Behavior of Deep Eutectic Solvents under External Electric Fields: A

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On the Behavior of Deep Eutectic Solvents Under External Electric Fields: A Molecular Dynamics Approach Mert Atilhan, and Santiago Aparicio J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b09714 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 12, 2016

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The Journal of Physical Chemistry

On the Behavior of Deep Eutectic Solvents Under External Electric Fields: A Molecular Dynamics Approach Mert Atilhan*a and Santiago Aparicio*b a

Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar b

Department of Chemistry, University of Burgos, 09001 Burgos, Spain

*

Corresponding authors: [email protected] (S.A.) and [email protected] (M.A.)

ABSTRACT: The properties of selected deep eutectic solvents comprising choline chloride as hydrogen bond acceptor and several types of hydrogen bond donors under static and dynamic external electric fields have been studied in this work using classic molecular dynamics simulations. The effects of field intensities under static conditions and of field frequencies for dynamic conditions were simulated. The response of the fluids to the external fields were analysed from the changes in dipolar arrangements, intermolecular interaction energies, nanoscopic arrangements and molecular diffusion. These results shows for the very first time the non-equilibrium behaviour of deep eutectic solvents under external electric fields.

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1. INTRODUCTION Deep Eutectic Solvents (DESs) are a class of materials that have attracted great academic attention these last years.1-4 These substances are produced by the proper mixing of two high melting point solids that leads to a eutectic mixture with low melting temperature.5 The mixing of a high melting point salt (acting as Hydrogen Bond Acceptor, HBA) develops many DESs, such as the commonly applied choline chloride, and suitable Hydrogen Bond Donors (HBD).1,6 The number of possible HBA / HBD combinations leading to DESs with melting points close to ambient temperature is very large, as the available literature shows,1,10 which confirms DESs as a suitable platform for developing task specific fluids. Therefore, the successful application of DESs has been proved for technologies such metal processing,2 polymerization,7 extraction,8 biodiesel production,9 or CO2 capture.10 The natural origin and the presence of some DESs in physiological processes have been reported,11 and thus, the development of DESs from natural sources has been proposed, leading to the so-called natural DES, NADES.3,6 DESs have many technological advantages such as their synthesis being 100 % atom economic,12 together with their low toxicity,13 high biodegradability,14 recyclability,15 and low cost,16 which justify the interest both in industry and academia. DESs have been considered as an alternative approach to ionic liquids (ILs)15 which can overcome some of the IL well-known problems.17 The application of DESs for large-scale industrial technologies requires an accurate knowledge of their most relevant physicochemical properties,10 and for this purpose a large number of experimental studies have been reported.10 Nevertheless, considering the large number of possible HBA / HBD combinations leading to DESs, and in order to obtain suitable structure – property relationships, a knowledge of DESs properties at the nanoscopic level is also required, which is still in its infancy.18 For this purpose, computational chemistry tools, mainly Density Functional Theory (DFT)18,19 and classical molecular dynamics simulations (MD), 20,21 have been successfully applied for obtaining a deep analysis of DESs properties together with the structuring and dynamics of DESs liquid phases at the molecular level. Likewise, these theoretical approaches have been applied for the characterization of relevant applications for DESs such as CO2 capturing,22 behaviour under confinement,23 adsorption on relevant solid surfaces,24 or properties at gas interfaces.25 The in silico studies on DESs can be used to explore new technologies and also to find the most relevant DESs molecular features in order to improve the applicability of these new applications.26 In this way, considering the properties of DESs we report in this work a study using MD on the behaviour of DESs under external electric fields (EEFs), both for static and 2


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dynamic EEFs. The analysis of the literature shows the absence of previous studies on the behaviour of DESs under EEFs. The literature on the related IL compounds is also scarce.27-33 The interest on the behaviour of DESs under EEFs stands on the possible application of these fluids in technologies such as microwave synthesis or dipolar heating.29 Moreover, it may be expected that the properties of DESs will change remarkably upon the application of EEFs, as similar to ILs,34 and thus property tuning through EEFs can be considered especially for properties which have a large impact on technological applications such as viscosity or molecular diffusion. In order to analyse the behaviour of DESs under EEFs, five different DESs were considered, Figure 1. These DESs share the same HBA (choline chloride), which was selected due to its wide use for DESs development, null toxicity and low cost.2,35 Five different HBDs were considered: glycerol (DES_GLY), levulinic acid (DES_LEV), malonic acid (DES_MAL), phenylacetic acid (DES_PAA) and urea (DES_URE). Relevant properties of the selected HBDs and freezing temperatures of the considered DES are reported in Table 1.36-39The selection of HBDs was done considering different molecular features and functionalities in order to infer their behaviour under EEFs. MD for these DESs were carried for static and dynamic EEFs, for several field intensities and frequencies. The reported results were analysed in terms of changes in structural and dynamic properties upon the application of EEFs and the effects of HBDs, field intensities and frequencies.

