Composition dependent structural organization in trihexyl(tetradecyl)phosphonium chloride ionic liquid-methanol mixtures.

Composition dependent structural organization in trihexyl(tetradecyl)phosphonium chloride ionic liquid-methanol mixtures Aditya Gupta, Shobha Sharma, and Hemant K. Kashyap Citation: The Journal of Chemical Physics 142, 134503 (2015); doi: 10.1063/1.4916308 View online: http://dx.doi.org/10.1063/1.4916308 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Communication: Unusual structure and transport in ionic liquid-hexane mixtures J. Chem. Phys. 142, 121101 (2015); 10.1063/1.4916388 Mixtures of protic ionic liquids and molecular cosolvents: A molecular dynamics simulation J. Chem. Phys. 140, 214502 (2014); 10.1063/1.4879660 Association structures of ionic liquid/DMSO mixtures studied by high-pressure infrared spectroscopy J. Chem. Phys. 134, 044506 (2011); 10.1063/1.3526485 Dynamics of ionic and hydrophobic solutes in water-methanol mixtures of varying composition J. Chem. Phys. 123, 234501 (2005); 10.1063/1.2137702 Composition-dependent dynamical structures of 1-propanol–water mixtures determined by dynamical dielectric properties J. Chem. Phys. 113, 9748 (2000); 10.1063/1.1321767

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THE JOURNAL OF CHEMICAL PHYSICS 142, 134503 (2015)

Composition dependent structural organization in trihexyl(tetradecyl)phosphonium chloride ionic liquid-methanol mixtures Aditya Gupta, Shobha Sharma, and Hemant K. Kashyapa) Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

(Received 10 January 2015; accepted 27 February 2015; published online 6 April 2015) This article reports results from the molecular dynamics simulations on the structural arrangement of the ions and molecules in the mixtures of trihexyl(tetradecyl)phosphonium chloride ([P666,14+][Cl−]) ionic liquid (IL) and methanol (MeOH) over the entire composition range. Effects of composition on the charge and polarity orderings have been investigated via computation of X-ray scattering structure function, S(q), and by using a partitioning scheme proposed for such multi-component mixtures. Except for the neat methanol liquid, the total S(q) shows two peaks in its intermolecular region for all the mole-fractions. The lowest q peak is dominated primarily by anion-anion, cation-anion, and methanol-anion correlations. Our results signify that the methanol bulk structure, which predominantly has short-distance characteristic correlations and is governed by polar group of methanol, is retained for x IL ≤ 0.1. Then, the mixture goes through gradual structural changes from methanol-like to the IL-like for 0.1 < x IL ≤ 0.7. The dipolar interaction between methanol molecules weakens in this range, and the structural landscape of the mixture is steered by strong ion-ion, anion-methanol, and nonpolar interactions. The IL-like structural arrangement is virtually recovered for x IL > 0.7. At all the compositions studied, while the cation head groups are predominantly solvated by anions and subsequently by methanol molecules, the polar hydroxyl group of methanol is preferentially solvated by the anions. The radial distribution functions of selected pair of atomic species have also confirmed these observations. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4916308]

I. INTRODUCTION

Room-temperature ionic liquids (RTILs) are a new class of materials that have recently found more attention due to their interesting and sometimes fascinating properties.1,2 Higher thermal stability, wide electrochemical window, and their ability to dissolve both polar and non-polar compounds make RTILs a good substitute for traditional solvents in synthesis, separation, and countless number of industrial applications.3–5 Their high density and short Debye screening length have rendered them to be able to screen the charged surfaces and therefore made them potential replacements for conventional electrolytes in electrical double layer capacitors and other electrochemical energy devices.5–7 Even though RTILs are homogeneous on macroscopic scale, these liquids exhibit a wide range of organizational landscape on molecular scale.8–15 These liquids can be intricately nanostructured in bulk, resulting from fine balance between electrostatic and nonpolar interactions.8,9,14,16–18 Nuclear magnetic resonance (NMR),19,20 X-ray,9,14–16,21–41 and neutron42–44 scattering experiments, as well as molecular dynamics (MD) simulations,8,16,34,40,45–56 have also confirmed these observations. An important feature of RTIL is that their properties can significantly be modified by taking different combinations of cations and anions and also by mutating the atoms of constituent cations or anions. Mixing RTILs with molecular solvents opens up another degree of freedom a)[email protected]

0021-9606/2015/142(13)/134503/10/$30.00

to fine-tune their physio-chemical properties and to better control the course and outcome of a chemical reaction in these media. However, RTIL+co-solvent mixtures exhibit anomalous properties at very low as well as at very high RTIL concentrations, especially non-monotonic dependence of ionic conductivity,57–61 dielectric relaxation times, and Stokes shift dynamics.62–67 These anomalies in the dynamics of RTIL+cosolvent systems are often linked to structural heterogeneity and transitions associated with them.57–59,61,68,69 Recently, a plethora of experimental and simulation studies have been focused on aqueous solutions of imidazolium cation based RTILs in order to shed light on structure and dynamics of these mixtures.70–75 While some of these studies, such as attenuated total reflection - infrared (ATR-IR),70 have highlighted the existence of preferential interaction of water with anion, other studies, mainly FTIR and Raman spectroscopic studies,75 suggested significant interaction of water with both positively charged head of cation and anion in [Xmim+][BF−4 ] (where Xmim+ is 1-alkyl-3-methylimidazolium). A handful of endeavors on ILs different than imidazolium ones when mixed with molecular solvents have emerged only recently.76 Phosphonium cation based ionic liquids render relatively higher thermal and electrochemical stability and offer several advantages over other types of ILs.77 These ILs are found to have better solubility of organic and inorganic solutes, including super-critical CO2.78–81 They also possess decent ionic conductivity and good solvation power even though these ILs are either nonpolar or less polar than other ILs.78,82–84 Trihexyl(tetradecyl)phosphonium chloride ([P666,14+][Cl−]) IL

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polar-apolar and charge orderings/alternations in neat IL get influenced by the addition of methanol is also explored here.

