Nanoscale Spatial Heterogeneity in Deep Eutectic Solvents - The

Article pubs.acs.org/JPCB

Nanoscale Spatial Heterogeneity in Deep Eutectic Solvents Supreet Kaur, Aditya Gupta, and Hemant K. Kashyap* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India ABSTRACT: In this article, we report a molecular dynamics simulation study on the X-ray and neutron scattering structures of deep eutectic solvents (DESs) and show that the DESs studied possess unique spatial heterogeneity on molecular length scales. The simulated X-ray and neutron scattering structure functions (S(q)s) of the DESs made of alkylamide + Li+/ClO4− display two peaks in the intermolecular region of the S(q)s. As a signature of nanoscale structural organization/heterogeneity, a prepeak is observed at 0.1 < q/Å−1 < 0.4. The principal peak observed at around 1.2 < q/Å−1 < 2 is rendered by short-distance inter- and intraspecies correlations. For the DESs studied, we demonstrate that nanoscale spatial heterogeneity is exhibited profoundly by the segregated domains of the constituent electrolyte, and the principal peak in S(q) is because of all sorts of close-contact correlations. The extent of nanoscale morphology as well as the strength of ion pairing is enhanced for the longer-tail alkylamide DES.

1. INTRODUCTION Deep eutectic solvents (DESs) are an emerging class of multicomponent liquids with a melting point much lower than that of either of the individual components. They share many physicochemical properties with relatively more popular ionic liquids (ILs) and also with other conventional solvents.1−4 Fine-tuning of the properties of DESs can be achieved by utilizing different combinations of the constituent species. For DESs, the choice of constituent species is not limited only to ions; one can have different combinations of H-bond donors and acceptors, salts, and other molecular compounds that are not necessarily liquids in their pure form. DESs render low toxicity, lower vapor pressure, and better biodegradability and sustainability than those of many other solvent media. These liquids are moderate conductors and possess good solvation power. Moreover, they are relatively cheap, nonflammable, highly pure, and safe solvents.1,2 They have an additional advantage in their ease of preparation, with the technique being 100% atom-economic, and also, they are easy to store, with the benefit of being recyclable and reusable.4 These characteristic properties of DESs have led to a wide range of applications in various fields. DESs are used as alternative media for the synthesis of organic compounds and in selective solvent separation, catalysis, biodiesel transformation, electrochemistry, metal-oxide processing, polymer synthesis, gas capturing and separation, and so forth.5−18 In the present nanotechnology field, DESs have proven to be promising solvents in the synthesis of nanomaterials.4,6 Recently, Oliviera et al. used choline-chloride-based DESs as extraction solvents in the liquid−liquid separation of azeotropic mixtures.9 As we are already familiar with the CO2-capturing ability of ILs, it has been found that DESs also show a similar © 2016 American Chemical Society

behavior, and further research activity is still emerging in this field.8 The extraction properties of various DESs have been thoroughly examined experimentally.7,9,13,16 In electroanalytical chemistry, DESs have been used for electrodeposition of many zinc-based alloys, and it is also observed that by varying the components of the DESs, the electrochemistry can be altered effectively.11 Furthermore, it has been shown that the amphiphilic nature of DESs could also be utilized for spontaneous self-assembly of surfactants as well as lipid molecules.19−21 Abbott and coworkers reported the first and most widely explored DES, comprising a mixture of urea and choline chloride in 2:1 mole ratio, also called reline.3,22 With the help of the NMR spectroscopy technique, the authors demonstrated the role of hydrogen bonding on the depression in the freezing point of this DES. To complement various computational studies conducted on this DES,23−25 Perkins et al. performed a comparative study between the atomistic molecular dynamics (MD) simulations and IR spectroscopic analysis to study the types and significance of interactions between moieties of the reline system.26 This led to further exploration of more halidesalt-based DESs. Recent endeavors of the Pandey group to understand the polarity power of halide-salt-based DESs helped traverse through the solvation properties of DESs more widely.27 Biswas and coworkers have recently attained a lot in experimental and theoretical research on DESs.28−34 The group carried out time-resolved fluorescence measurements, dielectric relaxation measurements, femtosecond Raman-induced Kerr Received: April 26, 2016 Revised: June 9, 2016 Published: June 17, 2016 6712