2. METHODS MD simulations were carried out for simulation boxes containing 250 choline chloride ion pairs and 500 HBD molecules for 1:2 DESs or 250 HBD molecules for 1:1 DESs (Figure 1), where i:j stands for HBA:HBD mole ratio. Simulations were developed in three stages: i) initial simulation boxes were built using the Packmol program,40 ii) non-equilibrium NPT simulations in presence of the corresponding EEFs at 350 K and 0.1 MPa were carried out for 10 ns allowing the system to reach steady states, and iii) with the average system sizes obtained from NPT simulations under each EEF, production runs in the presence of EEFs using the NVT ensemble were carried out. Six independent 20 ns production runs in the NVT ensemble at 350 K under static or dynamic EEFs were developed using the outputs of NPT simulations. This procedure is analogous to the one developed by Wang28 for simulating ionic liquids under EEFs. The analysis of the available literature on fluids simulations under EEFs shows that both NVT28-30 and NPT31,32 ensembles have been considered; nevertheless, results for both ensembles should be equivalent for large enough systems. In the case of fluids 3



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studied in this work, the used systems sizes (with the number of atoms in the 8250 to 13500 range) led to pressure fluctuations during the NVT production runs lower than 10 % of the total pressure, confirming the minor role of the ensemble selection in this case. The applied EEFs is described as: = 2

(1)

Where Emax and ω stand for the EEFs amplitude and frequency, respectively. In the case of static EEFs ω = 0 and = . EEFs amplitudes in the 0 to 0.25 V Å-1 (in 0.05 V Å-1 steps) were considered for simulation under static EEFs, whereas simulations under dynamic EEFs were carried out for Emax = 0.1 V Å-1 and ω = 2.45, 10, 50 and 100 GHz. ω = 2.45 GHz was considered because it is available in many commercial microwave reactors,29 whereas larger frequencies were applied for analyzing the response of DESs to largely oscillating fields and its relationship with intermolecular forces. The selected range of Emax for simulations under static EEFs was done in order to overcome the intrinsic electric fields in DESs in order to obtain relevant results. Simulations in the NVT and NPT ensembles were carried out with pressure and temperature controlled via the Nose–Hoover method. Ewald method41 was applied for coulombic interactions. The equations of motion were solved using the Tuckerman–Berne double time step algorithm (1 and 0.1 fs, for long and short time steps, respectively).42 Lennard-Jones cross terms were calculated using Lorentz-Berthelot mixing. The forcefield parameterizations used along MD simulations were obtained from our previous studies.21,25 MDynaMix v.5.2 software was used to carry out all the simulations reported in this work.43 Charge distribution in the studied molecules were obtained previously from quantum chemistry calculations of cation – anion – HBD complexes, and thus, atomic charges in [CH]+ cations or Cl- are different for each DES, leading to different dipolar moments for the cation depending on the considered HBD. Although it might be argued that the development of non-equilibrium MD simulations under EEFs might have some issues particularly with the use of kinetic thermostats such as Nose-Hoover method,34,44 the induced errors from the application of kinetic thermostats in non-equilibrium MD simulations under EEFs are negligible for fields with amplitudes lower than 0.1 V Å-1 and very minor for the 0.1 to 0.25 V Å-1, and thus Nose-Hoover thermostat can be applied for MD simulations under EEFs with amplitudes lower than 0.25 V Å-1.45

3. RESULTS AND DISCUSSION

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3.1 DESs under Static EEFs. The behavior of the studied DESs under EEFs was analyzed for intensities in the 0 to 0.25 V Å-1. Stronger fields, such as done in previous studies for ionic liquids,30,34 would lead to larger changes in fluids’ properties (overcoming the DESs intrinsic fields) but to apply these very strong field is not feasible for many realistic large scale industrial application, and thus the upper limit of simulations was fixed in 0.25 V Å-1. The five-studied DESs have very different molecular characteristics, which would lead to different behavior under EEFs. The EEFs were applied along the z-direction of the used simulation boxes, and thus, the total dipole moment along z-direction, µZ, was calculated to show the degree of molecular alignment with the EEFs. µZ was calculated as the vectorial sum of the dipole moments for all the considered molecules, eq. 2,34 Figure 2. = ∑ , (2) Where , stands for the z-component of the individual dipole moment of each individual molecule. Results in Figure 2 for static EEF with E = 0.25 V Å-1 show the both [CH]+ and HBD molecules rotate following the EEF leading to dipolar alignment with the field. The time required by [CH]+ cations to get the maximum alignment with the EEF is roughly 0.1 ns for E = 0.25 V Å-1 and it is almost independent of the type of HBD involved in the DESs. Nevertheless, µZ is dependent on the type of considered DESs, i.e. on the type of HBD involved, which leads to a different charge distribution in the cation. The lower values were obtained for DES_MAL followed by DES_URE, with DES_GLY, DES_LEV and DES_PAA showing similar values. The calculated dipole moments of isolated monomer [CH]+ in absence of EEF for the used forcefield parameterizations were 3.1, 3.1, 3.0, 3.0 and 2.8 D, for DES_GLY, DES_LEV, DES_MAL, DES_PAA and DES_URE, respectively. Therefore, although these values of dipole moment for monomers condition the µZ values reported in Figure 2a, it is obvious that the molecular ability to rotate following the EEF, produced by molecular size and shape, should also develop a pivotal role. Regarding the behavior of µZ for HBDs, the results show that the time required for HBDs to get the maximum alignment with the EEF is slightly larger than for [CH]+ being roughly 0.2 ns with the exception of MAL, which requires around 1 ns Figure 2b. The percentage of dipole alignment, %α, as defined by English et al.,29,34 eq. 3, was calculated to infer the effectiveness of molecular reorientation with EEFs. % = 100 ∙

