Comparison of the Structural Response to Pressure of Ionic Liquids

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A Comparison of the Structural Response to Pressure of ILs with Ether and Alkyl Functionalities Kamal B. Dhungana, and Claudio Javier Margulis J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b04038 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 23, 2017

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The Journal of Physical Chemistry

A Comparison of the Structural Response to Pressure of ILs with Ether and Alkyl Functionalities Kamal B. Dhungana and Claudio J. Margulis∗ Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States E-mail: [email protected]

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Abstract The response of ionic liquids to external perturbations including elevated pressure is a topic of current interest for applications such as tribology. Ionic liquids come in many classes, including those that are amphiphilic and some that are mostly polar having either cationic or anionic tails that are functionalized.

The current

study compares the effect of elevated pressure on two ionic liquids with different types of cationic tail. 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C10 mim+ /NTf− 2 ) is amphiphilic whereas isoelectronic ether-functionalized 1-(2-(2-(2methoxyethoxy)ethoxy)ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C7 O3 mim+ /NTf− 2 ) has cationic tails that are more polar and conformationally different. We find that the response to elevated pressure for these two systems is quite unsimilar. Intramolecular conformational changes as well as changes in the structure of liquid nanodomains appear to be significantly more prominent in the case of − − + + C10 mim+ /NTf− 2 . Whereas both the density of C10 mim /NTf2 and C7 O3 mim /NTf2

change at elevated pressure, the change is more dramatic for C10 mim+ /NTf− 2 . The very different response for each of these two types of ionic liquids can be clearly gleaned from distribution functions in real space and the partial subcomponents of the X-ray structure function, S(q), in reciprocal space. Liquid structure in the case of + C7 O3 mim+ /NTf− 2 , and the intramolecular conformational structure of C7 O3 mim in

particular, appear to be more resilient to pressure changes than those in the isoelectronic amphiphilic analog.

1

Introduction

Recent articles 1–4 have described structural properties of ionic liquids (IL) of significant academic and industrial appeal. Some of these properties can be achieved by the careful functionalization of the ions 5–12 or by controlling other factors such as the pressure or electric fields. 4,13–31 Part of the rationale for such studies is the desire to better understand whether ILs can be used as materials that respond to perturbations by drastically modify2 ACS Paragon Plus Environment

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ing their amphiphilic nature in the liquid phase, or their organization at interfaces. As an example, the group of Atkin 28 has shown that upon application of electric fields, tribology properties of ILs can be conveniently manipulated resulting in adjustable local friction at surfaces. Recent experimental 20,21,32–42 and computational 20,22,24,43–45 studies have focused on structural effects of pressure on IL intermediate range order. This is also the focus of the current work where we compare the effect of pressure for ILs with ether functionalized vs. alkyl tails. We will show that the structural effect is drastically different for these two types of systems with structural changes for the ether systems being much less pronounced. The nanoscale structure of ether functionalized ILs have been the subject of significant recent interest, 6–12,46–53 since their tail kinks and curling significantly diminish the chargetail alternation that gives rise to a pre-peak or first sharp diffraction peak in ILs with alkyl tails. Because of their lower viscosity, ether containing ILs have been shown to be particularly suitable for electrochemical and energy applications in general. 5,54,55 In the case of the more conventional ILs with alkyl tails, prior studies have shown that whereas anionic conformational changes can be expected, pressure predominantly affects the apolar liquid subcomponent and the integrity of charge networks is affected to a much lesser extent. 20,24 The significant decrease in the first sharp diffraction peak (or prepeak) in X-ray scattering 20,21,24 has to a large extent been linked to cationic alkyl tail changes. Conformers at higher pressure have been recently likened 20 to those of ionic liquids possessing ether tail functionalities. In other words, the prediction or analysis was that pressure would cause alkyl tail curling similar to that observed for ether tails and that this curling would result in a loss of polar-apolar domain alternation with obvious consequences to the amphiphilic nature of the IL. 20 In this article, we compare the effect of pressure on the structure of 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C10 mim+ /NTf− 2 ) with that on the structure of isoelectronic ether-functionalized C7 O3 mim+ /NTf− 2 ; structures of these are shown in Figure 1. Figure 1 provides also the numbering convention that we use in figures throughout the main



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text and in the SI.

