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Structural Properties and Aggregation Behavior of 1‑Hexyl-3methylimidazolium Iodide in Aqueous Solutions Paola D’Angelo,*,† Alessandra Serva,† Giuliana Aquilanti,‡ Sakura Pascarelli,§ and Valentina Migliorati† †

Dipartimento di Chimica, Università di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy Elettra-Sincrotrone Trieste S.C.p.A s.s., 14, km 163.5, I-34149 Basovizza, Trieste, Italy § European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France ‡

S Supporting Information *

ABSTRACT: The structural properties of 1-hexyl-3-methylimidazolium iodide ([C6mim]I)/water mixtures with molar ratios ranging from 1:1 to 1:200 have been investigated using molecular dynamics (MD) simulations with extended X-ray absorption fine structure (EXAFS) experimental data. The presence of a complex network of interactions among cations, anions, and water molecules has been highlighted from the MD simulations, even if water molecules have been found to interact preferentially with the I− anion. The EXAFS results show that, also for the 1:1 [C6mim]I/water mixture, the water molecules are placed next to the I− anion, and the I− hydration shell becomes more and more crowded with increasing water content. Tight ion pairs have been detected in the [C6mim]I/water mixtures with molar ratios from 1:1 to 1:12, while no ionic pairs were found in the most diluted solutions. The aggregation behavior has been determined from MD simulations with the aid of S(q) functions. For the most concentrated IL/water mixtures with molar ratios between 1:1 and 1:12 the existence of long-range structural correlations has been evidenced, even if the apolar chains are not completely segregated as expected for micelle-like structures. Conversely, for the 1:200 mixture, that is above the experimental critical aggregation concentration value, the alkyl chains are completely separated from each other.

1. INTRODUCTION Among the most exciting and successful materials developed and studied in the last 20 years, ionic liquids (ILs) are part of those that can certainly claim one of the richest fields of applications in industry and applied technologies.1−4 Indeed, even today, ILs remain a growing research area in academia because of their unconventional properties that defy a simple description under many points of view. Recently, a large plethora of experimental techniques have been applied to the study of ILs, and correspondingly, many peculiar and essentially unique features have shown up, thus explaining both the wide range of different applications of these materials and the call for further exploration.5−12 ILs are usually built up by a bulky, asymmetric cation such as 1-alkyl-3-methylimidazolium ([Cnmim]+ where n is the number of carbon atoms in the alkyl chain) and an inorganic anion that can be as simple as chloride, bromide, and iodide, or have a more complex structure. The versatility of ILs depends on the fact that their properties can be tuned by varying the molecular structure of the cation and of the anion. Among iodide containing ILs, [C4mim]I is the best characterized one,5,10,13−15 while a detailed molecular level understanding of the other members of the series is still missing. Recently, alkylimidazolium iodides have been considered as necessary components of electrolytes to obtain good performance of dye-sensitized solar cells (DSSCs).16−20 The efficiency and stability of DSSCs are very much dependent on © XXXX American Chemical Society

the properties of the electrolyte employed in the cells, and use of volatile organic solvents hampers their use in outdoor applications.21−23 The main advantages of the ILs for electrochemical devices are the nonvolatility, nonflammability, and high ionic strength. In particular, 1-hexyl-3-methylimidazolium iodide [C6mim]I shows the best performance among the 1-alkyl-3-methylimidazolium iodides (C3−C9).18,19 Many industrial applications require a mixture of ILs with water or organic solvents. The physical properties of ILs such as their viscosity, conductivity, and polarity are strongly affected by the presence of molecular liquids,24−27 and mixtures of ILs and organic solvents have been shown to display very promising performances in different fields such as lithium-ion battery technology and extraction of sulfur aromatic compounds from fuels.28,29 Therefore, the knowledge of the behavior of ILs in binary liquid mixtures is important both from fundamental and applied aspects. Several investigations are present in the literature aiming at providing a better characterization of binary mixtures of ILs and organic solvents. These studies range from the determination of properties such as viscosity, conductivity, heat capacity, speeds of sound, and refractive indices, to a microscopic characterization of these systems by means of molecular dynamics (MD) simulations, Received: September 7, 2015 Revised: October 18, 2015

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DOI: 10.1021/acs.jpcb.5b08739 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B and NMR, infrared, and Raman spectroscopy. 30−37 In particular, infrared and Raman spectroscopies have been used to study intermolecular interactions existing between ILs and organic molecules, providing information on the variation of the structural organization of ILs in these solutions as a function of the IL polarity.36,37 Moreover, several investigations have been devoted to the characterization of the IL/water mixtures, and it is now accepted that the ILs based on imidazolium cations and halide anions undergo aggregation beyond a critical concentration called critical aggregation concentration (CAC).38−47 1H NMR and fluorescence measurements were carried out on [Cnmim]+ ILs, and it was concluded that the short alkyl chain ILs (n ≤ 4) behave as simple salts, with transitional ones (n = 6) forming only a monolayer with no detectable aggregates, while the long alkyl chain ones (n ≥ 8) form aggregates in aqueous solutions.48 [C4mim]Br has been widely studied in aqueous solution, and while no aggregation has been detected for this IL, the existence of tight ion pairs has been evidenced.49−53 On the contrary it has been observed that [C8mim]Cl in water solution forms aggregates that are very different from the micelles of ionic surfactants at concentrations greater than CAC.54 Despite these results, detailed theoretical and experimental studies are further needed to shed light on the structural and aggregation properties of IL/water mixture also using innovative experimental approaches. In this context recently X-ray absorption spectroscopy (XAS) in combination with molecular dynamics (MD) simulations has been found to be an effective tool to provide detailed structural information on ILs both as pure liquids and in water mixtures.52,53,55−58 The potentiality of combining MD and EXAFS is now wellrecognized, and very accurate structural information can be obtained for disordered systems that are difficult to gain with different approaches.52,53,55,58−61 Moreover, the agreement between the experimental data and the MD results allows one to be confident on the theoretical framework used in the simulations. This is an important issue when using classical MD simulations to determine the structural properties of ILs due to their ionic character and high viscosity. Here, we will investigate the structural properties of [C6mim]I/water mixtures as a function of water concentration up to very high dilution. The integrated MD-XAS approach allowed us to have a global structural picture of the complex molecular organization of [C6mim]I/water mixtures both in proximity of the I− ion and in the longer distance range. The combined use of MD and XAS has been recently adopted to investigate the structural and dynamic properties of [C4mim]Br/water mixtures as a function of water concentration.52,53 The existence of a complex network of interactions among cations, anions, and water molecules has been highlighted, even if water molecules have been found to interact preferentially with the Br− anion. Moreover, the existence of tight ion pairs has been evidenced, even in the presence of a great excess of water.