II. COMPUTATIONAL DETAILS

All the simulations were carried out in the isothermalisobaric ensemble (NPT) using the GROMACS software.86,87 A total of 1000 molecules (ionic-couple + cosolvent) were used for each composition of the mixture. Please see the supplementary material88 for more information. Proper periodic boundary conditions and minimum image convention were applied to the simple cubic box used in the simulation. Parameters from all-atom optimized potentials for liquid simulations (OPLS-AA)89–92 were used for methanol and [P666,14+][Cl−]. Modifications introduced by Lopes and Pádua93,94 for the cation16 and anion parameters were incorporated. The torsional energy parameters for dihedrals involving only alkyl carbons and hydrogens in the cation were taken from the latest OPLS-AA parameter set.16,92,95 OPLS-AA √ √ type of mixing rules, ϵ i j = ϵ i ϵ j and σi j = σi σ j , for the off-diagonal Lennard-Jones parameters were applied. The cutoff radius for the short-range interactions was set to 12 Å. Electrostatic interactions were evaluated using Particle Mesh Ewald (PME)96,97 summation technique with interpolation order of 6 and Fourier grid spacing of 0.8 Å. The leap-frog algorithm was used for integration of equations of motion. The simulation box for each IL mole-fraction was first run for 1 ns at 400 K temperature and 100 bar pressure using Berendsen thermostat and barostat. To expedite the dynamics of equilibration, the partial charges at this temperature and pressure were scaled to 10% of the total ones. Subsequently, each system was run for another couple of nano-seconds with full charges at 400 K and 1 bar. Finally, the last configuration of the previous run was allowed to equilibrate for 10 ns at 295 K and 1 bar temperature and pressure using NoséHoover98–100 thermostat and Parrinello-Rahman101 barostat, respectively. Notice that at the initial step of this run, atomic velocities were randomly generated at 295 K from a Gaussian distribution. To ensure thorough equilibration for x IL = 0.9 and neat ionic liquid, the above protocol was repeated by taking the last configuration of the previous run. The final 5 ns of the trajectory was saved at every 10 fs for the computation of the mixture properties. The X-ray scattering static structure function S(q) was computed using

FIG. 1. Structures of (a) [P666,14+][Cl−] ionic liquid cation (P666,14+) and anion (Cl−) and (b) methanol (MeOH) molecule.

is one of the most readily available and widely used P666,14+ cation based ILs. It is extensively used as starting material for synthesizing other phosphonium based ionic liquids.77 Gontrani et al. studied trihexyl(tetradecyl)phosphonium chloride ([P666,14+][Cl−]) IL using X-ray scattering and molecular dynamics simulations and alluded existence of nanoscale segregation in this IL.30 Theoretical and experimental investigations done by MacFarlane78 and Triolo30 groups on the same ionic liquid have shown the importance of cation phosphorous atom and anion interaction on conductivity and viscosity of the IL. It is to be emphasized that the individuation of a molecular compound that succeeds in disrupting the strong ion-pairing might be interesting to enlarge the potential application of phosphonium based ILs. This work will be able to appreciate the role of methanol on ion pairing strength in [P666,14+][Cl−]. Extensive simulation studies on the structure and dynamics for [P666,14+][NTf 2−] IL have also been pursued by Liu et al.85 and Margulis group.16,18 To the best of our knowledge, most of the P666,14+ ILs are hydrophobic and are miscible with non-polar and polar aprotic solvents.77 In this article, we have simulated trihexyl(tetradecyl)phosphonium chloridemethanol mixtures (Fig. 1) over the entire composition range to appreciate the molecular-level structural landscape of mixture comprising a representative hydrophobic and nonpolar IL and an amphiphilic compound such as methanol. This study leads us to reckon the effects of a hydrophobic and nonpolar IL on the one-dimensional hydrogen bonded linear chain network of neat methanol liquid. How the characteristic features of

ρo S(q) =

n  n  i=1 j=1

x i x j f i (q) f j (q)

L/2  0

[

n  i=1

4πr 2[gi j (r) − 1] sinqrqr W (r)dr

x i f i (q)][

In Eq. (1), gi j (r) is partial radial distribution function (rdf) for the atomic species of type i and j, including intra- and intermolecular pairs. x i and x j are the mole-fractions of atoms

n  j=1

.

(1)

x j f j (q)]

of type i and j; f i (q) and f j (q) are the X-ray atomic form factors.102 ρo is total number density of the system and L is average box length. W (r) is a Lorch window function,


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W (r) = sin(2πr/L)/(2πr/L),103,104 which is used to reduce the effect of finite truncation of r. For a system of moderately larger size, this window function does not hinder the physical meaning of the peaks in S(q). The indices i or j run over H, C, O, P, and Cl atoms in this study, i.e., n = 5.

Decomposition of the above structure function may be valuable in order to better comprehend the basis for the different features in S(q). In practice, one can choose to split the structure function either into its atomic pair contributions n  n  such that total S(q) = Si j (q), where Si j (q) is given by i=1 j=1

ρo x i x j f i (q) f j (q)

L/2  0

Si j (q) = [

n  i=1

4πr 2[gi j (r) − 1] sinqrqr W (r)dr

x i f i (q)][

n  j=1

,

(2)

x j f j (q)]

or into its molecular (cationic, anionic, and cosolvent) sub-components as S(q) =

m  m  α=1 β=1

S α β (q), where the different molecular-

type sub-components of S(q) are given by ρo S α β (q) =

n  n  i=1 j=1

x αi x βj f i (q) f j (q)

m  m x α x β [g α β (r) − 1]  i j ij α=1 β=1

In Eq. (4), we have used the identity

xi x j n  m  i=1 α=1

x αi =

4πr 2[giαj β (r) − 1] sinqrqr W (r)dr

0 n  m  α [ x i f i (q)][ i=1 α=1 j=1 β=1 n  m 

In Eq. (3), indices α or β represent cation, anion, or cosolvent (i.e., m = 3), giαj β (r) is the partial radial distribution function for the ith type of atom in α and jth type of atom in β, and x αi is mole-fraction of atom of type i in α kind of ions or molecules. The above decomposition is possible from the fact that atomic form factors are independent of α, i.e., f i (q) = f iα (q) and one can derive that [gi j (r) − 1] =

L/2 

.

n  i=1

(4) x i = 1.

