Orientational Jumps in (Acetamide + Electrolyte) Deep Eutectics

Jul 1, 2015 - Simulated jump characteristics register a considerable dependence on the anion identity. For example, large angle jumps are relatively l...
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Orientational Jumps in (Acetamide + Electrolyte) Deep Eutectics: Anion Dependence Suman Das,† Ranjit Biswas,*,†,‡ and Biswaroop Mukherjee*,‡ †

Chemical, Biological and Macromolecular Sciences and ‡Thematic Unit for Excellence − Computational Materials Science, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700098, India S Supporting Information *

ABSTRACT: All-atom molecular dynamics simulations have been carried out to investigate orientation jumps of acetamide molecules in three different ionic deep eutectics made of acetamide (CH3CONH2) and lithium salts of bromide (Br−), nitrate (NO3−) and perchlorate (ClO4−) at approximately 80:20 mole ratio and 303 K. Orientational jumps have been dissected into acetamide−acetamide and acetamide−ion catagories. Simulated jump characteristics register a considerable dependence on the anion identity. For example, large angle jumps are relatively less frequent in the presence of NO3− than in the presence of the other two anions. Distribution of jump angles for rotation of acetamide molecules hydrogen bonded (H-bonded) to anions has been found to be bimodal in the presence of Br− and is qualitatively different from the other two cases. Estimated energy barrier for orientation jumps of these acetamide molecules (H-bonded to anions) differ by a factor of ∼2 between NO3− and ClO4−, the barrier height for the latter being lower and ∼0.5kBT. Relative radial and angular displacements during jumps describe the sequence ClO4− > NO3− > Br− and follow a reverse viscosity trend. Jump barrier for acetamide−acetamide pairs reflects weak dependence on anion identity and remains closer to the magnitude (∼0.7kBT) found for orientation jumps in molten acetamide. Jump time distributions exhibit a power law dependence of the type, P(tjump) ∝ A(tjump/τ)−β, with both β and τ showing substantial anion dependence. The latter suggests the presence of dynamic heterogeneity in these systems and supports earlier conclusions from time-resolved fluorescence measurements.

I. INTRODUCTION Minimum ecological footprint via biodegradation, economic viability through cheaper preparation protocols, extended moisture stability, and easy alteration of physicochemical properties are the principal requirements that a solvent or a class of solvents must satisfy to be in demand for large-scale industrial applications. Deep eutectic solvents (DESs), solvents that are molten mixtures of a few components with high individual melting points, meet several or all of the above stringent criteria for large-scale applications.1−5 DESs made of amides and electrolytes constitute an example of one such class of solvents that have found lately several industrial and technological applications.6−10 Extensive interspecies hydrogen bonding (H-bonding) and entropic gain in the subsequent process are believed to bring the high melting solid components into liquid phase. Acetamide, by virtue of possessing several functional groups, large molecular dipole moment (μ = 3.7 D11), and high dielectric constant in molten conditions (ε0 ≈ 61 at ∼367 K11,12), can dissolve a wide range of organic and inorganic compounds except cellulose.13 It is only natural then that molten mixtures that offer acetamide in liquid phase near room temperature will be a subject of intense research. In addition, engineering these media for tailoring © 2015 American Chemical Society

chemical reactions demands a molecular level understanding of medium effects on chemical reactions.14−16 Although (acetamide + electrolyte) DESs have been used in a few industrial applications, microscopic understanding of interaction and dynamics of these media are still lacking. Previous dielectric17−19 and other relaxation experiments20−22 have interpreted measured data in terms of microheterogeneity in these systems, although no structural measurements at molecular length scales exist to contest such a view. In the past few years several studies employing time-resolved fluorescence23−27 and optical Kerr spectroscopic28 measurements have revealed information on collective interaction and dynamics of these systems. A few simulation studies23,24,29 have revealed the nature of viscosity coupling of diffusive dynamics in these DESs and helped to interpret departure from hydrodynamics of diffusive time scales observed in time-resolved fluorescence measurements. However, no study exists that dissects the experimentally observed ensemble-averaged total information Special Issue: Biman Bagchi Festschrift Received: March 30, 2015 Revised: June 2, 2015 Published: July 1, 2015 11157