C13

C3 N2

N1

C1

C2

C10

C10mim+

C3 N1 C2

CT

C11

C9

C7

C5

C12

C10

C8

C6

C4

N2 C1

O2

C6

C5

O3

C8

C7

O1

C4

C9

CT

C7O3mim+ F

F

F

F N-

C

C

S

F O

S OO

F O

NTf2-

Figure 1: 1-decyl-3-methylimidazolium (C10 mim+ ), 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl) -3-methylimidazolium (C7 O3 mim+ ), and bis(trifluoromethylsulfonyl)amide (NTf− 2 ).

2

Computational Methods

For all simulations, we considered boxes with 1,000 ion pairs in which the minimum image convention and periodic boundary conditions were applied. All studies were carried out using the GROMACS package. 56,57 The force fields were based on the OPLS-AA 58–60 and Lopes and Pádua 61–63 parameterization. The dihedral coefficients for CT-CT-CT-CT, CT-CTCT-HC and HC-CT-CT-HC were taken from the improved OPLS-AA set. 60 For systems with ether tails, charges were unavailable and we fitted them using the CHelpG protocol implemented in the Gaussian09 package 64 (G09) at the MP2 level of theory using the augcc-pvtz basis set. Dihedral parameters were obtained from references 58, 60, 61, 65, and 66. For completeness, the full set of charges, L-J parameters and dihedral coefficients along the ether tail for the C7 O3 mim+ cation are provided in the SI (Fig. S1, and Tables S1, S2 and S3). When comparisons were possible, distributions of dihedrals were found to be consistent with those provided earlier by Shimizu and coworkers (see Figs. S2 in the Supporting Information in comparison with Fig. 6 of reference 49). We also carefully tested the dihedral coefficients 4 ACS Paragon Plus Environment

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for atom types NA-CT-CT-O and CT-CT-O-CT by generating classical and ab-initio energy profiles. This comparison, which is provided as Supporting Information in Fig. S3, followed the protocol suggested in reference 61. Classical calculations were done with the GROMACS package 56,57 and ab-initio calculations with G09 64 at the MP2 level of theory and the augcc-pvtz basis set. To equilibrate our systems, an initial crystal-like configurations was first energy-minimized with charges at 1% of their real value, and then constant pressure and temperature simulations were carried out with charges at 10% of their correct value for 200 ps at 295 K and 1 bar. This step was followed by 2 ns in the NPT ensemble at full charge. After this, for each system, we run annealing md simulation for 28 ns where the temperature was gradually raised and lowered multiple times between 295 K and 500 K. This was followed by production runs at 295 K and 1 bar for 22 ns in duration. Runs at less than 100% charge as well as the annealing portion of the equilibration were carried out using the v-rescale and Berendsen temperature and pressure coupling schemes as coded in GROMACS. 56,57 Runs at 100% charge and production simulations used the Nosé-Hoover thermostat 67–69 and the Parrinello-Rahman barostat. 70 During the last 2ns of the production run, data was saved every 100 fs for analysis. Simulations at higher pressures (0.3 GPa and 1.2 GPa) were started from the pre-production equilibrated system at 1 bar and were subjected to the same 28 ns annealing protocol. In all cases, the Leap-frog algorithm with a time step of 1 fs was used to integrate the equations of motions. Coulomb and Lennard-Jones cutoffs were set to 1.5 nm. The Particle Mesh Ewald (PME) method 71,72 with an interpolation order of 6 and Fourier grid spacing of 0.8 ˚ A was used to account for electrostatic interactions. The following equation was used to compute the structure function (S(q))

S(q) =

ρ0

P P i

j

R L/2

xi xj fi (q)fj (q)

0

P

[

W (r)dr 4πr2 (gij (r) − 1) sinqr qr

xi fi (q)]2

.