〈| |〉 〈!, 〉

(3)

This parameter defines the number of dipoles aligned with the EEFs, numerator in eq. 3, in comparison with a hypothetical case in which all the molecular dipoles were aligned with the 5



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field, denominator in eq. 3. Results in Figure 3 show the evolution of %α with the intensity of the applied EEFs showing a non-linear increase with field intensity both for [CH]+ and the HBDs. Results in Figure 3a show that %α for [CH]+ is strongly dependent on the type of HBD involved in the DESs, leading to remarkably lower alignments for [CH]+ in DES_MAL. It should be remarked that DES_MAL is the only DES with 1:1 stoichiometry studied in this work. Regarding %α for HBDs reported in Figure 3b, although the increase in %α is similar to that for [CH]+ the efficiency of the alignment is different to that for the corresponding cation, which is especially remarkable for PAA (high) and MAL (low). It may be expected that the molecular reorientation following the applied EEFs should lead to changes in the fluids structuring, especially considering that [CH]+ and HBD do not rotate in exactly the same way with the field as reported in Figure 3. For this purpose, a collection of relevant radial distribution functions, RDFs, and the corresponding number of molecules obtained in the first solvation shells, N, were calculated, eq. 4. *+*

" = #*+0 ,-.// $%&4 & ( )& (4) Where rshell stands for the radius of the first solvation shells. Results in Figure 4 compare relevant RDFs in absence of EEFs and for EEFs with E = 0.25 V Å-1. In a first approach, Figure 4 results show that although DESs structuring changes upon the application of EEFs, these changes are very minor. In the case of RDFs for [CH]+ - Cl- interactions, the intensity of the first RDF peak decreases with the application of the field, but the position of the peak remains unchanged, for all the studied DESs. This leads to a slight decrease in the number of Cl- anions in the first solvation shell of [CH]+ with the applied EEFs, Figure 5. Regarding HBD molecules in [CH]+ solvation shells, results reported in Figures 4 and 5 show that it remains almost unchanged with EEFs. The structuring of solvation shells around HBD molecules shows only changes in the involved Cl- cations, especially for LEV, MAL and PAA, in which anions are moved toward regions closer to HBD as show the shifting of the corresponding RDF peaks. Nevertheless, although results in Figures 4 and 5 show that the properties of solvation shells do not change remarkably with the applied EEFs when considering the number of molecules present and the average distances to a central molecule of the remaining ones, the molecular rotation following the EEFs should lead to changes in the molecular positions in solvation shell to allow an effective alignment with the field. This information cannot be inferred from RDFs, which report spherical averages, and thus spatial distribution functions, SDFs, were considered. Results in Figure 6 compare SDFs around a central [CH]+ in absence of EEFs and for EEFs with E = 0.25 V Å-1, showing that the spatial

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arrangement in the first solvation shell around [CH]+ changes with the EEFs application for all the studied DESs. These changes are very minor for Cl- anions, which remain almost in the same positions around the cation upon the application of the field, thus showing that anions move couple with cation because of the strength of coulombic forced between ions. On the contrary, the HBD arrangements around [CH]+ change remarkably upon the field application, which would justify the behavior of total dipole moment reported and percentage of HBD alignment reported in Figures 2 and 3. HBD molecules occupy external regions around [CH]+ cation, with Cl- being placed in inner regions of the solvation shells, and thus, a non-coupled rotation of the HBD as a response to the field leading to a redistribution of HBDs in the first solvation [CH]+ shells is obtained, but maintaining the number of HBD molecules in these shells as inferred from results in Figure 5. Molecular dipole rotate following the field with special efficiency for large field intensities, and thus leading to a rearrangement of molecular orientations with the z-axis (direction of applied EEFs) evolving from random orientation in absence of the field to preferred orientations. This behavior was quantified through the order parameter, S, calculated according to eq. 5 for the angle θ formed by the reported vectors in Figure 7 and z-axis: 1=〈

2 345 6 789 (

〉

(5)