Notice that the proposed partitioning scheme is general and can easily be applied to mixtures with any number or type of constituent species.

III. RESULTS AND DISCUSSION A. Density and excess enthalpy

Figure 2(a) shows a comparison between simulated and experimental67 densities for [P666,14+][Cl−]–MeOH mixture as a function of the IL mole-fraction, x IL. The agreement between simulation and experimental data is quantitative for methanol rich concentrations and fairly good near the concentrated IL solutions. Notice that the maximum deviation between the computation and experiment is only about 2.5%. Also note that this much deviation in the density of neat ILs does not show any impact on the agreement between simulated and experimental X-ray structure function of the ILs.16,18,30,34,95 The energetics of the mixtures have been investigated through simulated excess molar enthalpy, H E , which was computed

x βj f j (q)]

.

(3)

using Eq. (5). As depicted in Fig. 2(b), at all the mole-fractions investigated, H E is negative, indicating that on average, the IL–MeOH interactions in the mixture are stronger than the IL–IL and MeOH–MeOH interactions in the corresponding neat liquids, H E (x IL) = HMix − [x IL HIL + (1 − x IL)HMeOH ].

(5)

Here, HMix, HIL, and HMeOH are molar enthalpies of the mixture, neat ionic liquid, and neat methanol at the same temperature and pressure, respectively. B. Total structure function, S(q)

In order to see the overall nature of structural ordering in the mixture, in Fig. 3, we have shown the simulated X-ray scattering structure functions, S(q), at 295 K for the entire composition range. While neat methanol shows only one peak at around q = 1.7 Å−1 in the low q region (q < 2 Å−1) of S(q), two peaks are present for all the other mole-fractions of IL studied here. The composition dependence of these two peaks are depicted in Fig. 4 and corresponding real-space characteristic distances, which can be estimated via expression 2π/qPeak, are shown in Figure S1 of the supplementary material.88 It is important to emphasize here that two peaks are also common in neat RTILs16,27,30,33,41,95,105 with moderately large tail cations and their physical origin has been explored recently.16,17,30,37,53,95 Typically, the peak below q = 0.5 Å−1 corresponds to polarity alternations/ordering16,95 in neat ionic liquids and is called prepeak or first-sharp diffraction peak (FSDP). The peak at around 1.5 Å−1 is often associated with nearest neighbour or close-contact distances between counterions and is called adjacency peak.16,95 From both the panels of


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FIG. 3. Composition dependence of simulated total structure function, S(q), at 295 K.

FIG. 2. (a) Comparison of experimental (solid circles) and simulated densities (open circles), ρ, as a function of the IL mole-fraction. The experimental density data were taken from Ref. 67. (b) Simulated excess molar enthalpy, H E , as a function of the IL mole-fraction. Please note that for comparison no experimental data for the H E in the literature exist to the best of our knowledge. Numerical data for ρ and H E are listed in Tables S1 and S2 of the supplementary material.88

Fig. 4, one can appreciate that the lowest q peak position is almost insensitive to the IL addition for x IL ≤ 0.1. For composition with 0.1 < x IL < 0.7, this peak is much more sensitive to the IL addition, it shifts towards smaller q values more rapidly with increasing IL concentration in this range. The shift of this peak towards lower q values resembles increased characteristic length scales corresponding to this peak. The highest q peak shows slight decrease in its intensity for x IL = 0.02 but its intensity starts increasing gradually for x IL > 0.02. The convergence of both the peaks towards those of neat IL S(q) implies better organization of continuous IL phase in the mixtures near the IL rich concentrations.

from Fig. 6(a) that the MeOH–MeOH too have contribution to the peak at q < 0.6 Å−1 until very dilute concentration of methanol. In fact, the cation-cation partial S(q) can also have important contribution to this peak in the form of cation(head)cation(head) and cation(tail)-cation(tail) peaks and in the form of cation(head)-cation(tail) anti-peaks. But, due to cancellation effects, these constructive and destructive interferences may add to zero or negligibly small values. As it will be more apparent in Secs. III D and III E, this peak corresponds to distances between cationic heads, anions, and methanol polar groups that are separated by the nonpolar cationic tail groups. It is easy to perceive from Fig. 6(a) that the larger q peak above 1.2 Å−1 in the total S(q) is because of MeOH–MeOH short-distance correlations near the methanol rich concentrations. But for the higher mole-fractions of the IL, the MeOH–MeOH contribution to this peak fades and that of P666,14+–P666,14+ correlations increases (Fig. 6(b)). The MeOH–P666,14+ also appears to have small contribution to this peak for the lower concentrations of the IL (Fig. S2 of the supplementary material88).

C. Partial structure functions

By using the scheme proposed in Sec. II, in Figs. 5 and 6, we find that the anion-anion, cation-anion, and MeOHanion correlations are significant contributors to the peak below q = 0.6 Å−1 for all the compositions. It also appears

FIG. 4. Composition dependence of S(q) peak positions in the low q region.


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FIG. 6. Partial structure function for (a) MeOH–MeOH and (b) P666,14+– P666,14+ at different mole-fractions of [P666,14+][Cl−] IL.