Table 1. Composition of the [C6mim]I/Water Mixtures under Investigation Together with the Box Edge Length of the Simulated Systems IL/water molar ratio

ion pairs

water molecules

simulation box edge (Å)

1:1 1:3 1:6 1:12 1:70 1:200

343 343 216 216 64 27

343 1029 1296 2592 4480 5400

52.93 53.88 49.19 53.84 53.96 55.57

The Lopes and Padua force field63,64 was used for [C6mim]I, while the SPC/E65 one was used for water. The choice of the SPC/E model resides on the fact that it appropriately reproduces the structural and dynamic behavior of liquid water.66 The Lorentz−Berthelot combining rules were used to calculate the Lennard-Jones parameters for all of the different atoms, with the exception of those relating to the I−−water interaction. In this case the Lennard-Jones potential developed by Reif et al. has been used,67 as in a combined MD-EXAFS (extended X-ray absorption fine structure) investigation this interaction potential has been found to provide a very good description of the structural properties of halide aqueous solutions.68 An IL monomer, formed by a [C6mim]+ cation (see Figure 1) and an I− anion, was replicated in three dimensions, and

Figure 1. Molecular structure of 1-hexyl-3-methylimidazolium cation ([C6mim]+). Atom labeling, to which we refer in this Article, is shown. CR is the carbon atom of the imidazolium ring placed between the two nitrogen atoms, and CWs are the two ring carbon atoms bonded to each other and to a nitrogen atom. C1 refers to the first carbon atom in the side chains, and C2 is the second atom in hexyl side chain; CS refers to any secondary carbon of the hexyl side chain that is removed at least two bonds from the ring, and CT is the terminal carbon atom of the hexyl chain.

water molecules were added randomly to generate a very large cubic simulation box. The number of IL monomers and water molecules used in the six simulations are reported in Table 1. These initial configurations were left to evolve in a short NPT run subjected to a very high pressure (1000 atm). Then, the systems have been equilibrated under NPT conditions at 1 atm pressure for 1 ns to obtain an almost constant density. The edge length of the cubic boxes, reproducing those densities, are given in Table 1. The six systems have been then simulated in the canonical ensemble (NVT), using the Nosé−Hoover thermostat69,70 with a relaxation constant of 0.5 ps. A first equilibration run was carried out at 300 K for 3 ns for each simulation, followed by 6 ns of production runs. A time step of 1 fs was used, and a configuration was saved every 200 timesteps. Nonbonded interactions were calculated within a cutoff of 12 Å, and the Ewald summation method71 was used to deal with long-range electrostatic effects. The SHAKE algorithm was employed to constrain the stretching interactions involving hydrogen atoms. In order to describe the structural properties of the [C6mim]I/water mixtures, we calculated the radial distribution functions, gA−B(r):

2. METHODS 2.1. Molecular Dynamics Simulations. Classical MD was employed to study the [C6mim]I/water mixtures with six different water concentrations, as reported in Table 1. The simulations have been carried out at room temperature (300 K) and atmospheric pressure using the DL_POLY package,62 putting the ions and water molecules in a cubic box and applying periodic boundary conditions in three dimensions. B


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The Journal of Physical Chemistry B gA − B(r ) =

⟨ρB (r )⟩

1 = NAρBave

ρBave

NA

NB

∑∑ i∈A i∈B

EXAFS theoretical signals have been calculated starting from the MD results. In particular, the total χ(k) has been modeled as a function of the radial distribution function g(r) as:

δ(rij − r ) 4πr

2

(1)

Here ⟨ρB(r)⟩ is the particle density of type B at distance r around type A, and ρave B is the average number density of type B particles. The three-dimensional organization of the IL ions and water molecules has also been investigated by computing the spatial distribution functions. All the analyses have been performed using the TRAVIS software.72 Moreover, in order to characterize the long-range organization of the [C6mim]I/water mixtures, we have calculated the X-ray theoretical structure factors, S(q), from the MD trajectories, through the following equation:73−75 N

S(q) =

χ (k ) =

N

N

(2)

Here xi and xj represent the fractions of atomic species i and j. The equation also includes the terms q, f i(q), f j(q), and Hij(q), that are the momentum transfer, the atomic X-ray scattering factors of species i and j, and the partial structure factors, respectively. Hij(q) is defined in terms of the radial distribution functions of species i and j, gij(r), by the Fourier integral: Hij(q) = 4πρ0

∫0

rmax

r 2(gij(r ) − 1)

sin(qr ) dr qr

∞

dr 4πρr 2g (r )A(k , r )sin[2kr + ϕ(k , r )]

(4)

where A(k, r) and ϕ(k, r) are the amplitude and phase functions, respectively, and ρ is the density of the scattering atoms. I-O and I-HW χ(k) theoretical signals are calculated by introducing the MD g(r)’s in eq 4, and the validity of the theoretical framework is assessed by comparison with the experimental data. Note that while the hydrogen atoms have been found to provide a detectable contribution to the total χ(k) signal, the scattering signals associated with the carbon, nitrogen, and hydrogen atoms of the imidazolium cation have been found to have a negligible amplitude. The experimental χ(k) signals have been extracted from the raw data using an atomic background modeled by step-shaped functions accounting for multielectron resonances.77 During the minimization procedure only two nonstructural parameters, namely, the ionization threshold energy E0 and S20, are optimized.

∑i = 1 ∑ j = 1 xixjfi (q)f j (q)Hij(q) [∑i = 1 xifi (q)]2

∫0

3. RESULTS AND DISCUSSION 3.1. Anion−Water Interactions. 3.1.1. MD Results. Figure 2 shows the I-O and I-HW g(r)’s obtained from the MD trajectories of the [C6mim]I/water mixtures, where O and HW refer to the oxygen and hydrogen atoms of the water molecules, respectively. In all the simulated systems, a sharp first peak can be observed, showing the presence of a preferential interaction of the water molecules with the I− anions. Note that also for the most diluted mixture with a [C6mim]I/water ratio 1:1 the I-O

(3)

Here ρ0 is the numerical density of the system, and rmax is the integration cutoff, equal to half of the box edge length. The structure factors have been calculated using in-house written codes. 2.2. X-Absorption Measurements. [C 6 mim]I was purchased from Iolitec GmbH with a stated purity of >99%. The powder was dried under vacuum for 48 h, and a final water content of 200 ppm was measured by Karl Fischer tritation. [C6mim]I/water mixtures with different IL/water molar ratios (1:1, 1:3, 1:6, 1:12, 1:70, 1:200) were prepared by adding the proper amount of [C6mim]I to bidistilled water. A 0.1 M aqueous solution of tetramethylammonium iodide (TMAI) was also prepared. The I K-edge XAS spectra of the solutions were collected in transmission mode at RT at the BM23 beamline of the European Synchrotron Radiation Facility (ESRF). The samples were kept in cells with Kapton windows and Teflon spacers of 1.5 cm. Also, the XAS spectrum of pure [C6mim]I was collected. To avoid contact with water the data acquisition was carried out keeping the cell under nitrogen flux. The monochromator was equipped with two flat Si(311) crystals that were kept slightly detuned with a feedback system in order to reduce harmonic contamination. The incident and transmitted fluxes were monitored by ionization chambers filled with Kr gas. The storage ring was operating in 2/3 fill mode with a typical current of 200 mA after refill. 2.3. EXAFS Data Analysis. The analysis of the EXAFS data was carried out using the GNXAS code, and a thorough description of the theoretical framework can be found in ref 76. Selected MD configurations have been used to calculate the phase shifts, A(k, r) and ϕ(k, r), using muffin-tin potentials and advanced models for the exchange-correlation self-energy (Hedin−Lundqvist). The MT radius values are 0.2, 0.9, and 2.3 Å for hydrogen, oxygen, and iodine atoms, respectively.