Parallel arrangements of the reported vectors with z-axis (and thus with EEFs) would correspond to S = 1. Results reported in Figure 7a for [CH]+ orientation show that the analyzed vector, joining nitrogen and hydroxyl atoms (in parallel to the molecular dipole moment),34 rotates following the field but only for significant effects are only obtained for E > 0.1 V Å-1 for all the studied DESs, Figure 7a. Likewise, in the case of HBDs, Figure 7b, the vector reorientations are only produced for E > 0.1 V Å-1, being especially efficient for LEV and PAA. The molecular reorientations following the field led to subtle changes in the structuring of the solvation shells, and thus, the effect of these changes in the intermolecular interaction energies, Einter, were quantified and reported in Figure 8. These results show clearly that for the range of applied EEFs Einter do not change remarkably, the changes in SDFs reported in Figure 6 allow the molecules to rotate with the EEFs but maintaining the most remarkable features of fluids’ structuring, Figures 4 and 5, and also the strength of Einter for all the involved interacting pairs. Nevertheless, the rotation of molecules following the EEFs should affect hydrogen bonding, which is strongly dependent on molecular orientation between interacting pairs, and thus, the variation on the total number of hydrogen bonds upon 7



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the application of EEFs was calculated, Figure 9. Results in Figure 9a shows that the extension of hydrogen bonding decreases remarkably for DES_MAL, for which hydrogen bonding network is almost destroyed, but minor changes are obtaining for the remaining DESs, leading even to a hydrogen bonding reinforcing for DES_URE. These changes in hydrogen bonding are produced in a short time after the application of the EEFs as reported in Figure 9b. Previous studies on ionic liquids have showed that the EEFs led to changes on the dynamic properties of these fluids,29,30,34 and thus, self-diffusion coefficients, D, were calculated for the studied DESs. D in the plane perpendicular to the applied EEFs, Dxy, were obtained from the corresponding mean square displacements, msd, and Einstein’s equation, eq. 6. 9

: = ; lim?→A

〈∆*?6 〉 ?

6)

Where the quantity between brackets stands for msd. To assure reliable D values, calculations were carried out for the last 5 ns of the sims, and to assure fully diffusive regime β parameter (β=1), defined as the slope of the log-log plots of msd vs simulation time, were calculated, leading to values in the 0.98 to 1.00 range for all the studied systems. Previous literature studies on IL have showed that ionic self-diffusion should increase with the intensity of the applied EEFs, but with remarkable changes only for E > 0.1 V Å-1, 29,30, 34 but the behavior for E < 0.1 V Å-1 is not well defined. In the case of the studied DESs, all the systems show remarkably larger molecular diffusion when EEFs with E = 0.25 V Å-1 are applied in comparison with diffusion in absence of field. This increase in molecular diffusion upon application of EEFs cannot be justified by a weakening of intermolecular interactions, because results in Figure 8 shows almost constancy of this property in the studied range, and thus, the subtle changes in fluids structuring reported in Figure 6 following the EEF increase molecular diffusion. The dynamics of the studied DESs under static EEFs was analyzed using the self-diffusion coefficients obtained from mean square displacements, msds. The msds values for the studied DESs follows a complex pattern, as it is showed in Figure 10 for DES_GLY, leading to a decrease in molecular mobility for EEFs (E < 0.15 V Å-1) whereas mobility is increased for strong fields (E > 0.15 V Å-1). This behavior leads to the complex evolution of Dxy with the strength of applied EEFs reported in Figure 11. All the studied DESs, with the exception of DES_LEV, show a minimum in the evolution of Dxy with EEFs strength, Figure 11. For very weak EEFs Dxy are even lower than in the case of E = 0, and thus, it seems that the molecular reorientation produced for weak fields even hinders 8


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molecular diffusion. The comparison of results for [CH]+, Cl- and HBDs reported in Figure 11 shows that the three types of involved molecules follow almost parallel trends regarding to Dxy, and thus their diffusion is highly coupled. Moreover, results reported in Figure 11, together with the almost null changes in Einter reported in Figure 8, confirms that the strength of intrinsic electric fields in DESs, as in IL,29,30,34 requires the application of EEFs with E > 0.1 V Å-1 to obtain useful effects, although even for fields as intense as E = 0.25 V Å-1 the structuring of DESs does not changes remarkably, Figures 4 and 5, but the dynamics is clearly improved. This would be useful for improving dynamics – related properties, such as viscosity, through the application of EEFs. Larger changes on DESs properties could be obtained through the theoretical application of larger EEFs (e.g. up to E = 1 V Å-1 as done for IL)34 but their application for industrial technologies is highly unfeasible.26