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increases with an increase in alkylamide tail length. Furthermore, the computed three-dimensional spatial distribution functions (SDFs) also provide a better understanding of the structures of the DESs studied.

effect spectroscopy (fs-RIKES), as well as all-atom MD simulations to investigate the dynamics and heterogeneity aspects of various amide-based DESs.28−35 The fs-RIKES experiments revealed that some of the eutectics possess lowfrequency vibrational modes, which could be the consequence of intermolecular hydrogen bonding.36 The effect of the charged components on the DES relaxation dynamics was also examined.36 Edler’s group was the first to examine the liquid structure of DES reline using a neutron diffraction experiment and confirmed that DES reline indeed possesses a strong and complex hydrogen-bonding network between its constituent species.37 Despite being rich with several exquisite properties, the molecular-level structures of DESs have not been explored and are not well understood. In this regard, our work mainly emphasizes appreciating the structural landscape of a particular class of DESs: a mixture of alkylamides (RCONH2) and lithium perchlorate (LiClO4) (see Figure 1a,b). The components of

2. SIMULATION DETAILS To ensure better statistical averages at low-q features of the structure functions, very large simulation boxes have been used for acetamide + Li+/ClO4− and propionamide + Li+/ClO4−. In both cases, the number of electrolyte and alkylamide molecules were fixed at 4750 and 20 250 to attain corresponding mole fractions of 0.19 and 0.81. This amounts to a total number of atoms of 210 750 and 271 500 for the acetamide and propionamide systems, respectively. MD simulations were performed in isothermal−isobaric ensemble using the singleprecision GPU variant of the GROMACS-5.1.1 package.41,42 Periodic boundary conditions and minimum image convention were applied to the cubic boxes used for the simulations. Atomistic parameters from CHARMM43 have been used to model Li+ and the alkylamides. All of the parameters for the O atom as well as the charge of the Cl atom of the ClO4− anion were taken from Maginn’s work,44 and the values of sigma and epsilon for the Cl atom were taken as 0.4417 nm and 0.49283 kJ mol−1, respectively.45−48 The equations of motion were integrated using the leap-frog algorithm, with a 1 fs time step. The initial configuration for each system was generated using the PACKMOL package.49 The simulation boxes were equilibrated for at least 33 ns at a temperature of 323 K and pressure of 1 bar using a Nose−Hoover thermostat and Parrinello−Rahman50 barostat, respectively. The last 5 ns of the trajectories was saved at every 0.5 ps for computation of the properties of the DESs. The cutoff radius for the short-range interactions was set to 1.2 nm, with a switching function used from 1.0 to 1.2 nm. Electrostatic interactions were evaluated using the particle mesh Ewald51,52 summation technique, with an interpolation order of six and Fourier grid spacing of 0.08 nm. The equilibrium values of the bulk densities at 323 K for all systems were in excellent agreement with the experimental data. The simulated and experimental30 (in bracket) densities at 323 K were 1.231 (1.215) and 1.139 (1.138) g/cm3 for acetamide + Li+/ClO4− and propionamide + Li+/ClO4−, respectively. As the deviation of the simulated densities from the experimental data is well within the acceptable limit, this agreement certainly increases our confidence in using the present force field for the DESs studied. We have also verified the temperature-dependent behavior of simulated density, and it is observed that the simulated density also captures the experimental density trend.30 The X-ray scattering static structure function, S(q), and its subcomponents were computed using the general methodology proposed in the literature.53 Using the coordinates of the atoms, we first computed the radial distribution function (RDF), gij(r), for the atoms of types i and j. The computed RDFs include both intra- and intermolecular terms. The X-ray or neutron scattering static structure function, S(q), is calculated using eq 1

Figure 1. Molecular structures of the (a) alkylamides (RCONH2 with RCH3 and C2H5) and (b) electrolyte (Li+/ClO4−) constituting the DESs studied.