Figure 2. Radial distribution functions, g(r)’s, calculated from the MD simulations of the [C6mim]I/water mixtures with molar ratio of 1:1 (black line), 1:3 (red line), 1:6 (green line), 1:12 (blue line), 1:70 (brown line), and 1:200 (cyan line). In the upper panel (A), the I-O g(r)’s are reported; in the lower panel (B), the I-HW g(r)’s are reported. C


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The Journal of Physical Chemistry B g(r) shows a well-defined peak, thus meaning that the single water molecule per ionic pair interacts with the I− anion. Moreover, all of the I-HW g(r)’s show two well-defined peaks, the former at shorter and the latter at longer distances as compared with the I-O g(r). This indicates that one of the hydrogen atoms of the water molecules points toward the I− anion, while the second hydrogen atom interacts with other species of the system. A better understanding of the dilution effect on the I-water g(r)’s can be gained by considering the radial distribution functions multiplied by the numerical density ρ of the oxygen or hydrogen atoms in the solutions. In the case of a strong density variation such as that present in our systems, comparison of the g(r)’s can be misleading as the peak intensity is not related to the number of selected atoms at a given distance, but it depends mainly on the density. Therefore, in order to appreciate the variation of the first shell coordination number, the g(r)ρ functions are better suited as in this case the integral of the first peak is directly related to the number of atoms present in the first shell. The I-O and I-HW g(r)ρ functions are shown in Figure 3, and it can be easily seen

the I-O coordination number of the 1:1 mixture is 1.6, in this system the water molecules tend to interact simultaneously with two I− anions. The presence of water molecules that are shared between two I− anions is found also for the [C6mim]I/ water mixture with molar ratio of 1:3. Moreover, the I-O coordination number is very similar for the 1:70 and 1:200 solutions, suggesting that in both cases the ions are fully hydrated. This hypothesis can be confirmed by comparing the I-O g(r)ρ functions obtained for the 1:70 and 1:200 [C6mim]I/ water mixtures and the same function calculated from a MD simulation of I− ion in aqueous solution using the same interaction potentials as those adopted in this work. All details about the simulation of the I− aqueous solution can be found in ref 67. The comparison among the three g(r)ρ functions reported in Figure S1 of the Supporting Information shows that the first peaks of the I-O g(r)ρ calculated for the 1:70 and 1:200 solutions are almost identical to that of the I− aqueous solution. This result indicates that in a [C6mim]I/water mixture with molar ratio of 1:70 the water aggregation around anions reaches a saturation limit, and additional dilutions of the system do not cause significant variations of the I-O first shell coordination number. The I-O coordination numbers obtained in the present work for the 1:70 and 1:200 solutions are in line with the results of previous MD simulations of I− in water,78 while other MD investigations reported lower values.79 In this context, it is important to stress that the results reported in the literature on iodide hydration properties are inhomogeneous and I−O coordination numbers are significantly scattered (4.2−10.3).80 This is due to the complexity of the halide hydration phenomenon and to the difficulty of defining iodide coordination shells also as a consequence of the fast water exchange between the first and second hydration spheres. However, the coordination numbers we found for the highest water concentrations are within the range of different results published in the literature. We have also analyzed the variation of the I-O coordination number during the MD trajectories, by computing the instantaneous coordination number (n), as the number of oxygen atoms within the first minimum in the I-O g(r)’s. Figure S2 of the Supporting Information displays the results of this analysis: in the IL/water mixture with molar ratio of 1:1 n fluctuates in the range 0−4 with a maximum probability of 2. Then, going from low to high diluted conditions, the most probable configurations are shifted to larger values of n, until reaching the value of 7 for the mixtures with molar ratio 1:70 and 1:200. For a pictorial view of the I−−water interactions, representative simulation snapshots obtained from the MD trajectories are shown in Figure 4. For each investigated IL/ water mixture, a sample of I− ions and water molecules surrounding the anions is depicted. The water molecules have been selected within a cutoff distance that has been chosen as the first minimum of the I-O g(r). From this figure, it can be seen that the water molecules tend to interact preferentially with the I− anions also when the water content is very low, as in the case of the 1:1 mixture. Note that all of the selected I− ions are coordinated with water molecules, and water molecules acting as bridge between two anions can be detected. With increasing water content, the I− hydration shell becomes more and more populated, and for the 1:70 and 1:200 solutions each anion is fully hydrated. It is important to stress that the water molecules not only interact with the I− ions, but also with the imidazolium ring due

Figure 3. Radial distribution functions multiplied by the numerical densities, g(r)ρ, calculated from the MD simulations of the [C6mim]I/ water mixtures with molar ratio of 1:1 (black line), 1:3 (red line), 1:6 (green line), 1:12 (blue line), 1:70 (brown line), and 1:200 (cyan line). In the upper panel (A) the I-O g(r)ρ functions are reported, and in the lower panel (B) the I-HW g(r)ρ functions are reported.

that the I− first hydration shell becomes more and more populated with increasing water concentration. The first shell coordination numbers can be calculated by the integration of the first peak up to a cutoff distance that is usually defined as the position of the first minimum of the g(r)’s. The first maximum positions R and the coordination numbers N are listed in Table 2 for all of the investigated systems. Inspection of this table shows that the I− hydration number increases from 1.6 to 7.8 in going from the 1:1 to the 1:200 solutions. Since D


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Table 2. Structural Parameters of the Radial Distribution Functions, g(r)’s, Computed from the MD Simulations of the [C6mim]I/Water Mixtures with Different Molar Ratios: Position of the g(r) First Peak, R; Coordination Number, N; and Cutoff Distances Used in the Calculations of Na R (Å) I-O I-[C6mim]+ O-[C6mim]+ CT-O HCR-O CT-CT a

N

1:1

1:3

1:6

1:12

1:70

1:200

1:1

1:3

1:6

1:12

1:70

1:200

cutoff(Å)

3.52 5.31 4.48 3.73 2.61 4.01

3.52 5.35 4.48 3.73 2.61 4.02

3.52 5.39 4.48 3.73 2.64 4.05

3.52 5.45 4.53 3.80 2.65 4.05

3.52 5.51 4.53 3.80 2.67 4.07

3.52 5.51 4.53 3.80 2.67 4.07

1.6 4.0 2.0 0.7 0.91 4.4

3.7 3.4 4.6 2.9 2.2 4.1

5.2 2.6 8.2 4.4 3.3 3.6

6.4 1.7 11.4 7.4 4.4 3.3

7.7 0.3 14.5 16.3 5.5 0.6

7.8 0.1 15.1 18.0 5.8 0.2

4.2 6.4 5.3 5.5 4.2 7.0

Note that the cutoff distance has been chosen as the position of the first minimum of the g(r)’s.

Figure 4. Simulation snapshots of the investigated [C6mim]I/water mixtures, showing the first hydration shell of the oxygen atoms of water molecules (red) around the I− ion (yellow).

to the ability of the ring hydrogen atoms to form hydrogen bonds. The cation−water interactions will be discussed in section 3.2. 3.1.2. EXAFS results. The results of the MD simulations indicate that the water molecules interact with the iodide anions also in the presence of one water molecule per IL ionic pair. Moreover, the I− hydration shell becomes more and more crowded with decreasing [C6mim]I/water molar ratio. These findings can be verified by the XAS technique that is very sensitive to the local environment of the I− absorbing atom. The I K-edge EXAFS spectra of the [C6mim]I/water mixtures with IL/water molar ratio 1:1, 1:3, 1:6, 1:12, 1:70, and 1:200 are shown in the upper panel of Figure 5, together with the experimental data of pure [C6mim]I and I− in aqueous solution. The corresponding Fourier transforms (FTs) calculated in the interval k = 1.5−8.3 Å−1 are shown in the lower panel of the figure. The first observation that can be made is that the EXAFS spectrum of the 1:1 mixture is different from that of pure [C6mim]I. In particular the main oscillation has a higher frequency indicating that the first neighbor distance in pure [C6mim]I is longer as compared to that of the 1:1 water mixture. Moreover, the amplitude of the pure IL EXAFS spectrum is lower as compared to all of the solutions, thus meaning that the interaction existing between the I− ion and the surrounding atoms is weaker. This means that addition of only one water molecule per ion pair produces a clear modification of the EXAFS signal and, consequently, a variation of the local environment around the I− anion. In particular, the frequency of the EXAFS oscillations of the 1:1 mixture is