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acetamide dynamic structure factors in the DESs that successfully explained fractional viscosity dependence of diffusive time scales observed in time-resolved fluorescence measurements.23,24 The force field was constructed with DL_FIELD.52 The short-range van der Waals interaction was represented by the Lennard-Jones (LJ) potential.53 The longrange electrostatic potential was treated via the Ewald summation technique53 using an Ewald parameter of α = 0.2 Å−1 and a 6 × 6 × 6 k-point grid. The initial configurations were built using Packmol54 and equilibrated in the NPT ensemble at 1 atm pressure for 500 ps. The simulated densities for all three systems were found to be in good agreement with those from experiments23,28 (Table S2, Supporting Information). A Nose−Hoover thermostat and barostat55,56 were used to control the temperature and pressure with time constants of 0.4 and 1.0 ps, respectively. Subsequently, further equilibration of 2 ns followed by a production run of 5 ns was carried out in the NVT ensemble. Periodic boundary conditions were employed in all three directions, and the equations of motion were integrated using a time step of 0.5 fs employing the velocity Verlet algorithm.53 Snapshots were saved every 10 fs for data analyses. For some calculations, particularly those related to estimation of free energy barriers, snapshots were saved every 2 fs.

into the separate contributions from acetamide molecules that are interacting with the electrolyte present in such a medium and those that are not. Conventional time-resolved fluorescence and other frequency-dependent relaxation experiments cannot separate out these contributions because response measured by them is averaged over contributions from all length scales.30 However, such a study can be conducted via computer simulations, and we focus here on revealing the orientational features of acetamide molecules that are Hbonded to ions present in these DESs and those which are Hbonded only to other acetamide molecules. In order to carry out the proposed study, we have considered three different deep eutectics, namely, (CH3CONH2 + LiBr), (CH3CONH2 + LiNO3), and (CH3CONH2 + LiClO4), all at 78:22 mole ratio and at 303 K. This mole ratio has been chosen because several measurements exist for these DESs at this composition.23,24,26,28 Reorientation jumps of acetamide molecules in these media have been monitored via a method that closely follows the framework developed earlier for studying water reorientation dynamics in a variety of environments31−42 and employed later for molten neat acetamide.43 The anion dependence is explored via simulating jump angle and jump time distributions, free energy barrier at the transition state (involved in H-bond switching), changes in radial and angular distances during successful jumps, and difference in Hbond coordination between the initial and the final acceptors. Since these ions vary in both shape and size, effects are expected to be more complex than those arising from excluded volumes alone.34,44 As observed earlier,32,43,45 H-bond switching events in these systems have also been found to be associated with a significant amount of translation−rotation coupling.

III. ANALYSIS PROTOCOL The presence of angular jumps in the reorientational dynamics of acetamide molecules present in all these ionic DESs is evident in Figure 1, which shows the time series of the cosine of the angle made by the backbone of a randomly chosen CH3CONH2 molecule (connecting the C (−CH3) and N(−NH2) atom) with the Z axis of the simulation box. As found previously for molten neat acetamide,43 acetamide

II. SIMULATION DETAILS All-atom simulations were performed at a temperature of 303 K using the package DL_POLY, version 2.20.46 Each mixture comprised 512 molecules, of which there were 400 acetamide and 56 pairs of Li+ and X− where X− denotes the identity of the anion. This provides a mole ratio of 78:22 between acetamide and electrolyte. The potential function used has the following form: U (R ) =

∑

K r(r − req)2 +

bonds

∑

∑

K θ(θ − θeq)2 +

angles

Kϕ[1 + cos(nϕ − δ)]

dihedrals atoms

+

∑ i ηLiClO4) for these DESs. This is expected because search for a H-bond acceptor during “flight” involves angular displacements through the medium. Interestingly, the 11163

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The Journal of Physical Chemistry B Table 1. Anion Dependence of Changes in Radial Distance and Angle during Orientation Jumps in (Acetamide + LiX) DESs acetamide−acetamide

a

anion (X−)

radius (Å)

Br− ClO4− NO3−

1.82 2.4 1.96

b

Table 2. Anion Dependence of Changes in Hydrogen Bond Coordination Number, ΔnHB, for Acetamide in (Acetamide + LiX) DESs anion (X−)

acetamide−ion

distance (ΔR, Å)

ΔR/ σaceta

angle (deg)

distance (ΔR, Å)

ΔR/ σaceta

angle (deg)