these DESs are cheap, easily available, and safe; therefore, they have much potential in various applications, as discussed above. Lithium perchlorate, being polar in nature, has an exceptionally high solubility in organic solvents, and it also supports enhancement of the kinetic performances of DESs.38 The present study will shed some light on the microscopic structural landscape of lithium-ion-based DESs. Dielectric relaxation experiments for (electrolyte + acetamide)-based mixtures have been explored in the past by Berchiesi and coworkers.39,40 In these pioneering experiments, the authors observed a low-frequency dielectric relaxation mode, which they speculated to be because of spatial heterogeneity or the presence of extended domains of dipoles in the system.39,40 Guchhait et al.30 have explored the dynamics of these systems in great detail and also highlighted the spatial and temporal heterogeneities in (alkylamide + electrolyte)based deep eutectics and the effect of chain length by carrying out both experimental and simualtion studies. However, the structural landscape of such types of DESs has not been explored fully. In this article, we aim at a molecular-level structural study of acetamide + Li+/ClO4− and propionamide + Li+/ClO4−, having a molar ratio of 81:19. We have performed MD simulations of the two mentioned DESs and explored their structural landscapes. We have reported the first X-ray and neutron scattering studies on these systems, revealing spatial heterogeneity of the system. The formation of polar Li+/ClO4− domains is responsible for this heterogeneity, which in turn

S(q) n

=

n

ρo ∑i = 1 ∑ j = 1 xixjyi (q)yj (q)∫

0

L /2

4πr 2[gij(r ) − 1]

sin qr ω(r ) qr

dr

n n [∑i = 1 xiyi (q)][∑ j = 1 xjyj (q)]

(1) 6713

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Figure 2. Tail-length dependence of (a) X-ray scattering and (b) neutron scattering structure functions (S(q)s) for alkylamide + Li+/ClO4− systems at a temperature of 323 K. Acetamide + Li+/ClO4− is shown in black and propionamide + Li+/ClO4− in red.

Figure 3. Partial X-ray scattering S(q)s for (a) acetamide + Li+/ClO4− and (b) propionamide + Li+/ClO4−. +

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.54 For the neutron scattering structure function, we take yi(q) as bi, the neutron scattering length for an ith-type atom.55 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),56,57 which could be used to dampen the effects of finite truncation of r. In this study, we split total S(q) into its cationic, anion, and alkylamide components as58−64 +

S(q) = S Li

− Li+

−

−

+

(q) + S ClO4 − ClO4 (q) + 2S Li

+

+ 2S Li

S Li

= S Li

+

S Li

+

+S

+

RCONH 2 − RCONH 2

(q)

−

+

− ClO−4

(q)

(4)

/ClO−4 − RCONH 2

+

(q) = S Li

− RCONH 2

−

(q) + S ClO4 − RCONH2(q)

3. RESULTS AND DISCUSSION 3.1. X-ray and Neutron Scattering Structure Functions and Their Partial Components. Via Figure 2a,b, we demonstrate that both the DESs studied display a prepeak near q = 0.2 Å−1 and a principal peak at around 1.5 Å−1 in both X-ray and neutron scattering structure functions, S(q)s. The real-space characteristic length scale corresponding to the prepeak is ∼3 nm and to the principal peak is ∼0.4 nm. The observation of a prepeak in the S(q) indicates the existence of nanoscale organization/heterogeneity in the system. A prepeak or first sharp diffraction peak has also been observed in many solvent mixtures, ILs, alloys, glassy solids, polymer solutions, and systems possessing complex morphologies.53,65,66 However, its origin has been shown to be different for different kinds of

(q)

(2)

(q) + 2S Li

−

(q) + S ClO4 − ClO4 (q) + 2S Li

Other details of the methodology are well described in the existing literature.53

(q) + 2S ClO4 − RCONH 2(q)

/ClO−4 − Li+/ClO−4

− Li

(5)

In the above expression, it is advantageous to group the ionic components together such that S(q) = S Li

+

and

− ClO−4

+ S RCONH2 − RCONH 2(q)

(q)

+

−

− RCONH 2

/ClO−4 − Li+/ClO−4

/ClO−4 − RCONH 2

(q) (3)

In eq 3, we defined 6714

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Figure 4. Equilibrium snapshots of (a) acetamide + Li+/ClO4− and (b) propionamide + Li+/ClO4− DESs. The Li+/ClO4− groups are shown as red isosurfaces in all of the systems. The acetamide molecules are shown in green and propionamide in blue.