FIG. 5. Partial structure functions for (a) Cl−–Cl− (b) P666,14+–Cl−, (c) MeOH–Cl− at different mole-fractions of [P666,14+][Cl−] IL. Note that the multiplication factor 2 is also included in the P666,14+–Cl− and MeOH–Cl− partial structure functions.

Interestingly, the cation-cation, anion-anion, and cationanion partial structure functions also show intermediate peaks and anti-peaks above 0.6 Å−1 and below 1.2 Å−1 for x IL > 0.1. This is accompanied also by peaks in MeOH-anion

and MeOH–MeOH structure functions in the same range. Observation of peaks and anti-peaks in the partial S(q)s corresponds to some kind of density alternations/orderings that are off-phase.17,95 In neat IL, this ordering is so-called charge ordering or charge alternation where the charged part of each cation, i.e., cation(head) is separated by anions or vice-versa.16–18,95 These partial S(q)s hint that the intermediate peak belongs to separations between cationic head groups that are separated by methanol molecules and anions which are in close contact with the cationic head groups. In other words, this peak also corresponds to anion-anion or methanol-methanol distances that are due to cation head groups. In summary, while cationic head and anion are in close contact with each other for all the mixtures, so is also true for anion and methanol hydroxyl group. At dilute IL concentrations, the cation(head)-cation(head) and anionanion distances are large because of abundance of methanol molecules in-between. In the dilute IL concentrations, the separation between cationic head groups and anions due to methanol and due to nonpolar alkyl tails of the cation are, on average, qualitatively same. But as the mole-fraction of the IL is increased, the separations between cationic head


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groups and anions due to methanol decrease and those due to nonpolar cationic alkyl tails increase. The overall effect is manifested through shift of the intermediate peak towards the larger q values and the lowest q peak towards smaller values. For neat IL, the intermediate peak turns up into socalled charge ordering/alternation peak and the lowest q peak mimics prepeak or FSDP. D. Radial distribution functions involving cation(head), cation(tails), anion, methanol oxygen, and carbon atoms

In order to better appreciate the mixture structure, we also opted to look at the real space correlations involving

FIG. 7. Radial distribution function, g(r), for (a) P–P and (b) Cl−–Cl−. Please see supplementary material88 for blown-up view of close-contact peaks. Notice that P–P and Cl−–Cl− rdf’s both have well split features between 0.5 and 1 nm for the neat IL. This splitting was not that apparent in the study carried out by Gontrani et al. because of lower statistical averaging.30 However, as clearly mentioned by Gontrani et al., the shorter P–P (Cl–Cl) distance corresponds to two phosphorous (chlorine) atoms that are bridged by two chlorine (phosphorous) atoms. The longer P–P (Cl–Cl) distance corresponds to two phosphorous (chlorine) atoms that are separated by a chlorine (phosphorous) atom.

J. Chem. Phys. 142, 134503 (2015)

cation phosphorous atom (P), anion, and the oxygen (OMeOH) of methanol for the various compositions investigated. In Figs. 7(a) and 7(b) shown are the rdf’s, g(r), for P–P and Cl−–Cl− atomic pairs. Clearly, the position of the peaks corresponding to the first solvation shell in both the rdf’s are well shifted to longer distances for x IL ≤ 0.1. This is not the case for the P–Cl− (Fig. 8(a)) rdf wherein the first peak position shows very little changes over the composition range studied. The above scenario reveals that widely separated cation(head)-anion pairs exist for x IL ≤ 0.1. In addition, for x IL ≤ 0.1, the OMeOH–OMeOH (Fig. 8(b)), P–OMeOH (Fig. 9(a)), and Cl−–OMeOH (Fig. 9(b)) rdf’s atoms show negligible changes in the position of first peak but with enhanced correlations. Considering all the correlations shown in Figs. 7-9, we infer that in the mixtures with low concentration of the IL (x IL ≤ 0.1), the IL exists as associated cation(head)-anion pairs that are well solvated by the methanol molecules (Figs. 9(a) and 9(b)). Therefore, one can clearly

FIG. 8. Radial distribution function, g(r), for (a) P–Cl− and (b) OMeOH–OMeOH. Please see supplementary material88 for blown-up view of the close-contact peaks.


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FIG. 9. Radial distribution function, g(r), for (a) P–OMeOH and (b) Cl−–OMeOH. Please see supplementary material88 for blown-up view of closecontact peaks.

FIG. 10. Radial distribution function, g(r), for (a) CTS–CTS and (b) CMeOH–CMeOH.

anticipate that for x IL → 0, the P–P and Cl−–Cl− rdf peaks will continue to shift towards longer distances along with the reduction of the peak height. As the mole-fraction of the IL increases beyond 0.1 (x IL > 0.1), the rdf’s for the P–P and Cl−–Cl− indicate increased correlations at shorter distances. The same is also true for P–Cl−, P–OMeOH, and Cl−–OMeOH rdf’s, except for OMeOH–OMeOH and OMeOH–HOMeOH (see Fig. S4 of the supplementary material88) correlations which start decreasing for x IL > 0.08. This indicates that as IL molefraction increases, the IL cation(head)-anion pairs approach to shorter distances tending towards continuous IL-like arrangement. The formation of continuous IL-like structure is true for the cationic tails as well (Fig. 10(a)). Comparing the magnitude of the first peak of all the rdf’s involving methanol oxygen, it is clear that the methanol molecules interact more strongly with the Cl− ions than it does with the cation(head) or even with the other methanol molecules. However, from Fig. 8(a), we can clearly see that the cation(head)-anion pairs maintain its consistence upon methanol addition until very dilute IL