3.2 DESs under Dynamic EEFs. The results of the application of dynamic EEFs was also studied for ω = 2.45, 10, 50 and 100 GHz for Emax > 0.1 V Å-1. The evolution of µZ with the frequency of the applied field is reported for DES_GLY in Figure 12 and for the remaining DESs in Figure 13. The oscillatory behavior of µZ shows molecular dipole evolving back and forth following the dynamic EEFs. The comparison of results in Figures 2 and 12 shows that the largest values of µZ under dynamic EEFs (maxima of the peaks) are lower than under static EEFs, i.e. molecules in DESs are poorly orientated with the field under dynamic EEFs. Moreover, the maxima of the peaks reported in Figure 12 decrease with increasing EEFs frequency being almost negligible under ω = 200 GHz. This is in contrast with previous studies for IL for which the dipolar orientation under dynamic EEFs led to values for µZ close to those under static EEFs, and were almost independent of ω.34 This behavior shows that molecules in DESs have slower rotational dynamics than ions in previously studied IL, even when [CH]+ is considered.34 Therefore, the application of low frequency EEFs for the studied DESs should be considered if dipolar orientation is required. Regarding the response of [CH]+ to the dynamic EEFs is strongly dependent on the considered HBD, Figure 13, with minor changes in µZ for DES_MAL, Figure 13a. The rotation of HBD molecules with the field, Figure 13b, is similar to that for [CH]+ showing [CH]+ - HBD coupling. The molecular response to the dynamic field is characterized by a lag time, τlag, between the applied field and the rotating dipoles, calculated as previously defined.34 τlag was calculated according to eq. 7: ,,? = ,,?+0 ∙ cos FG − IJK LM (7) 9



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where µZ,i,t and µZ,i,t=0 stands for the dipole moments of the i molecule in the z-direction at times t and 0, respectively. The lag time decreases with increasing ω in non-linear way, both for [CH]+ and HBD, Figure 14, as previously reported for [CH]+ - containing IL, being in the 30 to 0.5 ps range, and thus being remarkably larger than for the previously reported IL.34 Therefore, the response of molecules in DESs to the dynamic EEFs is slower than in IL. Moreover, the comparison of results in Figures 14a and 14b shows similar τlag for [CH]+ and HBD, showing coupling of both types of molecules in agreement with results in Figure 13. The results in Figure 15 for %α under dynamic EEFs follow a parallel behavior to those in Figure 14 for τlag, and indicate a poor dipole orientation for high frequency fields, in agreement with µZ values reported in Figures 12 and 13. Therefore, although τlag under high frequency EEFs are almost zero, and thus, molecules try to rotate with the field, this evolution is poorly effective leading to modest dipolar orientation. Results in Figure 15 show that for the studied DESs under dynamic EEFs with ω = 200 GHz %α is roughly 1-2 %, whereas previous results for [CH]+ - containing IL under EEFs with the same ω but with Emax = 1 V Å1

led to %α in the 60 to 70 % range.34 Therefore, although larger effects on DESs under high

frequency EEFs could be obtained using larger Emax, this was discarded for practical reasons. Results in Figure 14 show that, although the capacity to orientate the dipoles with the evolving dynamic EEFs is not very effective (Figures 12, 13 and 15), DESs molecules are able to respond to the dynamic EEFs very quickly even for high frequencies. Therefore, changes in the energy of the system resulting from modifications in the intermolecular interaction energy should be expected under dynamic EEFs. To quantify this effect, the changes in the total non-bonded (intermolecular) potential energy with increasing frequency were calculated and compared with static EEFs under the same Emax, Figure 16. These results confirm that intermolecular interactions are weakened by the dynamic EEFs, especially for DES_GLY, but this effect is almost independent on ω for ω > 10 GHz. Surprisingly, in spite of the poor dipolar alignment with the field reported in Figure 15, the low lag times reported in Figure 14 show that dipole try to orientate with the field, although ineffectively, thus disrupting the DESs liquid structuring trying to couple with the EEFs. Nevertheless, results reported in Figure 16 (weakening of non-bonded potential energy lower than 5 % in most DESs) show minor changes in non-bonded potential energy and thus most of the DESs structuring remains even for the high frequency EEFs. Regarding the dynamics of DESs under dynamic EEFs, results reported in Figure 17 for msd show that oscillatory behavior for t < 0.1 ps is only obtained for high frequency fields, in agreement with previous results for IL that showed that oscillatory msd for e.g. ω = 10


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2.45 GHz were obtained only for large Emax.29 The evolution of msd with increasing ω reported in Figure 17 shows decreasing molecular mobility with increasing ω, which can be justified considering that the continuous dipole reorientation following the evolution of the EEFs hinder molecular diffusion, this effect being more remarkable for high frequency fields as studied in this work (e.g. ω = 50 and 200 GHz in Figure 17). Results in Figure 17 also show that oscillatory behavior for high frequency fields vanishes for t > 0.1 ns, in agreement with the behavior of IL,29,34 and thus self-diffusion coefficients were also calculated under dynamic conditions assuring β parameters close to unit, Figure 18. All the involved molecules show decreasing self-diffusion coefficients with increasing ω following a non-linear evolution almost parallel for the five studied DESs. Self-diffusion coefficients for [CH]+, Cl- and the corresponding HBD are almost the same for a fixed ω, showing coupled diffusion of these molecules under dynamic EEFs. The low diffusion rates under high frequency EEFs should be considered for any application of the studied DESs because it will lead to an increase of properties such as viscosity with the derived well-known drawbacks.