0.91 1.53 1.25

0.20 0.34 0.27

94 100 96

0.35 1.29 0.85

0.08 0.28 0.19

100 112 98

−

Br ClO4− NO3−

acetamide−acetamide

acetamide−ion

0.2 0.2 0.2

0.0 0.1 0.1

presence of Br− for acetamide−ion jumps is bimodal with peaks of nearly equal intensity at ∼64° and ∼88° and is completely different in shape from other distributions. However, the primary peak of the double-peak distribution almost coincides with the distribution for molten acetamide.43 To understand what kind of local structure renders this bimodality, we have selected two snapshots depicting the jump angles of the peak. Figure 11 presents the transition state configurations of the acetamide−ion jumps corresponding to above-mentioned orientational jumps in (acetamide + LiBr) DES. For ClO4− containing DES, a similar configuration is also present and renders a shoulder at higher angle. Such a diversity in jump angle distribution indicates complexity of interactions of these anions with acetamide molecules. Figure 12 shows the free energy (in units of kBT) as a function of time along the average jump trajectories shown in Figures 6 and 7. Note that these free energies have been obtained via projection of the average trajectory on the full free energy surface described by eq 2. It is possible to perform projections once we have calculated the free energy surface and the time dependence of the geometrical quantities characterizing the average trajectories. For each time instant of the average trajectory, we find the values of the reaction coordinates (ΔR, θ, Δn) and the corresponding free energies. This process is then repeated for the whole length of the average trajectory for obtaining an average free energy, which is

σacet = 4.52 Å (from ref 25). Ion radii are from ref 23. b

Figure 10 demonstrates the jump angle distributions for acetamide molecules calculated between starting and ending time of the microscopic jumps. The upper panel shows the anion dependence of jump angle distribution for acetamide− acetamide jump, while the lower panel shows the same for acetamide−ion jump. For comparison, a simulated jump angle distribution for molten neat acetamide43 is also shown. Note that the acetamide−acetamide jump angle distributions in these DESs, although possessing similar shape as that for molten acetamide, are shifted toward relatively lower angles, reflecting electrolyte effects. In addition, the dependence on anion identity of these distributions is weak, with the jump angles peaking around ∼55° for NO3− and ClO4− and ∼59° for Br−. Also, these distributions in the presence of electrolytes are broader than that for molten acetamide, with the maximum width for DES containing ClO4−. Except for the case of Br−, the distributions for acetamide−ion jumps are relatively narrower and shifted to lower angles compared with that for molten acetamide−acetamide case. The peak position in jump angle distribution is around 54° for NO3− and ClO4− like the corresponding acetamide−acetamide jumps. Note that a shoulder at a larger angle (∼80°) appears for acetamide−ion jumps in case of ClO4−. Interestingly, the distribution in the

Figure 8. Time evolution of the H-bond coordination number at 303 K. Columns starting from left show data from simulations of mixtures of acetamide and lithium bromide, lithium perchlorate, and lithium nitrate, respectively. The upper panels show the time evolution of the H-bond coordination number for the acetamide−acetamide orientational jumps, and the bottom panels show the same for acetamide−ion jumps. The red curves denote H-bond coordination for the initial (“I”) acceptor, the blue curves denote the final (“F”) acceptor, and the pink lines denote the same for the rotating (“R”) acetamide molecule. 11164

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Figure 9. Distribution of the jump times for the acetamide−acetamide (upper panel) and the acetamide−ion jumps (lower panel) for these DESs at 303 K. Fit parameters required for describing the power law dependence of long-time tails of jump time distributions are summarized in the insets. The exact form of the fitting function is provided in the text.

Figure 11. Transition state configurations corresponding to the two peaks of the bimodal acetamide−ion jump angle distribution for DES made of acetamide and lithium bromide at 303 K.

Figure 10. Simulated jump angle distribution of the N*−H* vector for the acetamide−acetamide (upper panel) and acetamide−ion jumps (lower panel) at 303 K. For comparison, jump angle distribution for molten neat acetamide is also shown (dashed line) in both panels.

Figure 12. Temporal evolution of the free energies corresponding to the average trajectories associated with acetamide−acetamide (upper panel) and acetamide−ion (lower panel) orientational jumps in these DESs at 303 K. Anion dependence of the barrier is summarized in Table 3 and discussed in the main text.

a function of time. We have chosen a convention similar to the one that has been used to calculate the average trajectories, where all transition states have been chosen to occur at time t = 0. This process of obtaining time-dependent free energies has been executed for both acetamide−acetamide and acetamide− ion jumps. The upper panel shows anion-dependent free energy profile for acetamide−acetamide jumps, and the lower panel presents the same for the ion−acetamide jumps. While the acetamide−acetamide free energy barrier is weakly dependent

on ion-identity, the ion−acetamide free energy barrier shows a significant anion dependence, with the free energy barrier for NO3− being the highest (∼1.0kBT) and ∼2 times larger than that for ClO4− (∼0.5kBT). Table 3 summarizes the anion dependent free energy barriers for orientation jumps in these DESs. Note that the free energy barrier simulated for 11165