Figure 5. RDFs for the (a) Li+ and oxygen (O) atom of ClO4−, (b) Li+ and chlorine (Cl) atom of ClO4−, (c) Li+−Li+, and (d) Cl−Cl.

into its subcomponents due to Li+/ClO4− and RCONH2 is advantageous for identifying the exact origins of the two peaks in the total S(q)s. As will be shown subsequently, the ClO4− anions always follow the Li+ cations (or vice versa) in their nearest solvation shell and therefore one can justify such a prejudice partitioning scheme used for this purpose (eqs 3 and 4). From Figure 3a,b, we unambiguously observe that the Li+/ ClO4−−Li+/ClO4− correlations significantly contribute to the

systems. For the DESs studied, the intensity of the prepeak increases with increasing tail length of the alkylamide. This means that the nanoscale ordering in these systems does depend on the alkylamide tail length. The principal peak shifts toward smaller wavenumbers for a longer-tail system. To gain an increased understanding about the origins of the peaks observed in the DESs studied, one usually examines the partial components of S(q). It appears that dissection of S(q) 6715

DOI: 10.1021/acs.jpcb.6b04187 J. Phys. Chem. B 2016, 120, 6712−6720

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The Journal of Physical Chemistry B prepeak, whereas the RCONH2−RCONH2 term also has a non-negligible contribution to the prepeak. This means that strong electrostatic interactions play a vital role in laying down the structural landscape of such systems. On the other hand, the principal peak has predominant contributions from RCONH2−RCONH2 correlations. But Li+/ClO4−−Li+/ClO4− as well as RCONH2−Li+/ClO4− contribute moderately to the principal peak. The figure also shows that the electrolyte contribution to the prepeak does not change with increasing alkylamide tail length. Similarly, the alkylamide contribution remains unaffected by tail modification. We notice that the increase in prepeak intensity for the propionamide + Li+/ClO4− system is due to a decrease in the depth of the Li+/ClO4−− RCONH2 partial S(q), which signifies enhancement in the cross-correlations between the ions and the atoms of propionamide. The Li+/ClO4− nanoscale size extended domains (shown in red) can be easily seen in the equilibrium snapshots of the DESs shown in Figure 4a,b. Significant heterogeneity, that is, nonuniform distribution of the alkylamide and the ions, is clearly visible in both systems. In several studies, lithium salts have been found to organize in a manner similar to that in pseudocrystalline materials and render some kind of phase segregation.67−70 In particular, it has been observed by Russina et al. that in the Li+/NO3− + ethyl ammonium nitrate system the Li+ ions not only reside in the polar domains of the IL but also undergo self-segregation.68,70 The extent of this segregation increases upon increasing the Li+/NO3− concentration, which was clearly reflected in the lowq peak of the measured X-ray scattering S(q).70 As shall be shown in the next section, our observation corroborates with their study to some extent, as Li+/ClO4− does tend to segregate together via very strong electrostatic interactions between the counterionic species, but this segregation is also facilitated by the polar group (carbonyl oxygen) of alkylamide through significant polar interactions. The formation of clusters and the heterogeneity enhancement in electrolyte-based DESs can lead to several possible applications, such as use of these DESs as selective solvents in synthesis and separation. 3.2. Ion Pairing and Hydrogen Bonding. The average local environments of the ions and alkylamide molecules can be visualized by examining the RDFs and SDFs around them. In Figure 5a−d, we have depicted the RDFs for Li+−O (Figure 5a), O being the oxygen atom of ClO4− and Li+−Cl (Figure 5b), Li+−Li+ (Figure 5c), and Cl−Cl (Figure 5d) all belonging to Li+/ClO4−. For both systems studied, evidence of very strong ion pairing can be seen in the RDFs for Li+ and ClO4− atoms (Figure 5a,b); the first peaks in the Li+−O and Li+−Cl RDFs are sharp and of very high magnitude. These peaks are at an approximate distance of 0.2 nm for Li+−O and 0.35 nm for Li+−Cl. From these figures, it is also apparent that the strength of ion pairing increases as we move from acetamide to propionamide. Figure 5c,d, showing cation−cation and anion− anion central atom RDFs, indicates stronger co-ion correlations, which is higher for the propionamide system. Overall, counterion correlations are stronger than those between coions and any other correlations present in both the systems. Cross-correlations between the electrolyte and the alkylamide molecules have been examined through RDFs for the Li+ and carbonyl oxygen (OR) of the alkylamides and the Li+ and amide nitrogen (N) of the alkylamides (see Figure 6a,b). The first sharp peak in the Li+−OR RDF reveals a significant shortdistance correlation between the two atoms. It is also clear that the probability of finding Li+ near the alkylamide oxygen atom

