (Fig. 6(b)) and Cl-Cl (Fig. 6(c)) RDFs for the acetamide+Li+ /ClO4− system show that upon increasing temperature the nearest neighbor peaks slightly shift towards the shorter distances. Also, there is appreciable increase in the peak heights of the counter- and co-ion correlations with increasing temperature, implying stronger ion-pairing and enhanced ion-pair or Li+ /ClO4− segregation at higher temperature. In Fig. 7, we have shown the RDFs for Li+ -OR (Fig. 7(a)), Cl-OR (Fig. 7(b)), Li+ -N (Fig. 7(c)) and HN-O (Fig. 7(d)), where O is ClO4− oxygen, OR is the alkylamide oxygen atom, HN and N are acetamide amide group hydrogen and nitrogen atoms, respectively. From these figures (Fig 7), we observe that with rise in temperature not only the peaks shift towards longer distances for few cases (see Table S6 and S7 in Supplementary Information) but also the peak intensities of all the RDFs increase, which is exactly opposite to the trend observed for correlations between the electrolyte species. This means that at higher temperature the cross-correlations between the ionic components and acetamide polar groups (carbonyl oxygen and amide) are depreciated along with slight increase in their 11



+


+

-


+

Li - Cl

-

+

40

5 303 K 323 K 333 K 343 K

35

303 K 323 K 333 K 343 K

4

g(r)

30

+

Li - Li

6

45

g(r)

25

3

20

2

15 10

1 5 0 0.2

0.25

0.3

0.4

0.35

0.45

0 0.1

0.5

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

r(nm)

r(nm)

(a)

(b) +


-

Cl - Cl

10 9 303 K 323 K 333 K 343 K

8 7 6

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 12 of 25

5 4 3 2 1 0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

r(nm)

(c)

Figure 6: Temperature dependent radial distribution functions for (a) Li+ -Cl, (b) Li+ Li+ and (c) Cl-Cl.

12


Page 13 of 25

+


-

+


+

Li - OR

20

Cl - OR

2.5

303 K 323 K 333 K 343 K

303 K 323 K 333 K 343 K

2

g(r)

15

g(r)

-

3

10

1.5 1

5 0.5 0

0 0

0.1

0.2

0.3

0.4

0.5

0.2

0.3

0.4

0.5

r(nm)

(a) +


0.6

0.7

0.8

r(nm)

(b)

-

+

+


Li - N

2.5

-

HN - O

1.5

303 K 323 K 333 K 343 K

2

303 K 323 K 333 K 343 K

1.25 1

1.5

g(r)

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


0.75

1 0.5

0.5

0

0

0.25

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

0

0.1

0.2

r(nm)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

r(nm)

(c)

(d)

Figure 7: Temperature dependent radial distribution functions for (a) Li+ -OR, (b) Cl-OR, (c) Li+ -N, (d) HN-O. OR is the acetamide oxygen atom, HN and N are acetamide amide group hydrogen, nitrogen atoms, respectively. O is ClO4− oxygen.

separation for some select pairs. Also, the hydrogen-bonding between the perchlorate oxygen atoms and acetamide amide group hydrogens reduces with rise in temperature although the ion-pairing and electrolyte segregation are enhanced. This observation can be supplemented with the temperature dependence of coordination numbers for all these pairs and are provided in Table S6 and Fig S5 ((a)-(j)). We clearly observe that the coordination number corresponding to first solvation shell of the pair of species belonging to electrolyte shows an increasing trend with temperature, while those corresponding to non-electrolyte are almost

13



insensitive towards any change in temperature and the cross terms show decrease in the coordination number with temperature. +

Acetamide+LiClO4


HN-OR

2.5

-

N - OR

2.5

303 K 323 K 333 K 343 K

2

303 K 323 K 333 K 343 K

2

1.5

g(r)

g(r)

1.5

1

1

0.5

0.5

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.1

1

0.2

0.3

0.4

0.5

r(nm)

(a) +


0.6

0.7

0.8

0.9

r(nm)

(b)

-

+


C-C

2.5

-

CT - CT

2

303 K 323 K 333 K 343 K

2 303 K 323 K 333 K 343 K

1.5

1.5

g(r)

g(r)

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Figure 8: Temperature dependent radial distribution functions for (a) HN-OR, (b) N-OR, (c) C-C and (d) CT-CT. HN, OR, C, N, and CT are the amide hydrogen, carbonyl oxygen, carbonyl carbon, nitrogen and methyl carbon atoms, respectively.