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nonexponentiality in dielectric relaxation of ionic liquids69,70 and orientation jumps would also be exciting future problems. It is to be mentioned here that the simulation results presented here have been obtained by using nonpolarizable potential functions. Inclusion of polarizabilities, therefore, may alter the angular displacement mechanism reflected by the presented analyses.71 Interestingly, very recent DR measurements of these and a few other DESs in the frequency window 0.2 ≤ ν ≤ 50 GHz have indeed revealed a strong dependence on ion identity of both static dielectric constant and relaxation times.72 The average ion-dependent DR relaxation times have been found to deviate strongly from the hydrodynamic predictions, hinting at orientation mechanisms that are different from Brownian rotational moves. In addition, reorientation jump picture for water have been found to remain unaltered even after inclusion of polarizability in the interaction potential.33 All these seem to indicate that the orientational jump mechanism for acetamide molecules in these DESs reflected by the present simulations may not be drastically altered upon inclusion of polarizability. Contribution of the orientation-coupled center-of-mass movements on the total translational dynamics73 of ions and acetamide molecules needs to be separated out for a quantitative understanding of mass transport in these heterogeneous and industrially important solvents.

Table 3. Anion Dependent Free Energy Barrier for Orientation Jumps in (Acetamide + LiX) DESs anion (X−)

acetamide−acetamide (in kBT units)

acetamide−ion (in kBT units)

Br− ClO4− NO3−

0.8 0.6 0.5

0.4 0.5 1.0

acetamide−acetamide jumps in these DESs is, on an average, very similar to the barrier (∼0.7kBT) found for molten neat acetamide.43 The free energy profile in the presence of Br− is not as sharp as in the presence of the other two anions. This can be connected to the lowest values for ΔR and Δn (see Tables 1 and 2) in the presence of Br−, which creates a relatively shallower and flatter free energy surface for the corresponding acetamide−ion jumps.

V. CONCLUDING REMARKS We summarize by stating that orientational jump characteristics of acetamide depend significantly on the identity of the anion present in these (acetamide + LiX) DESs. Jump reorientations have been separated into acetamide−acetamide and acetamide−ion contributions, and electrolyte effects have been observed for both components. Large angle jumps are found to be more frequent in systems containing ClO4− and Br− than NO3−. Acetamide−acetamide jump angle distributions are narrower and shifted to lower angles compared with those in molten neat acetamide.43 Interestingly, these distributions for acetamide−ion species show a bimodality for Br− and a shoulder for ClO4− but no such features in the case of NO3−. Substantial translation−rotation coupling has been observed for jump reorientation in all these systems with the extent depending on identity of the anion. In addition, change in relative radial distance (ΔR) and difference in H-bond coordination number (ΔnHB) between initial and final acceptors have been found to be significantly less than those found in molten neat acetamide.43 Jump time distributions follow power-law dependence at long times reflecting dynamic heterogeneity in these systems, with the tail decay rate being the slowest for Br− containing DES among the three such systems studied here. Free energy barrier for orientational jumps also registers electrolyte dependence with the electrolyte effects more pronounced on acetamide−ion jumps than on acetamide−acetamide jumps. Although in the present work we have separated out acetamide−acetamide and acetamide−ion jumps, the effects due to interaction between cation (Li+) and carbonyl oxygen of acetamide molecules on these jumps have not been analyzed. Also, exchange of H-bond partners between anions and subsequent effects on jump characteristics need to be investigated. Orientational relaxation in these heterogeneous molten mixtures and its connection to waiting time distribution would be interesting because that would provide a microscopic picture for understanding experimental dielectric relaxation (DR) data. These and other DESs, being industrially relevant and rich in important basic scientific aspects, warrant further experimental and simulation study not only for smarter large scale applications but also for examining the applicability of the language that describes dynamics of supercooled liquids.63−65 Exploring the interconnection between orientational relaxation and jumps for confined water 66−68 and that between

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ASSOCIATED CONTENT

S Supporting Information *

Force field parameters and representations of CH3CONH2, Li+, Br−, NO3−, and ClO4−, density comparison between simulations and experiments, simulated radial distribution functions, and various trajectories with error bars. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b03022.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax:+91 33 2335 3477. Phone: +91 33 2335 5706. *E-mail: [email protected]. Fax: +91 33 2335 3477. Phone: +91 33 2335 5706. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank anonymous reviewers for several insightful comments and suggestions. S.D. thanks Council of Scientific and Industrial Research (CSIR), India, for a research fellowship. Work reported here is performed using the computational facility provided by a TUE-CMS project at the Centre (SR/NM/NS29/2011(G)).

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REFERENCES

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DOI: 10.1021/acs.jpcb.5b03022 J. Phys. Chem. B 2015, 119, 11157−11168

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DOI: 10.1021/acs.jpcb.5b03022 J. Phys. Chem. B 2015, 119, 11157−11168