concentration. That is, methanol does not succeed in separating the cation(head)-anion pair. The increased close-contact peak heights (or enhanced correlations) in the OMeOH–OMeOH and OMeOH–HOMeOH (see Fig. S488) rdf’s also reveal that the IL acts as structure maker for x IL ≤ 0.1. This is because while anion-methanol interactions are dominant ones for all the mole-fractions, the methanol-methanol interactions are strong enough to keep the methanol-like structure persistent at least for x IL ≤ 0.1. Beyond x IL = 0.1, anion-methanol interactions continue to dominate in addition to cation(head)-anion, but methanol-methanol interactions become weaker than that of anion-methanol and cation(head)-anion. Therefore, the probability of finding two methanol molecules in close vicinity of each other decreases gradually with increasing IL concentration. This picture is also apparent in the form of decreased peak-heights (diminished correlations) in OMeOH–OMeOH and OMeOH–HOMeOH rdf’s for x IL > 0.1. A very close inspection of our simulation snap-shots also discards any aggregation of methanol molecule near IL rich concentrations. This suggest


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of three successive tail carbons of the P666,14+ cation tails (see Fig. S388). As shown in Fig. 10(a), for all the molefractions studied, the CTS-CTS rdf has an apparent shoulder at around 0.4 nm, a peak around 0.5 nm separation, and these two are accompanied by another peak at around 0.9 nm. It is important to notice that the height of the shoulder and the peak at the shorter distance increase up to x IL ≤ 0.2 and then remain insensitive to the further addition of the IL. This indicates that close-contact tail-tail correlations are saturated for x IL > 0.2. However, intermediate-range ordering peak at the longer distances continues to augment up to x IL = 0.7. It is to be noticed that IL with moderately longer alkyl tails shows adjacency as well as intermediate-range orderings. So, hydrophobic aggregation can be assumed as virtually complete only when both the orderings are attained, which in this case is occurring for x IL ≥ 0.7. It is also interesting to see from Fig. 10(b) that the peak height of CMeOH–CMeOH rdf also increases up to x IL ≤ 0.2, but it starts decreasing with increasing x IL further. E. Spatial distribution functions (sdf’s)

The notion of strong cation(head)-anion and anionmethanol correlations is also complimented by looking at the isodensity sdf’s of anions (green, solid) and methanol molecules (red, wire–frame) around P666,14+ cation, as depicted in Fig. 11(a). It is evident that the cation(head) groups are predominantly solvated by Cl− ions and subsequently by methanol molecules. The isodensity surface for Cl− (green, solid) and the cation phosphorous atoms (orange, solid) around a representative methanol molecule is shown in Fig. 11(b). It is clear that the methanol molecules are preferentially solvated by the anions near the hydroxyl (OH) group of methanol. We also observe that the anion sdf is of ring shape around the hydroxyl group of methanol. The Cl− ring is then capped with the isodensity surface of phosphorous atom of P666,14+ cation. In fact, it is not surprising that the polar OH group of methanol molecules prefers to have closer contact with anions. This is because, as in this case and in many other cases, the anions usually have small exclusive volume and possess very high density of negative (−1e) charge. These two factors can easily overcome the positive charge of the cation which is spreaded over the cation head part only.

FIG. 11. (a) sdf’s for anions (green, solid) and methanol molecules (red, wire–frame) around P666,14+ cation in the 50:50 composition of the [P666,14+][Cl−]–MeOH mixture. Note that all the 6 atoms of a methanol molecule have been used to construct the above sdf. (b) The sdf for anions (green, solid) and cation phosphorous atoms (orange, solid) around methanol molecule in the 50:50 composition of the [P666,14+][Cl−]–MeOH mixture. In all the cases, density isovalues were chosen to reflect the first solvation shell. The above sdf’s are averaged over 100 frames and 500 molecules.

that the methanol-like structure is gradually disrupted by the IL for x IL > 0.1 and the IL-like structural arrangement is virtually recovered for x IL > 0.7. The overall nature of cation tail-tail correlations was investigated by rdf’s for a judiciously chosen group (CTS)

IV. CONCLUSIONS

A detailed and systematic study on the structure of [P666,14+][Cl−]–MeOH mixtures has been carried out by means of MD simulations. Special attention was paid to the composition dependent X-ray scattering structure of the mixtures. With the exception of neat methanol, which shows only one peak in the intermolecular region of X-ray scattering structure function, two peaks are observed in total S(q) for all the compositions studied. Our MD results predict that the anions show closest packing around cation(head) for all the mole-fractions studied. The latter is subsequently surrounded by the methanol molecules. Also, each methanol molecule preferentially interacts with anions via strong interaction of


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its hydroxyl group and anion. Decomposition of total S(q) into its partial components reveals that the low q peak below 0.6 Å−1 is due to cation(head)-anion-methanol composites that are separated by nonpolar tails of the cation. This peak finally converges to prepeak for neat IL and corresponds to polarapolar ordering. The principal peak above 1.2 Å−1 is mainly because of cation-cation, cation-methanol, and methanolmethanol short-distance correlations. At very low IL molefraction, cation(head)-anion pairs are widely separated by the methanol molecules and polar interaction between methanol molecules dominates. As the IL mole-fraction is increased, the separation between cation(head)-anion pairs due to methanol gradually decreases and renders the intermediate peak to shift towards higher q values. The intermediate peak finally resembles to charge ordering/alternation in neat ionic liquid. Beyond x IL = 0.1, the polar interaction between methanol molecules weakens and strong ion-ion, methanol-ion, and tailtail interactions play vital role in defining the structure of the mixture. Our results also indicate that on increasing IL concentration, the degree of ion-pair aggregation increases. This realization corroborates well with very recent study done by Heitz and coworkers67 where they have observed increased excited state lifetime of a fluorescent probe with increasing IL mole-fraction. It would be interesting to explore the role of IL structure on Stokes shift dynamics as a semi-molecular theory predicts nearest neighbor correlations are of secondary importance for solvation energy relaxation in neat ILs and their binary mixtures with common dipolar solvents.106,107

ACKNOWLEDGMENTS

We sincerely thank the anonymous reviewers for constructive comments. H.K.K. thanks Professor Ranjit Biswas, Professor Claudio J. Margulis, and Professor Edward W. Castner, Jr. for helpful discussion and feedback. S.S. would like to thank CSIR-UGC, India for fellowship. We thank CSC, IIT Delhi for providing the HPC cluster facility. Financial support from the Department of Science and Technology (DST), India is gratefully acknowledged. 1T.