Figure 5. Top panel: EXAFS spectra of [C6mim]I/water mixtures with 1:1, 1:3, 1:6, 1:12, 1:70, and 1:200 molar ratios. The EXAFS spectra of pure [C6mim]I and of a 0.1 M aqueous solution of tetramethylammonium iodide providing I− ions in water are also shown. Bottom panel: non-phase-shifted corrected Fourier transforms of the experimental data.

different from that of pure [C6mim]I, and this is reflected on the position of the FT first peak. This result confirms the previous MD findings showing that water molecules tend to preferentially interact with the I− also for the lowest water concentration investigated in the present work. Similar results were previously found for [C4mim]Br/water solutions and other IL/water systems.45,52 Comparison of the EXAFS data of the IL/water systems highlights the existence of a trend of the amplitude of the spectra that increases at higher water concentrations, while the frequency of the signal remains the same. This finding indicates that the I− ion interacts mainly with water molecules at the same distance and the hydration shell becomes more crowded and structured when more water is added to the solution. Moreover, the EXAFS spectrum of the 1:200 mixture is almost identical to that of I− in aqueous solution, and this suggests that the iodide anion is fully hydrated in this system. All these findings are in perfect agreement and confirm the MD results reported above. However, a direct comparison between the MD and XAS data can be made to assess the accuracy of the theoretical E


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The Journal of Physical Chemistry B framework used in the simulations. To this end χ(k) theoretical signals have been calculated by introducing the I-O and I-HW g(r)’s in eq 4, for all of the mixtures under investigation. The comparison between the theoretical and experimental curves is shown in Figure 6. Note that the MD structural parameters

Figure 7. Radial distribution functions multiplied by the numerical densities, g(r)ρ’s, obtained from the MD simulations of [C6mim]I/ water mixtures with molar ratios of 1:1 (black line), 1:3 (red line), 1:6 (green line), 1:12 (blue line), 1:70 (brown line), and 1:200 (cyan line). The upper panel shows the anion−cation g(r)ρ’s, calculated between the geometrical center of [C6mim]+ cation and the I− ion. The lower panel shows the cation−O g(r)ρ’s, calculated between the geometrical center of [C6mim]+ and O, which is the oxygen atom of water molecules.

Figure 6. EXAFS experimental spectra (dotted red line) of [C6mim]I/ water mixtures with molar ratios of 1:1, 1:3, 1:6, 1:12, 1:70, and 1:200 compared with the theoretical signals (solid blue line) obtained from the MD I-O and I-HW g(r)’s. The experimental and theoretical EXAFS spectra of pure [C6mim]I and of a 0.1 M aqueous solution of tetramethylammonium iodide providing I− ions in water are also shown.

atoms of the imidazolium ring. Moreover, while in the less diluted mixtures up to a [C6mim]I/water ratio of 1:12, all of the ions exist as ionic pairs; for the last two solutions (1:70 and 1:200), only a small percentage of cations still form pairs with the I− anion. A different result was obtained in a recent study of [C4mim]Br/water mixtures where the presence of ionic pairs was detected also in more concentrated water in IL solutions.52 Most probably the relative polarizability of halide ions plays a key role on the nature of interactions existing in these IL/water mixtures. It is rather surprising that, in the most diluted mixture, although the iodide ion is surrounded on average by 7.8 water molecules, the cation−anion first shell distance does not significantly increase as compared to the 1:1 solution. This is because in the 1:200 solution a complex network of interactions takes place, in which one of the water molecules belonging to the iodide first hydration shell can simultaneously interact with the HCR atom of the cation. In other words, one water molecule can act as a bridge between the cation and the anion. In this configuration, the bridge water molecule places itself in a tilted arrangement that allows the counterions to remain in proximity of each other. A representative snapshot extracted from the 1:200 MD trajectory showing this cation−water− anion configuration is depicted in Figure S4 of Supporting Information. Figure S5 of the Supporting Information shows the instantaneous I-cation coordination numbers calculated from the MD trajectories by counting the number of I− ions around the [C6mim]+ cation at a distance shorter than the I-cation g(r) first minimum. In the mixtures with molar ratio of 1:70 and

have been kept fixed during the analysis and the only optimized parameters were S20 and E0, that were found equal to 0.9 and 3 eV above the first inflection point of the spectra, respectively, for all of the investigated systems. Least-squares fits of the EXAFS data have been carried out in the range k = 2.0−9.1 Å−1. In all cases the agreement between the experimental and theoretical EXAFS spectra is very good, proving the reliability of our results and the validity of the force field used in the MD simulations 3.2. Cation−Anion and Cation−Water Interactions. Although the iodide ion preferentially interacts with the water molecules present in the mixtures, an interaction between I− anions and imidazolium cations also takes place, as shown by the g(r)ρ’s depicted in the upper panel of Figure 7 and by the coordination number values N reported in Table 2. The g(r)ρ’s have been calculated between the I− anion and the geometrical ring center of the [C6mim]+ cation (the corresponding g(r)’s are shown in Figure S3 of the Supporting Information), while N has been computed by integration of the g(r)’s up to the first minimum. For the mixtures with low water content, the I-cation g(r)ρ’s show a distinct first shell peak suggesting the presence of strong interactions between the iodide species and the imidazolium ring. Conversely, the I-cation g(r)ρ’s of the 1:70 and 1:200 solutions are broad and featureless, and the coordination numbers become almost zero (0.3 and 0.1 for the 1:70 and 1:200 mixtures, respectively, see Table 2). Together, these results indicate that at this high water concentration the I− ions no longer interact with the hydrogen F


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The Journal of Physical Chemistry B 1:200 the most probable configuration is a zero-coordinated species, while for the other mixtures the preferential coordination number goes from 2 to 4 with decreasing water content. As expected, the [C6mim]+ cation not only interacts with the − I ion, but also with water molecules. The lower panel of Figure 7 shows the cation−water g(r)ρ’s, calculated between the geometrical ring center of the [C6mim]+ cation and the oxygen atom of water (the corresponding g(r)’s are shown in Figure S3 of the Supporting Information). A distinct peak can be observed in all cases, reflecting the presence of a first solvation shell of water molecules around the imidazolium cation. These peaks are less defined than those of the I-O and I-HW g(r)ρ’s, since the interaction of the water molecules with the cation is weaker as compared with the water−anion one. It is interesting to stress that the strongest interaction occurs between the water molecules and the most acidic hydrogen atom of the imidazolium ring (HCR). Figure 8 shows the HCR-O g(r)ρ’s

Figure 9. Spatial distribution functions, SDFs, of the I− ions (yellow) and oxygen atoms of water molecules (red), calculated from the MD simulations of the [C6mim]I/water mixtures. The same absolute densities have been used for all the investigated IL/water mixtures. Figure 8. HCR-O radial distribution functions multiplied by the numerical densities, g(r)ρ’s, obtained from the MD simulations of [C6mim]I/water mixtures with molar ratios of 1:1 (black line), 1:3 (red line), 1:6 (green line), 1:12 (blue line), 1:70 (brown line), and 1:200 (cyan line). HCR is the most acidic hydrogen of the cation, and O is the oxygen atom of water molecules.

