Local Depolarization in Hydrophobic and Hydrophilic Ionic Liquids

Article pubs.acs.org/JPCC

Local Depolarization in Hydrophobic and Hydrophilic Ionic Liquids/ Water Mixtures: Car−Parrinello and Classical Molecular Dynamics Simulation Mohammad Hadi Ghatee* and Amin Reza Zolghadr Department of Chemistry, Shiraz University, Shiraz 71454, Iran ABSTRACT: Car−Parrinello molecular dynamics (MD) was carried out to simulate pure [C4mim]PF6 and [C4mim]BF4 ionic liquids and their mixtures with polar water solvent to approach the admixing mechanism as well as hydrophobic and hydrophilic interactions from an electronic point of view. Initially, the results of density functional theory (DFT) on isolated ion pairs with partial charges assigned to atomic centers by various methods were analyzed. Next the trajectory of a 40 ps long Car−Parrinello MD were analyzed under bulk conditions. Water molecule influences substantially the hydrophilic ([C4mim]BF4) ionic liquid and the hydrophobic ([C4mim]PF6) to a different extent, which is evident by probing atomic charges of partnering constituents. The reduction in simulated dipole moment of water upon admixing with hydrophilic ionic liquid is larger than that with the hydrophobic one, which roots from stronger electrostatic screening. Water molecules tend to segregate when mixed with [C4mim]PF6 but mix with [C4mim]BF4 efficiently by interacting with BF4− anion, which interacts and resides on its cation [C4mim]+. When ionic liquids mixed with water, the average charge on each F atom in BF4− (PF6−) anion was −0.3261e (−0.1820e). The simulated charge on each H atom of pure water (0.3290e) can be evidently responsible for the effective H···F interaction in [C4mim]BF4 but ineffective in [C4mim]PF6. These results provide insight into the hydrophilic and hydrophobic character from an electronic point of view.

1. INTRODUCTION Room-temperature ionic liquids (RTILs) are salts in liquid phase. Despite high Columbic interactions, bulky ions in ionic liquids (ILs) are poorly coordinated and thus contrary to the inorganic salts are in the liquid state at room temperature.1,2 Regarding their hydrophobic and hydrophilic nature, RTILs have received various applications as green solvents involving electrochemistry,3,4 extraction processes,5 catalysis,6 and reaction media for organic synthesis.7 The availability of potentially hydrophobic and hydrophilic ILs attests to the ease of physical and chemical characteristics to be achieved.8 During our theoretical9−13 and experimental14,15 studies on bulk and surface of ILs and on surface of polar organic liquids,16 we found that both cations and anions greatly affect the hydrophilicity and hydrophobicity characters. Increasing alkyl chain length leads to a decrease in miscibility of IL in a solvent, which is rationalized in terms of large differences between the internal pressure of solvent and the ionic species.17 The technique of self-assembled monolayers (SAMs)18 could provide a simple method for quantifying the effects of counteranions on the surface hydrophilicity and hydrophobicity of ILs. Molecular simulations have been used extensively to study the bulk and interfacial properties of ILs. Lynden-Bell et al.19 have studied the interface between hydrophilic 1,3dimethylimidazolium chloride ([C1mim]Cl) IL and different Lennard−Jones fluids. They have also simulated the ILs/water © 2013 American Chemical Society

interfaces, with the total charge of +1e distributed over all [C1mim]+ cation atoms, keeping the IL neutral. Their studies on a water/[C1mim]Cl interface based on a 180 ps run indicate that the interface of IL with SPC/E water model is not thermodynamically stable. Wipff et al.20,21 have considered the interface between water and hydrophobic ILs including [C4mim]PF6, [C8mim]PF6, and [C4mim]Tf2N focusing on the questions of phase separation from water, the extent of IL/ water mixing in the bulk phases, and the properties of their interfaces with water. They have shown that the scaled [cation]0.9[anion]−0.9 model reaches equilibrium in a short time compared with the standard [cation]1[anion]−1 model. The procedure of reducing charges, which has been explained by Bhargava and Balasubramanian,22 leads to the faster dynamics of ILs. Recently, Feng and Voth23 have studied the role of alkyl chain length on the structure and dynamics of three hydrophobic IL/water mixtures at various water mole fractions. Klähn et al.24 have been proposed a water/ion interaction model and derived an equation that estimates the water/ion interaction strength based on the volume and surface charge of the ions. Received: May 31, 2012 Revised: January 8, 2013 Published: January 8, 2013 2066