(1)

Here, gij is the radial distribution function, which includes both intra and inter molecular pairs, for atomic species of type i and j . xi and xj are the fractions of atoms of type i 5 ACS Paragon Plus Environment


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and j, and fi (q) and fj (q) are the X-ray atomic form factors. 73 A Lorch function W(r) = sin(2πr/L)/(2πr/L), was used to reduce the effect of truncation of g(r) at large values of r . 74,75 ρ0 and L are the total number density and the periodicity of the simulation box, respectively. For our analysis, we further partitioned the total S(q) into corresponding subcomponents. A detailed description of possible partitioning schemes for S(q) is given in our prior papers. 76–79 In this article the head group includes the ring, the methyl group and up to the first methylene group of the longer side chain.

3

Results and Discussion

− + For C7 O3 mim+ /NTf− 2 and C10 mim /NTf2 , in the following subsections we provide an anal-

ysis of differences and similarities in structural response to pressure changes. Aspects associated with intermediate range behavior are better described in reciprocal space whereas changes in adjacency correlations as well as intramolecular changes are better described in real space using pair distributions.

3.1

Reciprocal Space Analysis

The most direct way to gauge the effect of pressure on our IL systems is by analyzing their computed X-ray structure functions S(q) shown in Figures 2a and 2b. At ambient pressure ˚−1 and as expected for C10 mim+ /NTf− 2 there are three distinct peaks in S(q) below 2 A . The lowest q peak (below q ∼ 0.5 ˚ A−1 ) is known as the first sharp diffraction peak or the pre-peak and is indicative of intermediate range order associated with the size of the apolar domain in ILs. 76,78,80,81 More precisely it has to do with the typical separation of filaments in a charge alternation network that are spaced by apolar pockets or domains. This peak appears missing or highly diminished in the case of C7 O3 mim+ /NTf− 2 . The two peaks at larger value of q have been thoroughly discussed in prior articles from our group 15,76–78,82 and correspond to ˚−1 ) and adjacency correlations at around a q value of 1.5 charge alternation (q below 1 A


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˚ A−1 . The charge alternation peak is associated with the typical separation between sametype charges when intercalated by an ion of opposite charge. The general features in Figures 2a and 2b are consistent with the X-ray diffraction data 12,83 and previous MD results 11,49 on ILs with alkyl tails and ether tails at ambient pressure. At higher pressures, the intensity of the prepeak in C10 mim+ /NTf− 2 significantly diminishes. This is consistent with prior studies by Sharma 24 and by Russina 20 on ILs with similar apolar features. Because of the increased pressure and smaller volume, the position of peaks –except the charge alternation peak– are shifted to higher q values (shorter distances). The ether system in Figure 2b also shows peaks of less intensity that are shifted to larger q values at higher pressures.

− + Figure 2: Pressure dependent total S(q) for (a) C10 mim+ /NTf− 2 and (b) C7 O3 mim /NTf2 at 295 K.

We only start to grasp how different the response to pressure in these liquids is, when in Figures 3 and 4 we dissect S(q) into its subcomponents associated with periodicities in the condensed phase. The math behind these partitionings has been described in detail in several prior studies. 76,77,82 Periodicities in the liquid phase manifest as two peaks and one antipeak in appropriately selected partitionings of S(q). 76,77,82 In the prepeak region the appropriate way to see this is by splittings S(q) into charge-charge, tail-tail and charge-tail subcomponents which corresponds to the polarity partitioning of S(q) in ILs with alkyl tails. Instead in the charge alternation region this is seen by splitting S(q) into positive-positive, negative-negative and positive-negative subcomponents. 7 ACS Paragon Plus Environment


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Figure 3: Charge-charge, charge-tail and tail-tail subcomponents of S(q) at 0.1 MPa and 1.2 − + GPa for (a) C10 mim+ /NTf− 2 and (b) C7 O3 mim /NTf2 . This corresponds to the polarity partitioning of S(q) in the case of ILs with apolar tails. The blue and black vertical lines in (a) indicate the position of polarity alternation at 0.1 MPa and 1.2 GPa respectively. No such shift is observed in (b). At ambient pressure, peaks and antipeaks in Figure 3a are sharper and of much larger intensity than those in 3b.