the three hydrogen atoms of the imidazolium ring, HCR, and HCWs (see Figure 1). Looking at the SDFs for the IL/water mixtures with 1:1 and 1:3 molar ratios, it can be seen that the strongest interaction of the anion is with the most acidic atom of the imidazolium ring, HCR. It can also be noticed that near the HCW1 and HCW2 atoms the anion isodensity surfaces are shifted toward the lateral groups, and in proximity of the hexyl chain the shift is less evident because of its steric hindrance. Furthermore, the anion near the HCR atom prefers not to be coplanar with the imidazolium ring plane, while in proximity of the HCW atoms it has the same probability of being or not being coplanar with the ring plane (see Figure S6 of the Supporting Information). This result is similar to that found in an MD simulation of pure [C4mim]I,55 [C6mim]I,58 and in a combined UV−vis, IR, and DFT study of [C4mim]X.82 As the water concentration increases, the probability of finding the anions around the imidazolium cation decreases, and in the most dilute mixture (molar ratio 1:200) only the I− distribution near HCR can be observed. As concerns the cation−water interactions, the SDFs reported in Figure 9 show that the favorite sites of interaction of the oxygen atoms are the HCR and HCW atoms of the imidazolium ring. From low to high dilution (IL/water mixtures with molar ratio 1:6 and 1:12), the isodensity surfaces of water molecules begin to wrap the imidazolium cation, and finally in the 1:70 and 1:200 mixtures, the water distribution around the [C6mim]+ cation becomes disordered and unstructured. 3.3. Tail−Tail Aggregation. It is well-known that alkylimidazolium cations with long alkyl chains (n ≥ 5) are

obtained from the MD simulations for all the investigated mixtures. A distinct first shell peak is found in all cases, with a nonzero coordination number also for the 1:1 solution (see Table 2). This result is in line with the ability of the HCR atom to promote hydrogen bonds. The importance of this issue has been previously pointed out in the literature.41−43 As far as pure ILs are concerned, it has been shown that hydrogen bonds formed between cations and anions via the imidazolium ring most acidic hydrogen atom have a strong impact on the IL macroscopic properties, and a significant variation of the IL melting points and viscosities is obtained when the HCR atom is substituted by a methyl group.81 Besides pure ILs, several authors have also inferred an interaction between the HCR atom and water in IL/water mixtures,41−43 in line with the results found in this work. Further information on the structural properties of the investigated systems is given by the spatial distribution functions (SDFs), which give the probability of finding the anions and water molecules in three-dimensional space around the [C6mim]+ cation. The SDFs are shown in Figure 9, where the isodensity surfaces of the I− anions and oxygen atoms are yellow and red, respectively. With regard to the anion−cation interaction, the favored contacts of the I− ion to the cation are G


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The Journal of Physical Chemistry B organized in polar and apolar domains, as a consequence of the alkyl tail aggregation. This behavior has been shown in several experimental and theoretical studies of imidazolium-based ILs with the bromide ion as counterion in water.49,50,83,84 The CTCT g(r)ρ functions reported on Figure 10 provide a first

Figure 11. X-ray structure factors S(q)’s calculated from the MD simulations of [C6mim]I/water mixtures with molar ratios of 1:1 (black line), 1:3 (red line), 1:6 (green line), 1:12 (blue line), 1:70 (brown line), and 1:200 (cyan line).

The most interesting result emerges from the q region below 1 Å−1, where the presence of the so-called ”prepeak” indicates the existence of long-range structural correlations. This peak is well-defined for the [C6mim]I/water mixtures between 1:1 and 1:12, while it becomes less pronounced and then disappears with increasing dilution. Note that the presence of a prepeak in the S(q) has been also highlighted in a small-angle neutron scattering study of [Cnmim]+-based ILs in water having the tetrafluoroborate, chloride, or iodide as anion.86 The fingerprints of the 1:200 S(q) suggest that there is not segregation of the alkyl tails in agreement with what was previously seen from the CT-CT g(r). This result is in agreement with the experimental CAC value determined for [C6mim]I (250.3 mmol/L)38 that corresponds to an IL/water molar ratio between 1:70 and 1:200. A nice way to have a glimpse into the tail−tail aggregation present in the mixtures is by looking at the MD snapshots using different colors to highlight either the polar or apolar regions (Figure 12). The polar regions include the cation heads, the water molecules, and the I− ions. From this representation it emerges that long-range structural correlations exist in all of the mixtures up to a IL/water ratio of 1:12, but the alkyl chains are not completely segregated in hydrophobic domains as expected for micelle-like structures, in agreement with a previous NMR study of [C6mim]Br and [C8mim]Br water solutions.85 Conversely, in the most diluted solutions the alkyl chains exist as isolated units, completely separated from each other. Finally, it is interesting to note that in previous works it has been shown that hydrogen bonding is the dominant interaction in IL/water systems.27,45,52 In particular, water molecules tend to reduce the electrostatic interaction between the cation and the anion, and hydrogen bonds are formed between water and the anion, and at higher water concentrations also between water and the cation. As the water concentration increases, the water state changes from isolated molecules, to string-like clusters.27 This behavior has also been evidenced in our systems, and a pictorial description is given in Figure 13, where representative snapshots are reported in which water molecules are highlighted. In the solution with IL/water ratio 1:1, water molecules are isolated, while in the 1:3, 1:6, and 1:12 mixtures the existence of string-like water clusters are evident as previously found in the literature.27 In the 1:70 IL/water mixture apolar domains are still present while in the last solution (1:200) no particular aggregation can be observed.

Figure 10. CT-CT and CT-O radial distribution functions, g(r)ρ’s, obtained from the MD simulations of the [C6mim]I/water mixtures with molar ratios of 1:1 (black line), 1:3 (red line), 1:6 (green line), 1:12 (blue line), 1:70 (brown line), and 1:200 (cyan line). CT is the terminal carbon atom of the alkyl chain of the [C6mim]+ cation, as shown in Figure 1, and O is the oxygen atom of water molecules.

indication of the presence of alkyl chain aggregation in our systems (the corresponding g(r)’s are shown in Figure S7 of the Supporting Information). All of the g(r)ρ’s show a distinct first peak around 4.0 Å, suggesting clustering of the hexyl chains. However, the terminal methyl group interacts also with water molecules, as evidenced by the presence of a peak at low distances in the CT-O g(r)ρ reported in the lower panel of Figure 10. The CT-O coordination number increases as a function of water dilution (Table 2), and it remains different from zero also for the most concentrated solutions (0.7 and 2.9 for the IL/water mixtures with molar ratio of 1:1 and 1:3, respectively). These results suggest that the nonpolar chains are not completely segregated in hydrophobic domains in agreement with a previous NMR study of [C6mim]Br and [C8mim]Br/water mixtures.85 The long-range structural organization and the aggregation behavior can also be analyzed by computing the X-ray structure factors from the MD g(r)’s, and the results are shown in Figure 11, where the structure factors, S(q)’s, are plotted as a function of the momentum transfer q. The oscillations in the region above 1 Å−1 are due both to intramolecular interactions and intermolecular short-range contributions existing in the solutions. When the amount of water becomes large (IL/ water mixtures with molar ratios of 1:70 and 1:200), a clear change of the S(q) is visible in this q region as the water contribution becomes predominant. H


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Figure 12. Snapshots obtained from the MD simulations of [C6mim]I/water mixtures. The polar components (water molecule, iodide ion, and cation head) are in red, and the apolar components (alkyl chain) are in cyan. In the right panels only the apolar components are shown for better visualization.