4. CONCLUSIONS The behaviour of deep eutectic solvents based on choline chloride salt mixed with glycerol, levulinic acid, malonic acid, phenylacetic acid and urea were studied under static and dynamic electric fields using classic molecular dynamics simulations. Properties under static fields showed moderate dipolar reorientation under the applied fields (E < 0.25 V Å-1) because of the large intrinsic fields in these fluids. Subtle changes in the structuring of solvation shells were inferred under the stronger fields although the composition of these shells and the corresponding intermolecular interaction energies suffer very minor changes. Disruption of intermolecular hydrogen bonding was minor with the exception of DESs containing malonic acid, which network was almost destroyed under intense fields. The molecular diffusion shows a complex evolution, evolving through minima with increasing field intensity and showing remarkably larger molecular mobility for intense fields in comparison with fluids in absence of external electric fields. The results under dynamics fields showed poor dipolar orientation for high frequency fields with moderate lag times decreasing with increasing field frequency. Perturbations of fluids structuring was minor for all the studied DESs but leading to a remarkable decrease in molecular diffusion under high frequency fields.

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ACKNOWLEDGEMENT This work was funded by Ministerio de Economía y Competitividad (Spain, project CTQ2013-40476-R), Junta de Castilla y León (Spain, project BU324U14) and NPRP grant # 6-330-2-140 from the Qatar National Research Fund (a member of Qatar Foundation). We also acknowledge The Foundation of Supercomputing Center of Castile and León (FCSCL, Spain) for providing supercomputing facilities. The statements made herein are solely the responsibility of the authors.

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Table 1. Properties of HBDs considered in this work: molar mass, M, dipole moment, μ, and melting point, mp. Likewise, freezing temperature, Tf, for the studied DESs are also reported HBD

M / g mol-1

μ/D a

b

mp / ºC

DES

c

DES_URE (1:2)

12 c

17.8 c

DES_GLY (1:2)

-33.47d

urea

60.06

4.13 (3.44)

glycerol

92.09

3.17 a (3.13) b

malonic acid

104.06

2.08 a (3.30) b

134 c

DES_MAL (1:1)

10 c

levulinic acid

116.11

1.96 a (3.14) b

32e

DES_LEV (1:2)

-11.87 f

phenylacetic acid

136.15

1.46 a (2.63) b

77 c

DES_PAA (1:2)

25 c

a

134

Tf / ºC

Values calculated for optimized structures of isolated HBDs in gas phase at B3LYP / 6-31g(2d,2p) theoretical

level using ORCA software; b Values calculated using charges in the forcefield parameterizations for HBDs used for MD simulations along this work (it should be remarked that these HBD charges are obtained from choline e

38 f

chloride + HBD 1:1 or 1:2 complexes); c Smith et al.;2 d AlOmar et al.;36 Maugeri and de María;

et al.

40

13


Florindo


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

Figure Captions.

Figure 1. Molecular structures of ions and HBDs in the DESs studied in this work. The acronyms used for each molecule used along this work are reported in blue together with the SALT:HBD mole ratio (in green) for the DESs considered in this work. Figure 2. z-component of total dipole moment, µZ, for (a) [CH]+ and (b) HBD under external static electric field with E = 0.25 V Å-1 as a function of simulation time, t. All values at 350 K for the reported DESs. Figure 3. Percentage of z-component of total dipole moment alignment, % α, as a function of the applied external static electric field, E, for (a) [CH]+ and (b) HBD. All values at 350 K for the reported DESs.

Figure 4. Radial distribution functions, g(r), between the center-of-mass of the reported pairs for the studied DESs as a function of the applied external static electric field, E. All values at 350 K.

Figure 5. Number of molecules around a central one, N, for the studied DESs as a function of the applied external static electric field, E. N was calculated from the integration of the corresponding radial distribution functions up to 7.5 Å. All values at 350 K. Figure 6. Spatial distribution functions of Cl- and the corresponding HBD around [CH]+ for the studied DESs as a function of the applied external static electric field, E. All values at 350 K.

Figure 7. Order parameter, S, for the angle formed between the corresponding molecular vectors and z-axis for the studied DESs as a function of the applied external static electric field, E. All values at 350 K.

Figure 8. Intermolecular interaction energy, Einter, for the corresponding pairs for the studied DESs as a function of the applied external static electric field, E. All values at 350 K.

Figure 9. (a) Percentage changes in the total number of hydrogen bonds, NH-bonds, on going from DESs in absence of external static electric field (E=0) to the application of external static electric field, E. (b) Evolution with simulation time for E = 0.25 V Å-1. The criterion for defining hydrogen bonding is 3.2 Å and 60o for donoracceptor separation and angle. All values at 350 K. Figure 10. Mean square displacement, msd, for [Ch]+ in DES_GLY under static external electric field as a function of electric field intensity, E. All values at 350 K.

Figure 11. Self-diffusion coefficients in the xy plane (perpendicular to the applied external electric field), Dxy, for the corresponding molecules in the studied DESs as a function of the applied external static electric field, E. All values at 350 K. Error bars are reported and they were calculated from the six independent production runs for each system under EEF. Errors are in the 10 to 20 % range for all the reported results.