dx.doi.org/10.1021/jp3053345 | J. Phys. Chem. C 2013, 117, 2066−2077

The Journal of Physical Chemistry C

Article

dealing mainly with thermodynamic and structural properties and single particle dynamics.37 There is a lack of explanation of water miscibility of ILs from an electronic point of view and by MD simulation. The use of classical force fields parameters of pure ILs in the simulation of the IL/water mixture is a crucial task, which requires much refinement for the best result of such a nature. The presence of polar water could significantly change the electronic structure of ILs; conversely, the presence of ILs with different ionic nature has a substantial effect on the locally induced electronbased polarization, the overall structure, and the bonding network of water in both hydrophobic and hydrophilic IL/ water mixtures. Seeking some details of mechanism and the extent of water miscibility of ILs with different nature, we performed CPMD simulation calculations for pure bulk ILs and binary ILs/water systems including hydrophobic [C4mim]PF6 and hydrophilic [C4mim]BF4 ILs. There are verities of hydrophobic and hydrophilic ILs, although two imidazolium-based ILs with nearly the same size anion are selected for this study. The primary aim is to investigate the interaction of anion and the cation with water. The partial charges across the gas phase, pure IL bulk, and IL/water scales, which were obtained using standard methods (e.g., CHELPG and ESP), are compared to evaluate the effect of neighboring molecules on the electrostatic properties. The focus is specifically on the changes in partial charges of both anions and cations in hydrophobic and hydrophilic ILs upon admixing with water as well as fluctuations in dipole moment of added water. The difference between the bond flexibility and hydrogen bond formation of hydrophobic and hydrophilic ILs/water mixtures will be studied in terms of molecular dipole moment fluctuations and cluster formation as well as the changes between water dielectric constant in pure water and in IL/water mixture calculated by CPMD simulations. The classical large-scale MD simulation was also performed to verify the influence of ensemble size on the molecular structure in comparison with the CPMD results. Additionally, the large-scale MD simulations of liquid/vapor interface of these ILs were explored. The organization of this article is as follows. In Section 2, computational methods containing ab initio gas-phase calculations, CPMD studies, force-field parameters, and details of MD simulation are presented. The results of bulk structure and properties are discussed in Section 3, followed by Section 4 containing the molecular orientation and properties for the [C4mim]PF6/water interface. The concluding remarks are presented at the end.

The advent of the Car−Parrinello MD (CPMD) method in 198525 enabled one to use a quantum mechanical description of the electronic degrees of freedom combined with a classical phase space trajectory involving a fictitious electronic mass parameter.26 Several ab initio molecular dynamics (AIMD) studies of pure IL can be found in the literature. AIMD simulations have been performed for the first time on the room-temperature organic IL [C1mim]Cl using density functional theory by Del Pópolo et al.27 They aimed to compare the liquid local structure with those obtained from two different classical force fields and from neutron scattering experiments. Bühl et al.28 have performed extensive CPMD simulations for a simple IL, 1,3-dimethylimidazolium chloride. The microstructure of the liquid has been analyzed in terms of suitable radial distribution functions and anisotropic site-specific cation−anion distributions. Bhargava and Balasubramanian29 have studied the intermolecular structure of 1,3-dimethylimidazolium chloride, using CPMD and classical MD simulations. Ghatee and Ansari have performed CPMD to simulate the structure and dynamics of [C4mim]I IL at 300 K. Site−site pair distribution functions reveal that the anion has a strong interaction with any three C−H’s of the imidazolium ring. Delle Site and coworkers30 have analyzed molecular polarization in the bulk of 30 ion pairs of [C1mim]Cl IL via the Car− Parrinello approach. Their analysis of the molecular electrostatic properties compared with the previous work on smaller number of ion pairs per unit cell shows that the immediate liquid environment predominantly affects the molecular electric dipole moments, whereas the bulk contributions appear to be minor. A detailed calculation of partial charges for the [C1mim] Cl with MP2 electronic structure calculations has been presented by Holm et al.31 They have provided the results of DFT calculation on isolated ion pairs and analyzed the trajectory of a 25 ps long CPMD run of 30 ion pairs under bulk conditions. Both the single ion pair and the bulk system resulted in a similar total ionic charge considerably less than unity. Recently, Delle Site et al.32 have investigated three different imidazolium-based ILs, [C1mim]Cl, [C2mim]SCN, and [C2mim][HN(CN)2], and a protic IL, monomethyl ammonium nitrate CH3NH3NO3, by CPMD simulations. They found that electrostatic properties were reduced and are very local yet dominated by large fluctuations. More recently, imidazolium-based ILs have been studied on different scales, going from the detailed quantum electronic scale to the classical atomistic scale by the same group.33 Their results confirmed the strong screening of Coulomb interactions, which gave reliable predictions for static and dynamic properties when incorporated into a classical force field. The properties of ILs mixed with water have not been widely studied by AIMD, and only limited number of studies have been reported to date. An AIMD study of a single [C2mim]Cl dissolved in 60 water molecules has been performed to explain the unusual association behavior of ILs in water.34 The liquid phase of pure tetramethyl ammonium fluoride (TMAF), its equimolar mixture with water, and its dilute solution in water have been investigated by AIMD method.35 Recently, Kirchner et al.36 have investigated the influence of a small amount of water in CH3NH3NO3 IL and the change of the properties of water following the solvation. Despite all of these studies on the solvation, the role of interaction of cation and the anion of IL with the water solvent is not yet fully understood. Just several computational studies of the properties of both pure ILs and IL/water mixtures have been reported in the recent years,