Welton, Chem. Rev. 99, 2071 (1999). Liquids in Synthesis, 2nd ed., edited by P. Wasserscheid and T. Welton (Wiley-VCH Verlag GmbH & Co. KGaA, 2008). 3W. Lu, A. G. Fadeev, B. Qi, E. Smela, B. R. Mattes, J. Ding, G. M. Spinks, J. Mazurkiewicz, D. Zhou, G. G. Wallace, D. R. MacFarlane, S. A. Forsyth, and M. Forsyth, Science 297, 983 (2002). 4D. R. MacFarlane, M. Forsyth, P. C. Howlett, J. M. Pringle, J. Sun, G. Annat, W. Neil, and E. I. Izgorodina, Acc. Chem. Res. 40, 1165 (2007). 5M. Armand, F. Endres, D. R. MacFarlane, H. Ohno, and B. Scrosati, Nat. Mater. 8, 621 (2009). 6H. Ohno, Electrochemical Aspects of Ionic Liquids (Wiley-Interscience, 2005), p. 408. 7T. Tsuda and C. L. Hussey, Electrochem. Soc. Interface (USA) 16, 42 (2007). 8J. N. Canongia Lopes and A. A. H. Pádua, J. Phys. Chem. B 110, 3330 (2006). 9A. Triolo, O. Russina, H.-J. Bleif, and E. Di Cola, J. Phys. Chem. B 111, 4641 (2007). 10M. Gomes, J. N. Canongia Lopes, and A. A. H. Pádua, Top. Curr. Chem. 290, 161 (2010). 11J. F. Wishart and E. W. Castner, Jr., J. Phys. Chem. B 111, 4639 (2007). 12E. W. Castner, Jr. and J. F. Wishart, J. Chem. Phys. 132, 120901 (2010). 2Ionic