14


Page 14 of 36

Page 15 of 36

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


Figure 12. z-component of total dipole moment, µZ, for [CH]+ and GLY in DES_GLY under dynamic external electric fields (Emax = 0.1 V Å−1) as a function of simulation time, t, and electric field frequency. All values at 350 K for the reported ionic liquids. Results are reported for the first 1 ns after system equilibration. Figure 13. z-component of total dipole moment, µZ, for (a) [CH]+ and (b) HBD in the studied DESs under dynamic external electric fields (Emax = 0.1 V Å−1, ω=2.45 GHz) as a function of simulation time. All values at 350 K.

Figure 14. Lag time, τlag, of the maximum dipole alignment with the applied dynamic external electric fields (Emax = 0.1 VÅ−1) as a function of electric field frequency, ω. Values are calculated for (a) [CH]+ and (b) HBDs for the studied DESs. All values at 350 K.

Figure 15. Percentage of z-component of total dipole moment alignment, % α, for the studied DESs with the applied dynamic external electric fields (Emax = 0.1 VÅ−1) as a function of electric field frequency, ω. Values are calculated for (a) [CH]+ and (b) HBDs for the studied DESs. All values at 350 K.

Figure 16. Percentage differences between non-bonded total potential energy under dynamic, Ep,w, and static, Ep,w=0, external electric field (Emax = 0.1 VÅ−1) as a function of electric field frequency, ω, for the studied DESs. All values at 350 K. Figure 17. Mean square displacement, msd, for [Ch]+ in DES_GLY under dynamic external electric field (Emax = 0.1 VÅ−1) as a function of electric field frequency, ω. All values at 350 K.

Figure 18. Self-difussion coefficients, D, for the corresponding molecules in the studied DESs under dynamic external electric field (Emax = 0.1 VÅ−1) as a function of electric field frequency, ω. All values at 350 K. Error bars are reported and they were calculated from the six independent production runs for each system under EEF. Errors are in the 10 to 20 % range for all the reported results.

15



H3C

OH

SALT

+

N

Cl

H3C

[CH]+

CH3

-

ClO

O CH3 HO

OH

O

O

OH

HBD

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 16 of 36

OH HO

GLY

LEV

1:2

1:2

HO

O

O

OH

H2N

NH2

MAL

PAA

URE

1:1

1:2

1:2

Figure 1.

16


Page 17 of 36

1200

1600

(a) [CH]+

(b) HBD 1200

800 µZ / D

µZ / D

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


400

0 0.001

DES_GLY DES_LEV DES_MAL DES_PAA DES_URE

0.01

0.1 t / ns

1

800

400

10

0 0.001

0.01

Figure 2.

17


0.1 t / ns

1

10


100

100 DES_GLY DES_LEV DES_MAL DES_PAA DES_URE

80

(a) [CH]+

(b) HBD 80

60

60 %α

%α

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 18 of 36

40

40

20

20

0

0 0

0.05 0.1 0.15 0.2 0.25 0.3 E / V Å-1

0 0.05 0.1 0.15 0.2 0.25 0.3 E / V Å-1

Figure 3.

18


Page 19 of 36

[CH]+ - Cl-

2

3 2 1

0 5

0 5

0 5

(f) g(r)

g(r)

2

2

2 0 5

(g)

4

3

3 1

2

3 2

1

1

1

1

0 5

0 5

0 5

0 5

2

2

1

1

1

0 5

0 5

0 5

(m)

4 g(r)

3 2

3 2

1

0

0

0

5

5

(r)

4 g(r)

3 2

2

1

0

0

0

0

5

10 r/Å

15

0

5

10

15

r/Å

(u)

4

2

1

2 0 5

(t)

3

1

3 1

4

3

(q)

4

5

(s) g(r)

4

5

2

1

2 0

(o)

3

1

3 1

4 g(r)

4

(n)

(l)

4

3

g(r)

g(r)

g(r)

2

(k)

4

3

g(r)

4

3

g(r)

(i)

(j)

(h)

4

3

g(r)

4

3

(d)

4 g(r)

3 1

(e)

5

(c)

4

1

4

g(r)

(b) g(r)

g(r)

g(r)

g(r)

DES-LEV

2

HBD - HBD

5

4

-1

3

4

g(r)

DES-MAL DES-PAA

Emax= 0.25 V Å

Cl- - HBD

[CH]+ - HBD 5

Emax= 0 V Å-1

(a)

4

g(r)

DES-GLY

5

DES-URE

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


3 2 1 0

0

5

10

r/Å

Figure 4.

19


15

0

5

10 r/Å

15


6


N

5 4 3

(a) HBD ar HBD

2 6

N

5 4 3 2 6

(b) Cl- ar HBD

N

5 4 3

(c) [CH]+ ar HBD

2

6 5 N

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

4 3

(d) Cl- ar [CH]+

2 0

0.05

0.1 0.15 E / V Å-1

0.2

0.25

Figure 5.

20


Page 20 of 36

Page 21 of 36

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


Figure 6.