2. COMPUTATIONAL METHODS The quantum chemical calculations on the isolated ion pairs were performed to determine the partial charge distributions. The structure of each IL was optimized by DFT method at B3LYP/6-311++G(d,p) level of theory, using the Gaussian 03 program. The structures of neutral ILs, cations, and anions were checked for vibrational frequencies38 to ensure that the optimized structures are minima on the potential-energy surface.39 We allocated a set of partial charges for each IL by fitting to the electrostatic potential surface with the CHELPG procedure. The Car−Parrinello molecular dynamics package (CPMD,40 Version 3.11.1) was used for simulation of pure water, pure ILs, and IL/water mixtures. A cubic box with periodic boundary conditions applied in three dimensions was employed to obtain 2067



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Table 1. Specification of the Ensembles Simulated by CPMD and Classical MD system [C4mim]BF4/water [C4mim]PF6/water [C4mim]BF4 [C4mim]PF6 [C4mim]BF4/water [C4mim]PF6/water [C4mim]BF4/water [C4mim]PF6/water

simulation type CPMD CPMD CPMD CPMD classical classical classical classical

MD MD MD MD

(bulk) (bulk) (slab) (slab)

cation

anion

5 5 5 5 512 512 324 324

5 5 5 5 512 512 324 324

water

box size (Å)

15 15

12.14 12.55 11.75 11.82 60.00 62.06 46.90 48.00

1536 1536 2800 2800

× × × × × × × ×

12.14 12.55 11.75 11.82 60.00 62.06 46.90 48.00

× × × × × × × ×

12.14 12.55 11.75 11.82 60.00 62.06 86.00 95.00

Table 2. Electrostatic Charges for [C4mim]BF4 and [C4mim]PF6 Atoms Obtained at B3LYP/6-311++G(d,p) Level of Theory by CHELPG Method for Isolated Molecules Together with ESP Charges Obtained from Bulk CPMD Simulationsa [C4mim]BF4 atom N1 C2 N3 C4 C5 C6 C7 C8 C9 C10 H2 H4 H5 H6 H6 H6 H7 H7 H8 H8 H9 H9 H10 H10 H10 B F F F F OW HW

a

[C4mim]PF6

q/e B3LYP CHELPG

q/e CPMD ESP pure

q/e CPMD ESP with water

0.1120 −0.0462 0.1902 −0.1657 −0.1668 −0.1715 −0.1148 0.2430 0.0843 −0.1922 0.1944 0.1761 0.1938 0.1108 0.1637 0.0700 0.0447 0.0951 −0.0019 −0.0581 −0.0303 0.0004 0.0518 0.0563 0.0292 1.0894 −0.4908 −0.4852 −0.5148 −0.4670

0.2420 −0.1796 0.2764 −0.1560 −0.1852 −0.2478 −0.1026 −0.0796 0.0558 −0.2816 0.2032 0.1798 0.1846 0.1060 0.1036 0.1298 0.0926 0.0746 0.0706 0.0572 0.0124 0.0266 0.0780 0.0832 0.0824 1.0514 −0.4678 −0.4650 −0.4786 −0.4664