EXAFS experiments and MD simulations. This joined approach allowed us to gain a clear description of the structural organization of these systems both in the short- and longdistance range. The EXAFS results unambiguously show that the water molecules are placed next to the I− anion also for the 1:1 [C6mim]I/water mixture, and the I− hydration shell becomes more and more crowded with increasing water content. This experimental evidence is confirmed by the MD simulations that indicate the existence of water molecules acting as bridge between two I− ions in the 1:1 and 1:3 IL/water mixtures. MD simulations show the presence of interactions also between water and the imidazolium cation for all of the investigated systems. An interesting result of the present investigation concerns the formation of cation−anion ion pairs in the mixtures. A small percentage of ionic pairs was found in the most diluted solutions ([C6mim]I/water molar ratio 1:70 and 1:200), suggesting that the larger size and more diffuse character of the I− anion play a fundamental role, and hamper the formation of strong cation−anion interactions. Finally, the aggregation behavior has been determined from MD simulations. For the most concentrated IL/water mixtures with molar ratio between 1:1 and 1:12 the S(q) functions indicate the existence of long-range structural correlations even if the apolar chains are not completely segregated as expected for micelle-like structures. Conversely, for the 1:200 mixture, that is above the experimental CAC value, the alkyl chains are completely separated from each other, as expected.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b08739. Additional figures (PDF)

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Figure 13. Snapshots obtained from the MD simulations of [C6mim]I/water mixtures. The water molecules are highlighted while the IL molecules are shown in blue.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

4. CONCLUSIONS In the present work we have carried out a thorough characterization of binary mixtures composed by [C6mim]I and water, in a wide range of concentrations (IL/water molar ratios of 1:1, 1:3, 1:6, 1:12, 1:70, and 1:200), by combining

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ACKNOWLEDGMENTS We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities, and we would I


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Hexyl-3-Methylimidazolium Iodide. Chin. Phys. Lett. 2006, 23, 724− 727. (18) Kubo, W.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Quasi-solid-state dye-sensitized solar cells using room temperature molten salts and a low molecular weight gelator. Chem. Commun. 2002, 374−375. (19) Kubo, W.; Kambe, S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Photocurrent-Determining Processes in QuasiSolid-State Dye-Sensitized Solar Cells Using Ionic Gel Electrolytes. J. Phys. Chem. B 2003, 107, 4374−4381. (20) Kawano, R.; Watanabe, M. Anomaly of charge transport of an iodide/tri-iodide redox couple in an ionic liquid and its importance in dye-sensitized solar cells. Chem. Commun. 2005, 2107−2109. (21) Bonhote, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts. Inorg. Chem. 1996, 35, 1168−1178. (22) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonhote, P.; Pettersson, H.; Azam, A.; Grätzel, M. The Performance and Stability of Ambient Temperature Molten Salts for Solar Cell Applications. J. Electrochem. Soc. 1996, 143, 3099−3108. (23) O'Regan, B.; Grätzel, M. A Low-cost, High-efficiency Solar Cell based on Dye-sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (24) Seddon, K. R.; Stark, A.; Torres, M.-J. Influence of Chloride, Water, and Organic Solvents on the Physical Properties of Ionic Liquids. Pure Appl. Chem. 2000, 72, 2275−2287. (25) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. Solution Thermodynamics of Imidazolium-Based Ionic Liquids and Water. J. Phys. Chem. B 2001, 105, 10942−10949. (26) Porter, A. R.; Liem, S. Y.; Popelier, P. L. A. Room temperature Ionic Liquids Containing Low Water Concentrations-a Molecular Dynamics Study. Phys. Chem. Chem. Phys. 2008, 10, 4240−4248. (27) Khan, I.; Taha, M.; Ribeiro-Claro, P.; Pinho, S. P.; Coutinho, J. A. P. Effect of the Cation on the Interactions between Alkyl Methyl Imidazolium Chloride Ionic Liquids and Water. J. Phys. Chem. B 2014, 118, 10503−10514. (28) Kühnel, R.-S.; Böckenfeld, N.; Passerini, S.; Winter, M.; Balducci, A. Mixtures of Ionic Liquid and Organic Carbonate as Electrolyte with Improved Safety and Performance for Rechargeable Lithium Batteries. Electrochim. Acta 2011, 56, 4092−4099. (29) Shekaari, H.; Zafarani-Moattar, M. T.; Behrooz, N. J. Volumetric, Acoustic, and Refractometric Properties of (Thiophene + Hexane/Cyclohexane) Solutions in the Presence of Some Imidazolium Based Ionic Liquids at T = 298.15 K. J. Chem. Thermodyn. 2015, 86, 188−195. (30) Li, W.; Zhang, Z.; Han, B.; Hu, S.; Xie, Y.; Yang, G. Effect of Water and Organic Solvents on the Ionic Dissociation of Ionic Liquids. J. Phys. Chem. B 2007, 111, 6452−6456. (31) Zhao, Y.; Wang, J.; Wang, H.; Li, Z.; Liu, X.; Zhang, S. Is There Any Preferential Interaction of Ions of Ionic Liquids with DMSO and H2O? A Comparative Study from MD Simulation. J. Phys. Chem. B 2015, 119, 6686−6695. (32) Chaban, V. V.; Voroshylova, I. V.; Kalugin, O. N.; Prezhdo, O. V. Acetonitrile Boosts Conductivity of Imidazolium Ionic Liquids. J. Phys. Chem. B 2012, 116, 7719−7727. (33) Liang, M.; Zhang, X.-X.; Kaintz, A.; Ernsting, N. P.; Maroncelli, M. Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Aprotic Liquid Mixture: 1-Butyl-3-methylimidazolium Tetrafluoroborate + Acetonitrile. J. Phys. Chem. B 2014, 118, 1340−1352. (34) Le, M. L. P.; Cointeaux, L.; Strobel, P.; Leprêtre, J.-C.; Judeinstein, P.; Alloin, F. Influence of Solvent Addition on the Properties of Ionic Liquids. J. Phys. Chem. C 2012, 116, 7712−7718. (35) Zhu, A.; Wang, J.; Han, L.; Fan, M. Measurements and Correlation of Viscosities and Conductivities for the Mixtures of Imidazolium Ionic Liquids with Molecular Solutes. Chem. Eng. J. 2009, 147, 27−35. (36) Rodrigues, F.; Santos, P. S. Effect of the Chain Length in the Structure of Imidazolic Ionic Liquids and Dimethylformamide

like to thank the staff of BM23 for assistance in using the beamline. This work was supported by the University of Rome “La Sapienza” (Progetto ateneo 2014, no. C26A14L7CX) and by the CINECA supercomputing center through the grant IscrCDIWA (no. HP10C2Q0F3).