This highlights a better defined charge-tail periodicity for

− + C10 mim+ /NTf− 2 . Since in C7 O3 mim /NTf2 the anions can solvate both the charge heads and

the polar ether groups in the tails, this result is not unexpected. 11 Although we see an enormous drop in intensity with increasing pressure in the subcomponents of S(q) in Figure 3a, we do not see the same effect in Figure 3b. In other words, whereas both C7 O3 mim+ /NTf− 2 and C10 mim+ /NTf− 2 display charge-tail alternations –clearly detected from the presence of two peaks and one antipeak– the effect of pressure on ether-functionalized tails in this q region is negligible. Further analysis of Figure 3a shows not only that peaks and antipeaks –signature of alternation– broaden and diminish in intensity, but also their maximum (minimum) values shift to larger q consistent with a decreased distance between strings of charge alternation separated by apolar domains. This result is consistent with prior work. 20,24 Instead, in Figure 3b, the charge-tail alternation distance appears to be quite insensitive to pressure. Even at ambient pressure, because of the well established curling of tails 11,49 in ether-containing ILs, the charge-charge distance separated by tails is smaller than in the case of ILs with alkyl tails. In Figure 3b, this distance (associated with 2π/q where q is


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˚−1 ), does not appear to significantly change with applied pressure; slightly larger than 0.5A it is as if at ambient pressure, ether tails were already pre-compacted. Ether containing ILs are generally of significantly larger density that their isoelectronic alkyl-tail counterparts; in − + our case the simulated ( experimental) densities for C10 mim+ /NTf− 2 and C7 O3 mim /NTf2

at ambient pressure are 1.33 (1.283 at 293.15 K) 84 g.cm−3 and 1.47 (1.43 at 298 K) 9 g.cm−3 respectively. A simple back-of-the-envelope calculation confirms that this difference in density is not simply due to a mass effect but instead to a molar volume effect. At 1.2 GPa, densities computed from simulation were 1.56 g.cm−3 and 1.69 g.cm−3 for C10 mim+ /NTf− 2 and C7 O3 mim+ /NTf− 2 respectively. Consistent with the idea that the ether system is to some extent pre-compacted at ambient pressure, the change in density for the alkyl system is about 17.5% but only 14.8% for the ether system.

Figure 4: Cation head-cation head, anion-anion, and cation head-anion partial subcompo− + nents of S(q) at 0.1 MPa and 1.2 GPa for (a) C10 mim+ /NTf− 2 and (b) C7 O3 mim /NTf2 . We also learn much by looking at the charge partitioning of S(q) in Figures 4a and 4b. In this case, the reader is instructed to focus on the region around q=0.85 ˚ A−1 and q=0.74 ˚ A−1 − + for the C10 mim+ /NTf− 2 and C7 O3 mim /NTf2 systems respectively. Whereas changes are

more significant for peaks and antipeaks in the case of C10 mim+ /NTf− 2 , these are nowhere as prominent as what is observed in Figure 3a in the case of charge-tail alternation. In general, this appears to indicate that the charge network is more resilient to changes due to increased pressure. 9 ACS Paragon Plus Environment


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Previously, Russina and co-workers have reported that high applied pressure mostly 20 affected the more weakly interacting apolar domains in C8 mim+ /BF− A recent article 4.

by Sharma and colleagues interpreted that pressure affected both polar and apolar re− 24 gions in Pyrr+ Based on Figures 3a and 4a, we conclude that in the case of 1,8/10 /NTf2 .

C10 mim+ /NTf− 2 , polar-apolar alternation is significantly more affected by pressure than charge alternation. Furthermore, we also conclude from Figures 3b and 4b, that in the case of C7 O3 mim+ /NTf− 2 , neither charge-tail alternation nor positive-negative alternation symmetries are significantly distorted due to a large increase in applied pressure.

3.2

Real Space Analysis

− + The effect of increased pressure on C10 mim+ /NTf− 2 and C7 O3 mim /NTf2 , can be both

intra- and inter-ionic. In the case of C10 mim+ /NTf− 2 , large changes in peaks and antipeaks in subcomponents of S(q) shown in Figure 3a may arise due to changes in organization within nano-domains in the liquid or because of major intramolecular changes.

