Unusual Temperature Dependence of Nanoscale Structural

Article pubs.acs.org/JPCB

Cite This: J. Phys. Chem. B 2018, 122, 5242−5250

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 S Supporting Information *

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 the 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.

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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, and biotransformation.1−16 Despite having similar characteristics to those of ILs,17−19 DESs are more economical and benign. The popularity of DESs as emerging solvents is quiet obvious, but still the study of structural and dynamic aspects of these promising solvents is lagging behind. Studies on the structural characteristics of DESs through X-ray/neutron scattering experiments as well as simulations have emerged only recently.20−24 Experimental and theoretical investigations on the (electrolyte + alkylamide)-based DESs have been carried out by Biswas and co-workers.25,26 Their work also points to the presence of various type of aggregates, which depends on the nature of the anion present in the DESs and temperature. As the physicochemical properties and the stability of DESs depend on their constituents, the use of hydrogen bond donors such as glycerols, alkylamides, and urea has been attracting 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 The study of DESs−water mixture has produced highly appreciating results as these solvents are nonreactive with water.21,30,31 From 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 © 2018 American Chemical Society

interactions at more than 50% water content and also resulting in a decrease in the viscosity of the mixture. With the electrolyte-based systems gaining a growing interest in the field of electrochemistry, a recent pathbreaking work carried out by Wang et al.32 on the Li+ salt-based electrolyte system has revealed many interesting properties of such systems. One of them is their use in Li+ batteries with an enhancement in the electrochemical window approximately from 4 to 5 V without the intervening obstacle of dissolution of metal ions at high voltage. In particular, the authors have investigated the 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 the excellent performance of such electrolytes at higher voltages. These observations were further confirmed by the molecular dynamics (MD) simulation studies, which showed that the aggregated clusters of the electrolyte predominate over individual ion pairs at higher salt concentrations. A similar study on Li+-based electrolyte systems has been reported by Qian and co-workers using lithium bis(fluorosulfonyl)imide (Li+/FSI−) plus dimethoxy ethane (DME),33 wherein they observed the characteristic ability of such systems to provide excellent high rate cycling stability of Li metal anode at higher concentrations of Received: March 11, 2018 Revised: April 27, 2018 Published: April 27, 2018 5242

DOI: 10.1021/acs.jpcb.8b02378 J. Phys. Chem. B 2018, 122, 5242−5250

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The Journal of Physical Chemistry B Li+/FSI− + DME electrolyte solution in batteries. In one of our recent studies,22 we also observed aggregation of Li+/ClO4− in alkylamide systems, which is similar to what was observed in the above-mentioned studies.32,33 Herein, we are interested in exploring the structures of DESs and examine their molecular-level structural arrangement toward 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 have been observed on the vapor 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 the molar ratio 0.81:0.19. Notice that the composition of the DESs studied here is very similar to the super-concentrated electrolyte solutions used in batteries.32 The effects of temperature on the densities, structure functions, and the radial distribution functions (RDFs) for both the systems have been explored and compared with experimental data wherever possible. These properties have been studied at four different temperatures: 303, 323, 333, and 343 K. As we are already aware of the tendency of Li+/ClO4− ions to segregate in the alkylamide, in the present analysis we have discussed the temperature dependence of this segregation. Since the prepeak in the X-ray scattering structure function is a marker of this nanoscale segregation, how the temperature change affects the nanoscale segregation is the primary goal of this study. We observe that the Li+/ClO4− electrolyte-based DESs show different behaviors in this aspect as compared with several pure solvents and ILs.36−40

1.2 nm. Electrostatic interactions were evaluated using the particle mesh Ewald54,55 summation technique. The equilibrium values of the simulated bulk densities at all four temperatures for both the systems were in excellent agreement with the experimental data.26 The comparison is provided in Table S1 and Figure 1. The maximum deviation between the simulated and experimental densities is about 2% for the acetamide system and 0.6% for the propionamide DES.

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

The X-ray/neutron scattering static structure function, S(q), and its subcomponents were computed using the general methodology proposed in the literature.56 We first compute the RDF, gij(r), for the atoms of type i and j. The computed RDFs include both intra- and intermolecular terms. By using the RDF, the X-ray/neutron scattering static structure function, S(q), can be calculated as

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SIMULATION DETAILS All 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 20 250 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 the GROMACS-5.1.1 (singleprecision) package.41,42 Periodic boundary conditions and minimum image convention were applied. The Li+ ion and both the alkylamide molecules were modeled using CHARMM43 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 ϵ = 0.4928 kJ mol−1.44−48 Further, for the sake of completeness, we have also provided all bonded and nonbonded parameters in Tables S2 through S4 of the Supporting Information. The equations of motion were solved by using the leap-frog algorithm with 1 fs time step. The PACKMOL49 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 a Nosé−Hoover thermostat50−52 and a Parrinello−Rahman53 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

n

S(q) =

n

ρo ∑i = 1 ∑ j = 1 xixjyi (q)yj (q)∫

L /2

0

4πr 2[gij(r ) − 1]

sin qr ω(r ) qr

dr

⎡∑n x y (q)⎤⎡∑n x y (q)⎤ ⎣ i = 1 i i ⎦⎣ j = 1 j j ⎦

(1)