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REFERENCES

(1) Freemantle, M. An Introduction to Ionic Liquids; RSC Publishing: London, 2009. (2) Ionic Liquids IIIA: Fundamentals, Progress, Challenges, and Opportunities: Properties and Structure; Rogers, R., Seddon, K., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005; Vol. 901. (3) Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and Opportunities: Transformations and Processes; Rogers, R., Seddon, K., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005; Vol. 902. (4) Rogers, R. D.; Seddon, K. R. Ionic Liquids-Solvents of the Future? Science 2003, 302, 792−793. (5) Katayanagi, H.; Hayashi, S.; Hamaguchi, H.; Nishikawa, K. Structure of an Ionic Liquid, 1-n-butyl-3-methylimidazolium Iodide, Studied by Wide-angle X-ray Scattering and Raman Spectroscopy. Chem. Phys. Lett. 2004, 392, 460−464. (6) Bodo, E.; Gontrani, L.; Triolo, A.; Caminiti, R. Structural Determination of Ionic Liquids with Theoretical Methods: C8mimBr and C8mimCl. Strength and Weakness of Current Force Fields. J. Phys. Chem. Lett. 2010, 1, 1095−1100. (7) Aoun, B.; Goldbach, A.; Kohara, S.; Wax, J.-F.; González, M. A.; Saboungi, M.-L. Structure of a Prototypic Ionic Liquid: Ethylmethylimidazolium Bromide. J. Phys. Chem. B 2010, 114, 12623− 12628. (8) Migliorati, V.; Ballirano, P.; Gontrani, L.; Triolo, A.; Caminiti, R. Thermal and Structural Properties of Ethylammonium Chloride and Its Mixture with Water. J. Phys. Chem. B 2011, 115, 4887−4899. (9) Bradley, A. E.; Hardacre, C.; Holbrey, J. D.; Johnston, S.; McMath, S. E. J.; Nieuwenhuyzen, M. Small-Angle X-ray Scattering Studies of Liquid Crystalline 1-Alkyl-3-methylimidazolium Salts. Chem. Mater. 2002, 14, 629−635. (10) Umebayashi, Y.; Hamano, H.; Tsuzuki, S.; Canongia Lopes, J. N.; Pádua, A. A. H.; Kameda, Y.; Kohara, S.; Yamaguchi, T.; Fujii, K.; Ishiguro, S.-i. Dependence of the Conformational Isomerism in 1-nButyl-3-methylimidazolium Ionic Liquids on the Nature of the Halide Anion. J. Phys. Chem. B 2010, 114, 11715−11724. (11) Migliorati, V.; Ballirano, P.; Gontrani, L.; Caminiti, R. Crystal Polymorphism of Hexylammonium Chloride and Structural Properties of Its Mixtures with Water. J. Phys. Chem. B 2012, 116, 2104−2113. (12) Gontrani, L.; Bodo, E.; Triolo, A.; Leonelli, F.; D’Angelo, P.; Migliorati, V.; Caminiti, R. The Interpretation of Diffraction Patterns of Two Prototypical Protic Ionic Liquids: a Challenging Task for Classical Molecular Dynamics Simulations. J. Phys. Chem. B 2012, 116, 13024−13032. (13) Kim, K.-S.; Shin, B.-K.; Lee, H. Physical and Electrochemical Properties of 1-butyl-3-methylimidazolium Bromide, 1-butyl-3-methylimidazolium Iodide, and 1-butyl-3-methylimidazolium Tetrafluoroborate. Korean J. Chem. Eng. 2004, 21, 1010−1014. (14) Nakakoshi, M.; Shiro, M.; Fujimoto, T.; Machinami, T.; Seki, H.; Tashiro, M.; Nishikawa, K. Crystal Structure of 1-Butyl-3methylimidazolium Iodide. Chem. Lett. 2006, 35, 1400−1401. (15) Katoh, R.; Hara, M.; Tsuzuki, S. Ion Pair Formation in [bmim]I Ionic Liquids. J. Phys. Chem. B 2008, 112, 15426−15430. (16) Kinoshita, S.; Okuda, K.; Machida, N.; Shigematsu, T. Additive Effect of Ionic Liquids on the Electrochemical Property of a Sulfur Composite Electrode for All-solid-state Lithium-sulfur Battery. J. Power Sources 2014, 269, 727−734. (17) Miao, W.; Qing-Li, Z.; Yu-Xiang, W.; Yuan, L.; Xu-Rui, X. Investigation of Mechanisms of Enhanced Open-Circuit Photovoltage of Dye-Sensitized Solar Cells Based the Electrolyte Containing 1J


Article

The Journal of Physical Chemistry B Solutions Probed by Raman Spectroscopy. Vib. Spectrosc. 2010, 54, 123−126. (37) Garcia, H. C.; de Oliveira, L. F. C.; Nicolau, B. G.; Ribeiro, M. C. C. Raman Spectra of Acetonitrile in Imidazolium Ionic Liquids. J. Raman Spectrosc. 2010, 41, 1720−1724. (38) Vaghela, N.; Sastry, N.; Aswal, V. Surface Active and Aggregation Behavior of Methylimidazolium-based Ionic Liquids of Type [Cnmim] [X], n = 4,6,8and[X] = Cl-, Br-, and I- in water. Colloid Polym. Sci. 2011, 289, 309−322. (39) Sastry, N. V.; Vaghela, N. M.; Macwan, P. M. Densities, Excess Molar and Partial Molar Volumes for Water + 1-Butyl- or, 1-Hexyl- or, 1-Octyl-3-methylimidazolium Halide Room Temperature Ionic Liquids at T = (298.15 and 308.15) K. J. Mol. Liq. 2013, 180, 12−18. (40) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular States of Water in Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192−5200. (41) Mele, A.; Tran, C. D.; De Paoli Lacerda, S. H. The Structure of a Room-Temperature Ionic Liquid with and without Trace Amounts of Water: The Role of C-H···O and C-H···F Interactions in 1-n-Butyl-3Methylimidazolium Tetrafluoroborate. Angew. Chem., Int. Ed. 2003, 42, 4364−4366. (42) Moreno, M.; Castiglione, F.; Mele, A.; Pasqui, C.; Raos, G. Interaction of Water with the Model Ionic Liquid [bmim][BF4]: Molecular Dynamics Simulations and Comparison with NMR Data. J. Phys. Chem. B 2008, 112, 7826−7836. (43) Miki, K.; Westh, P.; Nishikawa, K.; Koga, Y. Effect of an ”Ionic Liquid” Cation, 1-Butyl-3-methylimidazolium, on the Molecular Organization of H2O. J. Phys. Chem. B 2005, 109, 9014−9019. (44) Schröder, C.; Rudas, T.; Neumayr, G.; Benkner, S.; Steinhauser, O. On the Collective Network of Ionic Liquid/Water Mixtures. I. Orientational Structure. J. Chem. Phys. 2007, 127, 234503. (45) Hanke, C. G.; Lynden-Bell, R. M. A Simulation Study of WaterDialkylimidazolium Ionic Liquid Mixtures. J. Phys. Chem. B 2003, 107, 10873−10878. (46) Köddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. The Association of Water in Ionic Liquids: A Reliable Measure of Polarity. Angew. Chem., Int. Ed. 2006, 45, 3697−3702. (47) López-Pastor, M.; Ayora-Cañada, M. J.; Valcárcel, M.; Lendl, B. Association of Methanol and Water in Ionic Liquids Elucidated by Infrared Spectroscopy Using Two-Dimensional Correlation and Multivariate Curve Resolution. J. Phys. Chem. B 2006, 110, 10896− 10902. (48) Blesic, M.; Marques, M. H.; Plechkova, N. V.; Seddon, K. R.; Rebelo, L. P. N.; Lopes, A. Self-Aggregation of Ionic Liquids: Micelle Formation in Aqueous Solution. Green Chem. 2007, 9, 481−490. (49) Goodchild, I.; Collier, L.; Millar, S. L.; Prokeš, I.; Lord, J. C.; Butts, C. P.; Bowers, J.; Webster, J. R.; Heenan, R. K. Structural Studies of the Phase, Aggregation and Surface Behaviour of 1-alkyl-3methylimidazolium Halide + Water Mixtures. J. Colloid Interface Sci. 2007, 307, 455−468. (50) Zhao, Y.; Gao, S.; Wang, J.; Tang, J. Aggregation of Ionic Liquids [Cnmim]Br (n = 4, 6, 8, 10, 12) in D2O: A NMR Study. J. Phys. Chem. B 2008, 112, 2031−2039. (51) Singh, T.; Kumar, A. Aggregation Behavior of Ionic Liquids in Aqueous Solutions: Effect of Alkyl Chain Length, Cations, and Anions. J. Phys. Chem. B 2007, 111, 7843−7851. (52) Migliorati, V.; Zitolo, A.; D’Angelo, P. Using a Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of Imidazolium Based Ionic Liquids/Water Mixtures. 1. MD Simulations. J. Phys. Chem. B 2013, 117, 12505−12515. (53) D’Angelo, P.; Zitolo, A.; Aquilanti, G.; Migliorati, V. Using a Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of Imidazolium Based Ionic Liquids/Water Mixtures. 2. EXAFS Spectroscopy. J. Phys. Chem. B 2013, 117, 12516− 12524. (54) Miskolczy, Z.; Sebok-Nagy, K.; Biczok, L.; Gokturk, S. Aggregation and micelle formation of ionic liquids in aqueous solution. Chem. Phys. Lett. 2004, 400, 296−300.