J. Chem. Phys. 142, 134503 (2015) 13E.

W. Castner, Jr., C. J. Margulis, M. Maroncelli, and J. F. Wishart, Annu. Rev. Phys. Chem. 62, 85 (2011). Russina and A. Triolo, Faraday Discuss. 154, 97 (2012). 15O. Russina, A. Triolo, L. Gontrani, and R. Caminiti, J. Phys. Chem. Lett. 3, 27 (2012). 16H. K. Kashyap, C. S. Santos, H. V. R. Annapureddy, N. S. Murthy, C. J. Margulis, and E. W. Castner, Jr., Faraday Discuss. 154, 133 (2012). 17H. K. Kashyap and C. J. Margulis, ECS Trans. 50, 301 (2013). 18H. K. Kashyap, C. S. Santos, R. P. Daly, J. J. Hettige, N. S. Murthy, H. Shirota, E. W. Castner, and C. J. Margulis, J. Phys. Chem. B 117, 1130 (2013). 19F. Castiglione, M. Moreno, G. Raos, A. Famulari, A. Mele, G. B. Appetecchi, and S. Passerini, J. Phys. Chem. B 113, 10750 (2009). 20H. Y. Lee, H. Shirota, and E. W. Castner, J. Phys. Chem. Lett. 4, 1477 (2013). 21A. E. Bradley, C. Hardacre, J. D. Holbrey, S. Johnston, S. E. J. McMath, and M. Nieuwenhuyzen, Chem. Mater. 14, 629 (2002). 22A. Triolo, A. Mandanici, O. Russina, V. Rodriguez-Mora, M. Cutroni, C. Hardacre, M. Nieuwenhuyzen, H.-J. Bleif, L. Keller, and M. A. Ramos, J. Phys. Chem. B 110, 21357 (2006). 23A. Triolo, O. Russina, B. Fazio, R. Triolo, and E. DiCola, Chem. Phys. Lett. 457, 362 (2008). 24R. Atkin and G. G. Warr, J. Phys. Chem. B 112, 4164 (2008). 25S. Fukuda, M. Takeuchi, K. Fujii, R. Kanzaki, T. Takamuku, K. Chiba, H. Yamamoto, Y. Umebayashi, and S.-i. Ishiguro, J. Mol. Liq. 143, 2 (2008). 26K. Fujii, S. Seki, S. Fukuda, T. Takamuku, S. Kohara, Y. Kameda, Y. Umebayashi, and S.-i. Ishiguro, J. Mol. Liq. 143, 64 (2008). 27D. Xiao, L. G. Hines, Jr., S. Li, R. A. Bartsch, E. L. Quitevis, O. Russina, and A. Triolo, J. Phys. Chem. B 113, 6426 (2009). 28A. Triolo, O. Russina, B. Fazio, G. B. Appetecchi, M. Carewska, and S. Passerini, J. Chem. Phys. 130, 164521 (2009). 29O. Russina, A. Triolo, L. Gontrani, R. Caminiti, D. Xiao, L. G. Hines, Jr., R. A. Bartsch, E. L. Quitevis, N. Pleckhova, and K. R. Seddon, J. Phys.: Condens. Matter 21, 424121 (2009). 30L. Gontrani, O. Russina, F. L. Celso, R. Caminiti, G. Annat, and A. Triolo, J. Phys. Chem. B 113, 9235 (2009). 31T. Pott and P. Méléard, Phys. Chem. Chem. Phys. 11, 5469 (2009). 32K. Fujii, T. Mitsugi, T. Takamuku, T. Yanaguchi, Y. Umebayashi, and S. Ishiguro, Chem. Lett. 38, 340 (2009). 33W. Zheng, A. Mohammed, L. G. Hines, D. Xiao, O. J. Martinez, R. A. Bartsch, S. L. Simon, O. Russina, A. Triolo, and E. L. Quitevis, J. Phys. Chem. B 115, 6572 (2011). 34C. S. Santos, H. V. R. Annapureddy, N. S. Murthy, H. K. Kashyap, E. W. Castner, Jr., and C. J. Margulis, J. Chem. Phys. 134, 064501 (2011). 35C. S. Santos, N. S. Murthy, G. A. Baker, and E. W. Castner, Jr., J. Chem. Phys. 134, 121101 (2011). 36B. Aoun, A. Goldbach, M. A. Gonzalez, S. Kohara, D. L. Price, and M.-L. Saboungi, J. Chem. Phys. 134, 104509 (2011). 37K. Fujii, R. Kanzaki, T. Takamuku, Y. Kameda, S. Kohara, M. Kanakubo, M. Shibayama, S. ichi Ishiguro, and Y. Umebayashi, J. Chem. Phys. 135, 244502 (2011). 38R. Hayes, S. Imberti, G. G. Warr, and R. Atkin, Phys. Chem. Chem. Phys. 13, 13544 (2011). 39Y. Umebayashi, H. Hamano, S. Seki, B. Minofar, K. Fujii, K. Hayamizu, S. Tsuzuki, Y. Kameda, S. Kohara, and M. Watanabe, J. Phys. Chem. B 115, 12179 (2011). 40S. Li, J. L. Banuelos, J. Guo, L. Anovitz, G. Rother, R. W. Shaw, P. C. Hillesheim, S. Dai, G. A. Baker, and P. T. Cummings, J. Phys. Chem. Lett. 3, 125 (2012). 41A. Triolo, O. Russina, R. Caminiti, H. Shirota, H. Y. Lee, C. S. Santos, N. S. Murthy, and E. W. Castner, Jr., Chem. Commun. 48, 4959 (2012). 42A. Triolo, O. Russina, V. Arrighi, F. Juranyi, S. Janssen, and C. Gordon, J. Chem. Phys. 119, 8549 (2003). 43C. Hardacre, J. D. Holbrey, M. Nieuwenhuyzen, and T. G. A. Youngs, Acc. Chem. Res. 40, 1146 (2007). 44C. Hardacre, J. D. Holbrey, C. L. Mullan, T. G. A. Youngs, and D. T. Bowron, J. Chem. Phys. 133, 74510 (2010). 45S. M. Urahata and M. C. Ribeiro, J. Chem. Phys. 120, 1855 (2004). 46M. G. Del Popolo and G. A. Voth, J. Phys. Chem. B 108, 1744 (2004). 47Y. Wang and G. A. Voth, J. Am. Chem. Soc. 127, 12192 (2005). 48Y. Wang and G. A. Voth, J. Phys. Chem. B 110, 18601 (2006). 49O. Borodin and G. D. Smith, J. Phys. Chem. B 110, 11481 (2006). 50B. L. Bhargava, R. Devane, M. L. Klein, and S. Balasubramanian, Soft Matter 3, 1395 (2007). 51B. Bhargava, M. Klein, and S. Balasubramanian, ChemPhysChem 9, 67 (2008).

14O.


134503-10 52J.


N. Canongia Lopes, K. Shimizu, A. A. H. Pádua, Y. Umebayashi, S. Fukuda, K. Fujii, and S.-i. Ishiguro, J. Phys. Chem. B 112, 1465 (2008). 53H. V. R. Annapureddy, H. K. Kashyap, P. M. De Biase, and C. J. Margulis, J. Phys. Chem. B 114, 16838 (2010). 54L. J. A. Siqueira and M. C. C. Ribeiro, J. Chem. Phys. 135, 204506 (2011). 55H. K. Kashyap, C. S. Santos, N. S. Murthy, J. J. Hettige, K. Kerr, S. Ramati, J. Gwon, M. Gohdo, S. I. Lall-Ramnarine, J. F. Wishart, C. J. Margulis, and E. W. Castner, J. Phys. Chem. B 117, 15328 (2013). 56K. Shimizu, C. E. S. Bernardes, and J. N. Canongia Lopes, J. Phys. Chem. B 118, 567 (2014). 57Y. Jiang, H. Nadolny, S. Kashammer, S. Weibels, W. Schroer, and H. Weingartner, Faraday Discuss. 154, 391 (2012). 58V. V. Chaban, I. V. Voroshylova, O. N. Kalugin, and O. V. Prezhdo, J. Phys. Chem. B 116, 7719 (2012). 59V. V. Chaban and O. V. Prezhdo, J. Phys. Chem. Lett. 2, 2499 (2011). 60T. Koddermann, S. Klembt, D. Klasen, D. Paschek, U. Kragl, and R. Ludwig, ChemPhysChem 13, 1748 (2012). 61R. Jan, G. Rather, and M. Bhat, J. Solution Chem. 43, 685 (2014). 62X.-X. Zhang, M. Liang, J. Hunger, R. Buchner, and M. Maroncelli, J. Phys. Chem. B 117, 15356 (2013). 63M. Liang, X.-X. Zhang, A. Kaintz, N. P. Ernsting, and M. Maroncelli, J. Phys. Chem. B 118, 1340 (2014). 64T. Pal and R. Biswas, J. Chem. Phys. 141, 164502 (2014). 65S. Daschakraborty and B. Ranjit, J. Phys. Chem. B 115, 4011 (2011). 66S. Daschakraborty and R. Biswas, J. Phys. Chem. B 118, 1327 (2014). 67K. M. Barra, R. P. Sabatini, Z. P. McAtee, and M. P. Heitz, J. Phys. Chem. B 118, 12979 (2014). 68C. Roth, A. Appelhagen, N. Jobst, and R. Ludwig, ChemPhysChem 13, 1708 (2012). 69F. Bardak, D. Xiao, L. G. Hines, P. Son, R. A. Bartsch, E. L. Quitevis, P. Yang, and G. A. Voth, ChemPhysChem 13, 1687 (2012). 70L. Cammarata, S. G. Kazarian, P. A. Salter, and T. Welton, Phys. Chem. Chem. Phys. 3, 5192 (2001). 71M. Moreno, F. Castiglione, A. Mele, C. Pasqui, and G. Raos, J. Phys. Chem. B 112, 7826 (2008). 72S. Feng and G. A. Voth, Fluid Phase Equilib. 294, 148 (2010). 73C. Schroeder, T. Rudas, G. Neumayr, S. Benkner, and O. Steinhauser, J. Chem. Phys. 127, 234503 (2007). 74H. Shirota and R. Biswas, J. Phys. Chem. B 116, 13765 (2012). 75O. Russina, B. Fazio, G. Di Marco, R. Caminiti, and A. Triolo, The Structure of Ionic Liquids, Soft and Biological Matter, edited by R. Caminiti and L. Gontrani (Springer International Publishing, 2014), pp. 39–61. 76O. Russina, A. Sferrazza, R. Caminiti, and A. Triolo, J. Phys. Chem. Lett. 5, 1738 (2014). 77K. J. Fraser and D. R. MacFarlane, Aust. J. Chem. 62, 309 (2009). 78K. J. Fraser, E. I. Izgorodina, M. Forsyth, J. L. Scott, and D. R. MacFarlane, Chem. Commun. 2007, 3817. 79J. Huang and T. Ruether, Aust. J. Chem. 62, 298 (2009).