In eq 1, xi is the mole fraction of an atom of type i. We take yi(q) as the X-ray atomic form factors (f i(q)) for the calculation of the X-ray scattering structure function.57 For the neutron scattering structure function, we take yi(q) as bi, the coherent part of the neutron scattering length for the 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 the finite truncation error. In this study, we split the total S(q) into its cationic, anionic, and alkylamide components as22,36,61−66 +

S(q) = S Li

−Li+

(q) + S ClO4

+ 2S

−

−ClO4 −

Li+−RCONH 2

(q) + 2S

+

(q) + 2S Li

−ClO4 −

(q)

ClO4 −−RCONH 2

(q)

+ S RCONH2−RCONH 2(q)

(2)

In the above expression, it is advantageous to collate the electrolyte components together such that22 +

S(q) = S Li

/ClO4 −−Li+/ClO4 −

+S

+

(q) + 2S Li

RCONH 2−RCONH 2

(q)

/ClO4 −−RCONH 2

(q) (3)

In eq 3, we defined 5243


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The Journal of Physical Chemistry B

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

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

S Li

liquids and also in several ILs.37−40 However, the current observation is similar to that of trihexyltetradecylphosphonium ([P666,14]+)-based68,69 and ammonium-based36 ILs. In addition, it is also observed that the prepeak position shifts toward a lower q value with the rise in the temperature as is observed in most cases (Figure 3a,b). Accordingly, as depicted in Figure 3a,b, the real space characteristic length scale increases with increasing temperature. Another interesting observation is the dependence of the prepeak position on the alkylamide chain length, that is, nanoscale segregation occurs at shorter length scale in the propionamide + Li+/ClO4− system as compared to the acetamide + Li+/ClO4− system. For both the systems, the principal peak at around 1.5 Å−1 shows a very minor variation with increasing temperature when compared with the prepeak, displaying almost the same intensities and positions for all temperatures. From our discussion on 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 an impact on the prepeak or nanoscale heterogeneity of these DESs. A similar low q peak dependence on temperature is observed in the neutron scattering structure functions for both the DESs (please see Figure S2 in Supporting Information). To give a more clear picture of the peaks observed in total S(q), we have also examined the partial components of the Xray scattering structure functions at all temperatures, but results for only three are presented for clarity. Herein, we have used two distinct types of partitioning schemes to understand the origin of the peaks, that is, component-based and polarity-based

/ClO4 −−Li+/ClO4 −

(q)

+

= S Li

−Li+

(q) + S ClO4

−

−ClO4 −

+

(q) + 2S Li

−ClO4 −

(q)

(4)

and +

S Li

/ClO4 −−RCONH 2

=S

(q)

Li+−RCONH 2

(q) + S ClO4

−

− RCONH 2

(q)

(5)

One more informative partitioning scheme could be polaritybased, which is given by +

S(q) = S Li

/ClO4 − /CONH 2−Li+/ClO4 − /CONH 2

+ 2S

(q)

Li+/ClO4 − /CONH 2−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

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RESULTS AND DISCUSSION X-ray and Neutron Scattering Structure Functions and Partial Components. In Figure 2a,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 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 observes in neat and simple 5244


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

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

Figure 6. Temperature-dependent RDFs for (a) Li+−Cl, (b) Li+−Li+, and (c) Cl−Cl.

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

as listed in Table S5. From the table, we can observe that the real space part of the electrostatic (short-range) attractions between Li+ and ClO4− is the maximum and increases with the rise in temperature, whereas the other interactions are still weaker than those of Li+−ClO4−. Hence, the overall sum of these partial components leads to an increased prepeak intensity or spatial heterogeneity at higher temperature because of the increase in Li+−ClO4− interactions. Similar effects are observed for the propionamide + Li+/ClO4− system (see Figures S3a−c and S4a−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 RDFs 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 been discussed in our previous work, so herein we focus only on exploring the effect of temperature on these correlations and interactions. The Li+− Cl (Figure 6a), Li+−Li+ (Figure 6b), and Cl−Cl (Figure 6c) RDFs for the acetamide + Li+/ClO4− system show that upon increasing the temperature, the nearest neighbor peaks slightly shift toward the shorter distances. Also, there is an appreciable increase in the peak heights of the counter- and co-ion correlations with increasing temperature, implying stronger ion pairing and enhanced Li+/ClO4− segregation at higher temperature. In Figure 7, we have shown the RDFs for Li+− OR (Figure 7a), Cl−OR (Figure 7b), Li+−N (Figure 7c), and HN−O (Figure 7d), where O is the ClO4− oxygen, OR is the alkylamide oxygen atom, and HN and N are acetamide amide group hydrogen and nitrogen atoms, respectively.