Figure 6. RDFs for the (a) Li+ and carbonyl oxygen (OR) of the alkylamides and the (b) Li+ and amide nitrogen (N) of the alkylamides.

is more than that of finding Li+ near the alkylamide nitrogen. The preference of the Li+ ion for the OR group is also supported by the SDFs around acetamide. The ClO4− oxygens (O) and carbonyl oxygen (OR) of RCONH2 both show concomitant H-bonding with the hydrogens (HN) of the amide group of the alkylamides (see Figure 7a,b). For both the systems, the first solvation shell peaks or H-bonding peaks are clearly visible at a distance of around 0.2 nm in HN−OR and HN−O RDFs. From these RDFs, it is easy to appreciate that HN−OR hydrogen bonding dominates over HN−O. The strength of these two H-bonds is enhanced for the propionamide system. The extent of nonpolar or apolar segregation was examined through the RDFs for the methyl carbons (CT) of the two alkylamides. From these RDFs (data not shown), we observe that the short-distance methyl−methyl correlations are almost similar for the two systems. A unified picture of the three-dimensional arrangement of ions in the DESs studied can be conceived by looking at the SDFs for Li+ (green solid) and the oxygen atoms (red transparent) of acetamide around ClO4− in Figure 8a. The isodensity surfaces shown here are to reflect only the nearest 6716

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Figure 8. Average three-dimensional density isosurfaces reflecting the nearest solvation shells around (a) ClO4− and (b) acetamide for the acetamide + Li+/ClO4− system. The Li+ isosurfaces are shown in green, carbonyl oxygen (OR) in transparent red, and perchlorate oxygen (O) in a red wireframe.

Figure 7. Intermolecular RDFs for the (a) amide hydrogens (HN) and carbonyl oxygen (OR) atom of the alkylamides and the (b) amide hydrogens (HN) and oxygen (O) atoms of ClO4−.

atoms around Li+ increases, which is also the case for the anionic oxygens. Electrolyte-based DESs have important applications in batteries and electrochemical devices, in which better kinetic performance as well as thermodynamic properties of the electrolyte is desirable. In this regard, it is very crucial to first examine the structural correlations exhibited by the DES, which could have a significant impact on the microscopic dynamics of the DES. Our study supports that in lithium-salt-based DESs Li+/ClO4− undergoes self-segregation, leading to a unique nanoscale spatial heterogeneity, due to very strong electrostatic interactions between the counterions. This segregation is further facilitated by the lithium ion and carbonyl oxygen interaction. This study also provides sufficient evidence to support the existence of extended domains of polar moieties that was speculated by Berchiesi and coworkers several years ago.39,40