21



0.6

(a) [CH]+

0.4 S

H3C OH

0.2

+

N


0 -0.2 0.3

H3C CH3

OH

(b) HBD HO

OH

0.2

OH O

S

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 22 of 36

O

0.1

CH3 HO O

O

0

O

O

H2N HO

-0.1 0

0.05

0.1 0.15 E / V Å-1

0.2

0.25

Figure 7.

22


OH

NH2

Page 23 of 36

Einter / kJ mol-1

0


-40 -80 -120 -160 (a) [CH]+ - HBD

-200

Einter / kJ mol-1

40 0 -40 -80 -120

(b) Cl- - HBD

0 Einter / kJ mol-1

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


-10 -20 -30 -40 (c) HBD - HBD

-50 0

0.05

0.1 0.15 E / V Å-1

0.2

0.25

Figure 8.

23



50

50

(a) 100(NH-bonds,E-NH-bonds,E=0)/NH-bonds,E=0

100(NH-bonds,E-NH-bonds,E=0)/NH-bonds,E=0

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 24 of 36

0

-50


-100 0

0.05

0.1 0.15 E / V Å-1

0.2

0.25

(b)

0

-50

-100 0.001

0.01

Figure 9.

24


0.1 t / ns

1

10

Page 25 of 36

100000 10000 1000 msd / Å2

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


100 E = 0 V Å-1 E = 0.05 V Å-1 E = 0.10 V Å-1 E = 0.15 V Å-1 E = 0.20 V Å-1 E = 0.25 V Å-1

10 1 0.1 0

0.2

0.4 0.6 t / ns

0.8

Figure 10.

25


1


10

10

(a) [CH]+

10

(b) Cl-

(c) HBD

1

1

0.1

0.1

0.1

0.01 0.001


0.0001 1E-005 0

0.05

0.1 0.15 E / V Å-1

0.2

109 Dxy / m2 s-1

1 109 Dxy / m2 s-1

109 Dxy / m2 s-1

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 26 of 36

0.01 0.001 0.0001

0.001 0.0001

1E-005

0.25

0.01

1E-005 0

0.05

0.1 0.15 E / V Å-1

0.2

0.25

Figure 11.

26


0

0.05

0.1 0.15 E / V Å-1

0.2

0.25

Page 27 of 36

800 [CH]+ GLY

µZ / D

400 0 -400 -800 800 0

(a) 2.45 GHz

0.2

0.4 0.6 t / ns

0.8

1

0.4 0.6 t / ns

0.8

1

0.4 0.6 t / ns

0.8

1

0.4 0.6 t / ns

0.8

1

µZ / D

400 0 -400 (b) 10 GHz

-800

800 0

0.2

µZ / D

400 0 -400 (c) 50 GHz

-800 800 0

0.2

400 µZ / D

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 -400 (d)200 GHz

-800 0

0.2

Figure 12.

27



800

(a)

CH in DES_GLY CH in DES_LEV CH in DES_MAL CH in DES_PAA CH in DES_URE

µZ / D

400 0 -400 -800 0 800

0.2 (b)

0.4 0.6 t / ns

0.8

1

GLY in DES_GLY LEV in DES_LEV MAL in DES_MAL PAA in DES_PAA URE in DES_URE

400 µZ / D

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 28 of 36

0 -400 -800 0

0.2

0.4 0.6 t / ns

0.8

1

Figure 13.

28


Page 29 of 36

τlag / ps

30

[CH]+ in DES_GLY [CH]+ in DES_LEV [CH]+ in DES_MAL [CH]+ in DES_PAA [CH]+ in DES_URE

(a)

20

10

0

30 1

10 (b)

τlag / ps

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


100 ω / GHz

20

1000


10

0 1

10

100 ω / GHz

Figure 14.

29


1000


25

(a)

[CH]+ in DES_GLY [CH]+ in DES_LEV [CH]+ in DES_MAL [CH]+ in DES_PAA [CH]+ in DES_URE

%α

20 15 10 5 0 25 1

10 (b)

100 ω / GHz

20 %α

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 30 of 36

15

1000


10 5 0 1

10

100 ω / GHz

Figure 15.

30


1000

Page 31 of 36

0

100 (Ep,w-Ep,w=0)/Ep,w=0

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


-4


-8

-12 0

40

80 120 ω / GHz

160

Figure 16.

31


200


10000 1000

msd / Å2

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 32 of 36

ω= 0 GHz ω=2.45 GHz ω=10 GHz ω=50 GHz ω=200 GHz

100 10

1 1 (a) 0.1 0.001

(b)

0.01

0.1

1 0.001

t / ns

Figure 17.

32


0.01 0.1 t / ns

1

Page 33 of 36

109 D / m2 s-1

1

(a) [CH]+

109 D / m2 s-1


0.1

0.01

0.001 1 1 (b) Cl-

10

100

1000

100

1000

100

1000

ω / GHz

0.1

0.01

0.001 1 1

10 (c) HBD

109 D / m2 s-1

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


ω / GHz

0.1

0.01

0.001 1

10 ω / GHz

Figure 18.

33



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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Behavior of Deep Eutectic Solvents under External Electric Fields: A

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