partial X-ray scattering structure functions, which are given by eqs 3 and 6, respectively. Polarity-based partial and total X-ray structure functions of acetamide + Li+/ClO4− system is shown in Figure 4a−c. The figure indicates that whereas polar−polar components contribute positively to the prepeak, the polar− apolar components contribute negatively to the prepeak at all temperatures. Moreover, polar−polar correlations have an overwhelming positive contribution to the prepeak than their apolar−apolar counterparts. Thus, the enhancement in interactions between the polar−polar components at higher temperature is much more than the depreciation in the polar− apolar interactions, which consequently leads to enhancement in prepeak with temperature. On the other hand, as can be clearly observed from Figure 4, the apolar−apolar correlations are less likely to be affected by temperature alterations. From the second partitioning scheme, that is, the electrolyteand alkylamide-based partial structure functions, as shown in Figure 5a−c, one can observe a significant positive contribution to the total S(q) from the Li+/ClO4−−Li+/ClO4− component, which increases with increasing temperature. Conversely, the peak corresponding to the 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 decrease upon increasing temperature. The RCONH2−RCONH2 partial S(q) also shows a slight increment with the rise in temperature. Further, these observations are confirmed through residue-wise interaction energy calculations 5246


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Figure 8. Temperature-dependent RDFs 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.

CT correlation (Figure 8d), CT being alkylamide methyl carbon, is observed for the nearest neighbor as the temperature is increased, the CT−CT correlation is enhanced in the second solvation shell. Overall, negligible changes are observed for all cases. Various propionamide + Li+/ClO4− correlations have also been provided in Figure S6a−i, which apparently show very similar trends with rising temperature as acetamide + Li+/ ClO4− system.

From Figure 7, we can observe that with the rise in temperature, not only the peaks slightly shift toward longer distances for a few cases (see Tables S6 and S7 in Supporting Information) but also the peak intensities of all RDFs decrease, which is exactly opposite to the trend observed for correlations between the electrolyte species. This means that at a higher temperature, the cross-correlations between the ionic components and acetamide polar groups (carbonyl oxygen and amide) are depreciated along with a slight increase in their separation for some select pairs. Also, the hydrogen bonding between the perchlorate oxygen atoms and acetamide amide group hydrogens weakens with the 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 Figure S5a−j. We can clearly observe that the coordination number corresponding to the first solvation shell of the pair of species belonging to the electrolyte shows an increasing trend with temperature, whereas those corresponding to the nonelectrolyte are almost insensitive to any change in temperature and the cross-terms show a decrease in the coordination number with temperature. The pair correlations involving only acetamide atoms are shown in Figure 8a−d. Clearly, there is a very minor appreciation in the N−OR correlations (Figure 8b) with increasing temperature. The hydrogen bonding between the amide hydrogen and carbonyl oxygen atoms (Figure 8a) also tend to increase slightly with increasing temperature in contrast to that between amide hydrogens and perchlorate oxygen (Figure 7d), as discussed previously. Although decreased CT−

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CONCLUSION

The present MD study focused on the temperature dependence of the nanoscale heterogeneity in lithium perchlorate salt + alkylamide-based DESs. 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 crosscorrelations, leading to enhancement of the prepeak intensity with increasing temperature. In summary, the change in the behavior of DESs with the change in temperature is due to the 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 systems at elevated temperatures is one the major results of the current study. In addition, our MD results showed that whereas the hydrogen 5247


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bonding between the amide hydrogens of alkylamide and the oxygens of perchlorate ion is diminished, that between the amide group hydrogens and carbonyl oxygen shows a slight increase with increasing temperature. Therefore, we can say that the structure of the DESs studied is predominated not only by electrostatic interactions but also by significant hydrogen bonding. The current observations corroborate well with the recent studies on Li+ salt-based electrolyte systems, Li+/FSA− + carbonate ester and Li+/FSI− + DME, wherein the formation of aggregates and a peculiar three-dimensional network of these aggregates was the major outcome.32,33

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.8b02378. Tables for density, partial charges, bonded and nonbonded parameters, residue-wise interaction energies, coordination number, and RDF’s maximum/minimum at different temperatures; and figures for molecular structures, neutron scattering S(q)s, partial S(q)s, coordination number, and RDFs for the two DES studies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-(0)1126591518. Fax: +91-(0)11-26581102. ORCID

Hemant K. Kashyap: 0000-0001-9124-2918 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank Professor Ranjit Biswas, Professor Claudio J. Margulis, and Professor Edward W. Castner Jr. for valuable discussion and feedback. S.K. thanks UGC, India, for fellowship. The authors thank the IIT Delhi’s HPC facility for computational resources. This work is supported by the Department of Science and Technology (DST), India, through a grant awarded to H.K.K. (grant no. SB/FT/CS-124/2014).

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