neighbor shells. The isodensity surfaces in this figure show that the Li+ ions and ClO4− oxygens are structured around the ClO4− ions in a tetrahedral arrangement, which seems quite natural, considering the chemical structure of ClO4−. Although not shown in the figure, the remaining space in between ClO4− and the atoms surrounding it is covered uniformly by the amide groups of the acetamide molecules. This tetrahedral structural arrangement is invariably the same for both the DESs studied, but the probabilities are enhanced for the propionamide system (see Figure 5a,b). The SDFs for Li+ (green solid), oxygen atoms (transparent red) of the acetamide, and oxygen atoms (red wireframe) of ClO4− around the acetamide in Figure 8b give a clear image of the polar interactions and hydrogenbonding interactions between the components and are the same for the other system as well. From the SDFs around the alkylamides and ClO4− ions (Figure 8) and the RDFs (Figures 5 and 6), we notice that the first solvation shell of the Li+ ion is shared by the oxygen atoms of both ClO4− and RCONH2, but the probability of finding ClO4− oxygens is more than that of finding the RCONH2 oxygen. Moving from acetamide to propionamide, the probability of finding the RCONH2 oxygen and nitrogen

4. CONCLUSIONS In this work, we have studied the X-ray and neutron scattering structures of (electrolyte + alkylamide)-based DESs by means of very large scale MD simulations. We demonstrated that the DESs studied exhibit nanoscale heterogeneity/organization, which is primarily due to the segregated domains of Li+/ClO4−. 6717

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(3) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. Deep Eutectic Solvents Formed Between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids. J. Am. Chem. Soc. 2004, 126, 9142−9147. (4) Abo-Hamad, A.; Hayyan, M.; AlSaadi, M. A.; Hashim, M. A. Potential Applications of Deep Eutectic Solvents in Nanotechnology. Chem. Eng. J. 2015, 273, 551−567. (5) 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. (6) Wagle, D. V.; Zhao, H.; Baker, G. A. Deep Eutectic Solvents: Sustainable Media for Nanoscale and Functional Materials. Acc. Chem. Res. 2014, 47, 2299−2308. (7) Gu, T.; Zhang, M.; Tan, T.; Chen, J.; Li, Z.; Zhang, Q.; Qiu, H. Deep Eutectic Solvents as Novel Extraction Media for Phenolic Compounds from Model Oil. Chem. Commun. 2014, 50, 11749− 11752. (8) Garcia, G.; Aparicio, S.; Ullah, R.; Atilhan, M. Deep Eutectic Solvents: Physicochemical Properties and Gas Separation Applications. Energy Fuels 2015, 29, 2616−2644. (9) Oliveira, F. S.; Pereiro, A. B.; Rebelo, L. P. N.; Marrucho, I. M. Deep Eutectic Solvents as Extraction Media for Azeotropic Mixtures. Green Chem. 2013, 15, 1326−1330. (10) 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. (11) Abbott, A. P.; Harris, R. C.; Ryder, K. S. Application of Hole Theory to Define Ionic Liquids by Their Transport Properties. J. Phys. Chem. B 2007, 111, 4910−4913. (12) Abbott, A. P.; Capper, G.; McKenzie, K. J.; Ryder, K. S. Electrodeposition of Zinc−Tin Alloys from Deep Eutectic Solvents Based on Choline Chloride. J. Electroanal. Chem. 2007, 599, 288−294. (13) Kareem, M. A.; Mjalli, F. S.; Hashim, M. A.; Hadj-Kali, M. K.; Bagh, F. S. G.; Alnashef, I. M. Phase Equilibria of Toluene/Heptane with Deep Eutectic Solvents Based on Ethyltriphenylphosphonium Iodide for the Potential Use In the Separation of Aromatics from Naphtha. J. Chem. Thermodyn. 2013, 65, 138−149. (14) Liao, J.-H.; Wu, P.-C.; Bai, Y.-H. Eutectic Mixture of Choline Chloride/Urea as a Green Solvent in Synthesis of a Coordination Polymer: [Zn(O3PCH2CO2)] NH4. Inorg. Chem. Commun. 2005, 8, 390−392. (15) Wang, S.-M.; Chen, W.-L.; Wang, E.-B.; Li, Y.-G.; Feng, X.-J.; Liu, L. Three New Polyoxometalate-based Hybrids Prepared from Choline Chloride/Urea Deep Eutectic Mixture at Room Temperature. Inorg. Chem. Commun. 2010, 13, 972−975. (16) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Shikotra, P. Selective Extraction of Metals from Mixed Oxide Matrixes using Choline-Based Ionic Liquids. Inorg. Chem. 2005, 44, 6497−6499. (17) Hizaddin, H. F.; Sarwono, M.; Hashim, M. A.; Alnashef, I. M.; Hadj-Kali, M. K. Coupling the Capabilities Oxide of Different Complexing Agents into Deep Eutectic Solvents to Enhance the Separation of Aromatics from Aliphatics. J. Chem. Thermodyn. 2015, 84, 67−75. (18) Patil, U. B.; Singh, A. S.; Nagarkar, J. M. Choline Chloride Based Eutectic Solvent: An Efficient and Reusable Solvent System for the Synthesis of Primary Amides from Aldehydes and from Nitriles. RSC Adv. 2014, 4, 1102−1106. (19) Edler, K. J.; Bowron, D. T. Combining Wide-angle and Smallangle Scattering to Study Colloids and Self-assembly. Curr. Opin. Colloid Interface Sci. 2015, 20, 227−234. (20) Sanchez-Fernandez, A.; Edler, K. J.; Arnold, T.; Heenan, R. K.; Porcar, L.; Terrill, N. J.; Terry, A. E.; Jackson, A. J. Micelle Structure in a Deep Eutectic Solvent: A Small-angle Scattering Study. Phys. Chem. Chem. Phys. 2016, 18, 14063−14073. (21) Bryant, S. J.; Atkin, R.; Warr, G. G. Spontaneous Vesicle formation in a Deep Eutectic Solvent. Soft Matter 2016, 12, 1645− 1648.