J. Chem. Phys. 142, 134503 (2015) 80P.

J. Carvalho, V. H. Alvarez, I. M. Marrucho, M. Aznar, and J. A. P. Coutinho, J. Supercrit. Fluids 52, 258 (2010). 81L. Pison, J. N. Canongia Lopes, L. P. N. Rebelo, A. A. H. Pádua, and M. F. Costa Gomes, J. Phys. Chem. B 112, 12394 (2008). 82D. R. MacFarlane, M. Forsyth, E. I. Izgorodina, A. P. Abbott, G. Annat, and K. Fraser, Phys. Chem. Chem. Phys. 11, 4962 (2009). 83H. Jin, G. Baker, S. Arzhantsev, J. Dong, and M. Maroncelli, J. Phys. Chem. B 117, 7291 (2007). 84N. Ito, S. Arzhantsev, M. Heitz, and M. Maroncelli, J. Phys. Chem. B 108, 5771 (2004). 85X. Liu, G. Zhou, S. Zhang, and G. Yu, Mol. Simul. 36, 79 (2010). 86B. Hess, C. Kutzner, D. van der Spoel, and E. Lindahl, J. Chem. Theory Comput. 4, 435 (2008). 87D. van der Spoel, E. Lindahl, B. Hess, G. Groenhof, A. E. Mark, and H. J. C. Berendsen, J. Comput. Chem. 26, 1701 (2005). 88See supplementary material at http://dx.doi.org/10.1063/1.4916308 for information regarding tables for simulated bulk densities, excess molar enthalpies, number of constituent ions and molecules at each molefraction, and supporting figures. 89W. L. Jorgensen, D. S. Maxwell, and J. Tirado-Rives, J. Am. Chem. Soc. 118, 11225 (1996). 90G. Kaminski and W. L. Jorgensen, J. Phys. Chem. 100, 18010 (1996). 91R. C. Rizzo and W. L. Jorgensen, J. Am. Chem. Soc. 121, 4827 (1999). 92M. L. P. Price, D. Ostrovsky, and W. L. Jorgensen, J. Comput. Chem. 22, 1340 (2001). 93J. N. Canongia Lopes and A. A. H. Pádua, J. Phys. Chem. B 108, 16893 (2004). 94J. N. Canongia Lopes and A. A. H. Pádua, J. Phys. Chem. B 110, 19586 (2006). 95H. K. Kashyap, J. J. Hettige, H. V. R. Annapureddy, and C. J. Margulis, Chem. Commun. 48, 5103 (2012). 96T. Darden, D. York, and L. Pedersen, J. Chem. Phys. 98, 10089 (1993). 97U. Essmann, L. Perera, M. L. Berkowitz, T. Darden, H. Lee, and L. G. Pedersen, J. Chem. Phys. 103, 8577 (1995). 98S. Nose, J. Chem. Phys. 81, 511 (1984). 99S. Nose, Mol. Phys. 52, 255 (1984). 100W. G. Hoover, Phys. Rev. A 31, 1695 (1985). 101M. Parrinello and A. Rahman, J. Appl. Phys. 52, 7182 (1981). 102International Tables for Crystallography Vol. C, edited by E. Prince (International Union of Crystallography, 2006). 103E. Lorch, J. Phys. C: Solid State Phys. 2, 229 (1969). 104J. Du, C. J. Benmore, R. Corrales, R. T. Hart, and J. K. R. Weber, J. Phys.: Condens. Matter 21, 205102 (2009). 105E. Bodo, L. Gontrani, R. Caminiti, N. V. Plechkova, K. R. Seddon, and A. Triolo, J. Phys. Chem. B 114, 16398 (2010). 106S. Daschakraborty and R. Biswas, J. Chem. Phys. 137, 114501 (2012). 107S. Daschakraborty, T. Pal, and R. Biswas, J. Chem. Phys. 139, 164503 (2013).


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