Intraionic Changes We begin by showing that at ambient pressure the most substantial difference in tail confor− + mation between C10 mim+ /NTf− 2 and C7 O3 mim /NTf2 occurs at dihedrals angles associated

with N-C-C-O vs. N-C-C-C and O-C-C-O vs. C-C-C-C. In the case of C7 O3 mim+ , the preference for gauche instead of trans conformations can be gleaned from Fig. S2 in the SI and results in the well understood tail curling described for example in references 11 and 49. Figure 5 shows low and high pressure intramolecular probability distributions of the dis− + tance between the center of ring and terminal carbons for C10 mim+ /NTf− 2 and C7 O3 mim /NTf2 .

Consistent with results in Figures 3b and 4b, little change is seen in the low and high pres− + sure distributions for C7 O3 mim+ /NTf− 2 , but larger changes are observed for C10 mim /NTf2 .

A similar pressure dependent behavior has been reported previously for C8 mim+ /BF− 4 by Russina and collaborators. 20 In the case of C7 O3 mim+ , two principal conformer families exist 10 ACS Paragon Plus Environment

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Figure 5: Intramolecular distribution function for the distance between cationic center of ring (COR) and the terminal carbon (CT) of the longest cationic tail at two different pressures − + for (a) C10 mim+ /NTf− 2 and (b) C7 O3 mim /NTf2 . Representative conformers at different maxima are shown for illustrative purposes. In each graph, red and blue colors are for 0.1 MPa and 1.2 GPa respectively. of which a member is shown in each case for illustrative purposes in Figure 5b. Interestingly, applying high pressure in the case of C7 O3 mim+ /NTf− 2 favors to some small extent the stretching of tails. The opposite is true for C10 mim+ /NTf− 2 where at high pressures the distribution of distance populates tail conformations that have more kinks and are therefore more curled. The curling is nowhere as pronounced as for C7 O3 mim+ , which at all pressures has significant population in the 0.5 nm range in Figure 5b. Another way of seeing that whereas ether tails somewhat elongate at high pressure, alkyl tails curl is by comparing high and low pressure populations of trans and gauche dihedrals in Figures S4 and S5 of the SI. We see from S4 that at high pressure the population of most trans states is lower and that of gauche states is higher in the case of C10 mim+ . The exact opposite is true for N-C-C-O and O-C-C-O dihedrals in C7 O3 mim+ . For completeness we also provide changes in anionic conformation with pressure in Figure S6. Two more stable conformers with the trans and gauche configurations exist in the case 24,38,79,85 of NTf− As pressure is increased the trans configuration looses population to other 2.

regions in the dihedral landscape. The changes we have recorded are consistent with those noted for other ILs contaning NTf− 2 where the C-S-S-C pseudo-dihedral shifts away from the 11 ACS Paragon Plus Environment


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trans conformation at higher pressure. 24,38

Interionic Changes To begin understanding the relation between tails and the charge network at low and high pressure we show in Figure 6 the pair distribution between C12, C9 and anionic N as well as cationic N1. The analogous correlations in the case of ether tails correspond to O3, O2 and anionic N as well as cationic N1. These pair correlations do not include any intramolecular components and therefore report on localization changes of tail components as related to the charge network.

Figure 6: Upper panel: For C10 mim+ /NTf− 2 , radial distribution functions between C12 and anionic N, C9 and anionic N, C12 and cationic N1, C9 and cationic N1. Lower Panel: for C7 O3 mim+ /NTf− 2 , radial distribution functions between O3 and anionic N, O2 and anionic N, O3 and cationic N1, O2 and cationic N1. Red and blue colors are for 0.1 MPa and 1.2 GPa respectively. Atomic positions are as shown in Figure 1. In all cases, at closest contact changes are significant for the alkyl system but minor for the ether system. Alkyl tails get closer to the charge network at higher pressures. This is consistent with prior results in reference 24 where tails are seen to interact more strongly with anions at higher pressures. An interesting question is if in the case of C10 mim+ this 12 ACS Paragon Plus Environment

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implies a major disruption of the charge network or a different organization within the apolar domains. Figures 7 (a) and (b) provide a qualitative answer to this. At low pressure, for the most part alkyl tails tend to point radially inwards towards the center of the hydrophobic domain. Instead, at high pressure tails deform in an attempt to lay flat at or parallel to the interface with the charge network. This results in apolar pockets of smaller volume but not broken charge networks. This is clearly consistent with the fact that the q value for polarity alternation changes to larger q in Figure 3a but changes are very minor in the charge alternation region of Figure 4a.