(55) Migliorati, V.; Serva, A.; Aquilanti, G.; Olivi, L.; Pascarelli, S.; Mathon, O.; D'Angelo, P. Combining EXAFS Spectroscopy and Molecular Dynamics Simulations to Understand the Structural and Dynamic Properties of an Imidazolium Iodide Ionic Liquid. Phys. Chem. Chem. Phys. 2015, 17, 2464−2474. (56) D’Angelo, P.; Zitolo, A.; Migliorati, V.; Bodo, E.; Aquilanti, G.; Hazemann, J. L.; Testemale, D.; Mancini, G.; Caminiti, R. X-Ray Absorption Spectroscopy Investigation of 1-alkyl-3-methylimidazolium Bromide Salts. J. Chem. Phys. 2011, 135, 074505. (57) Zitolo, A.; Migliorati, V.; Aquilanti, G.; D’Angelo, P. On the Possibility of Using XANES to Investigate Bromide-Based Ionic Liquids. Chem. Phys. Lett. 2014, 591, 32−36. (58) Migliorati, V.; Serva, A.; Aquilanti, G.; Pascarelli, S.; D'Angelo, P. Local Order and Long Range Correlations in Imidazolium Halide Ionic Liquids: a Combined Molecular Dynamics and XAS Study. Phys. Chem. Chem. Phys. 2015, 17, 16443−16453. (59) Mancini, G.; Sanna, N.; Barone, V.; Migliorati, V.; D'Angelo, P.; Chillemi, G. Structural and Dynamical Properties of the Hg(2+) Aqua Ion: A Molecular Dynamics Study. J. Phys. Chem. B 2008, 112, 4694− 4702. (60) D’Angelo, P.; Migliorati, V.; Mancini, G.; Barone, V.; Chillemi, G. Integrated Experimental and Theoretical Approach for the Structural Characterization of Hg2+ Aqueous Solutions. J. Chem. Phys. 2008, 128, 084502. (61) Migliorati, V.; Zitolo, A.; Chillemi, G.; D'Angelo, P. Influence of the Second Coordination Shell on the XANES Spectra of the Zn2+ Ion in Water and Methanol. ChemPlusChem 2012, 77, 234−239. (62) Smith, W.; Forester, T. DL_POLY_2.0: A General-Purpose Parallel Molecular Dynamics Simulation Package. J. Mol. Graphics 1996, 14, 136−141. (63) Canongia Lopes, J. N.; Deschamps, J.; Pádua, A. A. H. Modeling Ionic Liquids Using a Systematic All-Atom Force Field. J. Phys. Chem. B 2004, 108, 2038−2047. (64) Canongia Lopes, J. N.; Pádua, A. A. H. Molecular Force Field for Ionic Liquids III: Imidazolium, Pyridinium, and Phosphonium Cations; Chloride, Bromide, and Dicyanamide Anions. J. Phys. Chem. B 2006, 110, 19586−19592. (65) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269−6271. (66) Mark, P.; Nilsson, L. Structure and Dynamics of the TIP3P, SPC, and SPC/E Water Models at 298 K. J. Phys. Chem. A 2001, 105, 9954−9960. (67) Migliorati, V.; Sessa, F.; Aquilanti, G.; D’Angelo, P. Unraveling Halide Hydration: A High Dilution Approach. J. Chem. Phys. 2014, 141, 044509. (68) Reif, M. M.; Hünenberger, P. H. Computation of MethodologyIndependent Single-Ion Solvation Properties from Molecular Simulations. IV. Optimized Lennard-Jones Interaction Parameter Sets for the Alkali and Halide Ions in Water. J. Chem. Phys. 2011, 134, 144104. (69) Nosé, S. A. Unified Formulation of the Constant Temperature Molecular Dynamics Methods. J. Chem. Phys. 1984, 81, 511−519. (70) Evans, D. J.; Holian, B. L. The Nosé-Hoover Thermostat. J. Chem. Phys. 1985, 83, 4069−4074. (71) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577−8593. (72) Brehm, M.; Kirchner, B. TRAVIS - A Free Analyzer and Visualizer for Monte Carlo and Molecular Dynamics Trajectories. J. Chem. Inf. Model. 2011, 51, 2007−2023. (73) Pings, C. J.; Waser, J. J. J. Chem. Phys. 1968, 48, 3016. (74) Migliorati, V.; Ballirano, P.; Gontrani, L.; Russina, O.; Caminiti, R. Crystal Polymorphism of Propylammonium Chloride and Structural Properties of Its Mixture with Water. J. Phys. Chem. B 2011, 115, 11805−11815. (75) Migliorati, V.; Ballirano, P.; Gontrani, L.; Materazzi, S.; Ceccacci, F.; Caminiti, R. A Combined Theoretical and Experimental Study of Solid Octyl and Decylammonium Chlorides and of Their Aqueous Solutions. J. Phys. Chem. B 2013, 117, 7806−7818. K


Article

The Journal of Physical Chemistry B (76) Filipponi, A.; Di Cicco, A. X-ray-absorption Spectroscopy and nbody Distribution Functions in Condensed Matter. II. Data Analysis and Applications. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 15135−15149. (77) D’Angelo, P.; Zitolo, A.; Migliorati, V.; Pavel, N. V. Measurement of x-ray multielectron photoexcitations at the I− K edge. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 144105. (78) Karmakar, A.; Chandra, A. Water in Hydration Shell of an Iodide Ion: Structure and Dynamics of Solute-Water Hydrogen Bonds and Vibrational Spectral Diffusion from First-Principles Simulations. J. Phys. Chem. B 2015, 119, 8561−8572. (79) Fulton, J. L.; Schenter, G. K.; Baer, M. D.; Mundy, C. J.; Dang, L. X.; Balasubramanian, M. Probing the Hydration Structure of Polarizable Halides: A Multiedge XAFS and Molecular Dynamics Study of the Iodide Anion. J. Phys. Chem. B 2010, 114, 12926−12937. (80) Ohtaki, H.; Radnai, T. Structure and Dynamics of Hydrated Ions. Chem. Rev. 1993, 93, 1157−1204. (81) Fumino, K.; Wulf, A.; Ludwig, R. Strong, Localized, and Directional Hydrogen Bonds Fluidize Ionic Liquids. Angew. Chem., Int. Ed. 2008, 47, 8731−8734. (82) Shukla, M.; Srivastava, N.; Saha, S. Theoretical and spectroscopic Studies of 1-butyl-3-methylimidazolium Iodide Room Temperature Ionic Liquid: Its Differences with Chloride and Bromide Derivatives. J. Mol. Struct. 2010, 975, 349−356. (83) Wang, J.; Wang, H.; Zhang, S.; Zhang, H.; Zhao, Y. Conductivities, Volumes, Fluorescence, and Aggregation Behavior of Ionic Liquids [C4mim][BF4] and [Cnmim]Br (n = 4, 6, 8, 10, 12) in Aqueous Solutions. J. Phys. Chem. B 2007, 111, 6181−6188. (84) Bhargava, B. L.; Klein, M. L. Molecular Dynamics Studies of Cation Aggregation in the Room Temperature Ionic Liquid [C10mim][Br] in Aqueous Solution. J. Phys. Chem. A 2009, 113, 1898−1904. (85) Cesare Marincola, F.; Piras, C.; Russina, O.; Gontrani, L.; Saba, G.; Lai, A. NMR Investigation of Imidazolium-Based Ionic Liquids and Their Aqueous Mixtures. ChemPhysChem 2012, 13, 1339−1346. (86) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C.; Heenan, R. K. Aggregation Behavior of Aqueous Solutions of Ionic Liquids. Langmuir 2004, 20, 2191−2198.

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