The pair correlations involving only acetamide atoms are shown in Figs. 8(a)-8(d). From these figures, it is clearly evident that there is very minor appreciation in the N-OR correlations (Fig. 8(b)) with increasing temperature. The hydrogen bonding between the amide hydrogen and carbonyl oxygen atoms (Fig. 8(a)) also tend to increase slightly with increasing temperature in contrast to that between amide hydrogens and perchlorate oxygen (Fig. 7(d)), as discussed previously. Whereas, decreased CT-CT correlation (Fig. 8(d)), CT being 14


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alkylamide methyl carbon, is observed for nearest neighbor but beyond the nearest neighbor shell the CT-CT correlations are enhanced as the temperature is increased. Overall, negligible changes are observed for all the cases. Various propionamide+Li+ /ClO4− correlations have also been provided in Fig. S6((a)-(i)) which apparently show very similar trends with rising temperature as acetamide+Li+ /ClO4− system.

Conclusion The present molecular dynamics study focused on the temperature dependence of the nanoscale heterogeneity in lithium perchlorate salt+alkylamide based deep eutectic solvents. The simulated X-ray and neutron scattering structure functions reveal that this temperature dependence of the nanoscale domain organization, formed by self-segregation of Li+ /ClO4− , is rendered by increased Li+ /ClO4− -Li+ /ClO4− correlations, decreased Li+ /ClO4− -alkylamide cross-correlations and minor changes in the alkylamide-alkylamide correlations. In addition, we have shown that the increased Li+ /ClO4− -Li+ /ClO4− correlations at higher temperatures outplay the decrease in the Li+ /ClO4− -alkylamide cross-correlations, leading to enhancement of the prepeak intensity with increasing temperature. In summary, the behavior of DESs with change in temperature is due to appreciated ion-pair segregation at higher temperatures and thus it is responsible for the increasing nanoscale heterogeneity in both the DESs. The enhanced formation of Li+ /ClO4− domains or increased heterogeneity in these system at elevated temperatures is one the major results of the current study. In addition, our MD results showed that while the hydrogen bonding between the amide hydrogens of alkylamide and the oxygens of perchlorate ion is diminished, the same interaction between the amide group hydrogens and carbonyl oxygen shows slight increase with increasing temperature. So we can say that the structure of the DESs studied is not only predominated by electrostatic interactions but also by significant hydrogen bonding. The current observations corroborates well with the recent studies on Li+ salt based electrolyte systems, Li+ /FSA− +carbonate ester

15



and Li+ /FSI− +DME, wherein the formation of aggregates and a peculiar three dimensional network of the same was the major outcome. 32,33

Supplementary Information Tables for density, partial charges, bonded and non-bonded parameters, residue wise interaction energies, coordination number and RDF’s maximum/minimum at different temperatures. Figures for molecular structures, neutron scattering S(q)s, partial S(q)s, coordination number, RDFs for the two DESs studies. Notes: The authors declare no competing financial interest.

Acknowledgements We sincerely thank Professor Ranjit Biswas, Professor Claudio J. Margulis and Professor Edward W. Castner Jr. for valuable discussion and feedback. SK thanks UGC, India for fellowship. Authors thank the IIT Delhi HPC facility for computational resources. This work is supported by the Department of Science and Technology (DST), India, through a grant awarded to HKK (Grant No. SB/FT/CS-124/2014).

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Unusual Temperature Dependence of Nanoscale Structural

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