Figure 7: Isosurface representing the charge network in bulk C10 mim+ /NTf− 2 at (a) 0.1 MPa and (b) 1.2 GPa. At 1.2 GPa, alkyl tails appear to lay more parallel to the charge network. Cationic tail is in green. Red and yellow colors represent cation head and anion components respectively. Figure 8 shows the correlation between terminal carbon atoms in the cation both for − + C10 mim+ /NTf− 2 and C7 O3 mim /NTf2 . This figure highlights that at high pressure the

changes in the tail domain are much more significant for C10 mim+ than for C7 O3 mim+ . In fact changes have opposite trends since the CT-CT correlation diminishes for the alkyl system but it increases in the case of C7 O3 mim+ . This is likely related to the fact that whereas alkyl tails kink and to some extent lay parallel to the charge network at higher 13 ACS Paragon Plus Environment


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pressures, ether containing tails become less curled.

Figure 8: Radial distribution functions between terminal carbon (CT) of the longest cationic − + tail for (a) C10 mim+ /NTf− 2 and (b) C7 O3 mim NTf2 at 0.1 MPa and 1.2 GPa.

4

Conclusions

We have studied ionic liquid structural dependence on pressure for C10 mim+ /NTf− 2 and C7 O3 mim+ /NTf− 2 . We found that both liquids have charge-tail and positive-negative alternation signatures in the partial subcomponents of S(q). The charge-tail alternation which is much better defined for C10 mim+ /NTf− 2 is also the liquid feature that is most prominently affected by elevated pressure. Peaks and antipeaks associated with this same feature in the case of C7 O3 mim+ /NTf− 2 do not significantly change in intensity or q value suggesting that tail nanostructure is more resilient to elevated pressure for ILs with ether containing tails. For both liquids, changes in the relevant subcomponents of S(q) associated with charge alternation are less significant. This indicates that in the case of C10 mim+ /NTf− 2 , it is not the charge network but instead the apolar regions that are most prone to changes. Both at high and low pressure, C7 O3 mim+ exists mainly as two conformer families associated with shorter and longer ring to tail-end distance. Instead, at high pressure many new conformers are populated with extra kinks in the tail when the cation is C10 mim+ . Even at high pressure, the degree of curling in C10 mim+ is smaller than in C7 O3 mim+ (at high or low pressure). One 14 ACS Paragon Plus Environment

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of the most relevant questions we attempted to answer was whether tail interactions with the charge network significantly changed for either system at high pressure; this has obvious implications for nanodomain segregation and the polar-apolar nature of these systems. We found that at contact with the charge network changes were much more prominent for the alkyl-based system than for the ether-based system. Alkyl tails approach the charge network at high pressures making the nanodomains smaller in volume but not removing them. Qualitatively it would appear that alkyl tails change from a more radial orientation towards the middle of the apolar nanodomains to a more flat on the surface of the charge network conformation. Seen as a whole, the results indicate that even pressures on the order of 1.2GPa deform but do not remove polar-apolar alternation in the case of C10 mim+ /NTf− 2. In the case of C7 O3 mim+ /NTf− 2 , pressure does little to distort liquid structure. Because of intramolecular kinks and curls in C7 O3 mim+ that already exist at ambient pressure, this liquid and probably other ether-containing ILs are more dense and at a molecular level more compact. This makes them less prone to pressure effects.

5

Acknowledgments

This work was supported by Grant No. CHE-1362129 from the US National Science Foundation awarded to C.J.M. We thank Professor Juan Carlos Araque from Benedictine College for insightful discussions.

6

Supporting Information

Force field details, dihedral angle distributions. This information is available free of charge at the ACS Publication Website.



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Graphical TOC Entry

Ionic Liquids Under Pressure Ether vs. Alkyl Tails


Comparison of the Structural Response to Pressure of Ionic Liquids

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