The alkylamide groups also show a non-negligible contribution to the prepeaks in the X-ray and neutron scattering S(q)s. The contributions of Li+/ClO4− and the alkylamide extended domains to the prepeak are more or less same in both systems. However, the increase in prepeak intensity in the total S(q) for the longer-tail system is due to increased cross-correlations between the atoms of the electrolyte and alkylamide. The principal peaks in the S(q)s are due to alkylamides and Li+/ ClO4− short-distance correlations. The pronounced and sharp peaks in the RDFs for counterionic species clearly suggest that because of the very high charge density the strength of the interaction between Li+ and the ClO4− ion is higher than that for any other interaction in the system. In addition, stronger Li+−OR correlations also suggest that the lithium ion has a significant number of carbonyl oxygens in its first solvation shell. The electrostatic interactions among the counterions and between Li+ and OR are so strong that a sort of loosely packed domain of electrolytes, also containing the carbonyl oxygen of alkylamide, develops. For all systems studied, the SDFs confirmed that the first solvation shell of ClO4− consisted of Li+, followed by the carbonyl oxygen of the alkylamide. The probability of finding the carbonyl oxygen of the alkylamide around the amide group hydrogens is more than that for ClO4− oxygens, showing preferential intraspecies hydrogen bonding in such a system. On the other hand, the probability of finding ClO4− atoms around Li+ or vice versa is higher than that for any other atom in the system, which clearly indicates stronger ion paring and ion-pair aggregation in the DESs studied. To the best of our knowledge, this is the first study on the Xray and neutron scattering structures of (alkylamide + electrolyte)-based DESs via MD simulations. X-ray and neutron scattering, NMR, Raman scattering, and any such experiment to understand the structure of electrolyte-based DESs are still scarce in the literature.37,70,71 Therefore, the present study will encourage experimentalist to explore DES structures using advanced techniques and compare their data against the predictions made in the current study. Further studies on temperature-dependent X-ray and neutron scattering structures of other DESs via MD simulations are under progress and shall be reported elsewhere.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank Professor Ranjit Biswas and Professor Claudio J. Margulis for helpful discussions and feedback. S.K. and A.G. thank CSIR-UGC, India, for fellowship. The authors thank the IIT Delhi HPC facility for computational resources. This work was 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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REFERENCES

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