0.1234 −0.2808 0.2886 −0.1050 −0.2880 −0.2318 −0.1854 −0.1508 −0.2282 −0.1708 0.1618 0.1350 0.1718 0.1618 0.1378 0.0984 0.1664 0.1068 0.0792 0.1606 0.1350 0.0982 0.1500 0.1000 0.1142 0.5582 −0.3166 −0.3212 −0.3246 −0.3422 −0.6700 0.3290

atom N1 C2 N3 C4 C5 C6 C7 C8 C9 C10 H2 H4 H5 H6 H6 H6 H7 H7 H8 H8 H9 H9 H10 H10 H10 P F F F F F F OW HW

q/e B3LYP CHELPG

q/e CPMD ESP pure


0.1352 −0.0692 0.1842 −0.1554 −0.1695 −0.1429 −0.1334 0.2568 0.0901 −0.2078 0.2001 0.1744 0.1946 0.1130 0.1434 0.0654 0.0489 0.0945 −0.0181 −0.0574 −0.0295 −0.0019 0.0625 0.0611 0.0318 1.3982 −0.3605 −0.3663 −0.3877 −0.3949 −0.4057 −0.3540

0.1334 −0.0914 0.3222 −0.2284 −0.1440 −0.3114 −0.0184 −0.0708 −0.0096 −0.2374 0.1660 0.1894 0.1646 0.1290 0.1462 0.1246 0.0654 0.0798 0.0456 0.0634 0.0368 0.0384 0.0784 0.0772 0.0674 1.3480 −0.3594 −0.3692 −0.3956 −0.3928 −0.3302 −0.3172

0.1214 −0.1050 0.2348 −0.1336 −0.1904 −0.2200 −0.1562 −0.1132 −0.1672 −0.1856 0.1808 0.1834 0.1796 0.0860 0.0998 0.0792 0.0732 0.0366 0.0422 0.1006 0.0950 0.1030 0.1550 0.1510 0.1264 0.3160 −0.1856 −0.1710 −0.2006 −0.1906 −0.1806 −0.1638 −0.6760 0.3360

OW and HW are oxygen and hydrogen of water molecule.

IL. The electronic orbitals were expanded in a plane-wave basis set with an energy cutoff of 100 Ry. All simulations were performed using a fictitious electron mass of 800 au and at 298.15 K for 40 ps. The kinetic energy of the electrons and ions was controlled using Nosé−Hoover chain thermostats. The initial configurations for the CPMD runs were taken from a 5 ns classical MD simulation using the NPT ensemble. Partial charges obtained by different methods including by CPMD simulations are shown in Table 2.

bulk behavior. Ensembles used for CPMD simulation under boundary condition are specified in Table 1. The general gradient-corrected density functional BP8641,42 together with norm-conserving pseudopotentials generated according to the Troullier and Martins procedure43 and transformed into the Kleinman-Bylander form were employed. The employed density functional probably neglects dispersion interactions while recently Kirchner et al.44 have shown the importance of dispersion and induction interactions in an imidazolium-based 2068



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For the two binary systems, [C4mim]PF6/water and [C4mim]BF4/water, classical MD was performed to simulate the bulk and liquid/liquid interface using the DL_POLY program version 2.17 at ambient pressure, 1.01325 × 105 Pa.45 The force field used for the ILs is in the form of the explicit fully flexible, all-atom force field developed by Lopes et al.46 The electrostatic charges for each mixture were taken from our CPMD simulation (Table 2) previously described. The SPC/E model was used for water in all simulations. The equations of motion were solved using Verlet-Leapfrog integration algorithm under the periodic boundary conditions. The Columbic longrange interactions were calculated using Ewald’s method with 1 × 10−5 precision. The potential cutoff distance value of 15 Å was used for bulk and interface simulations. For bulk simulations of ILs, a single ion pair with geometry optimized by ab initio calculation was replicated to obtain a simulation box containing 512 ion pairs; subsequently, 1536 water molecules were added at random positions. The simulation had to begin with short time step between (1 and 5) × 10−4 ps for 200 ps and with 1 × 10−3 ps following the equilibration. After the initial equilibration with short time step at 500 K, the temperature of the system was decreased at intervals of 100 K to a final value of 298.15 K. At each temperature, the system was simulated for 500 ps under NPT conditions with the Nosé-Hoover thermostat/barostat algorithm and the modification of Melchionna et al., as implemented in the DL_POLY program. The relaxation times for the thermostat and barostat are 0.1 and 2.0 ps, respectively. A 2 ns production run was performed for each binary system following the equilibration for 1 ns. To compare the water miscibility of the ILs, classical MD simulation of [C4mim]PF6/water and [C4mim]BF4/water binary systems was performed at 450 K. The initial cubic ensemble of 324 IL ion pairs was extended in the z direction, producing a slab after an initial equilibration. The empty spaces on either side of the slab were filled with 2800 water molecules. The simulations were started in NPT ensemble, under Berendsen thermostat.47 The total energy of system indicates that the state of equilibrium is reached satisfactorily in ∼4 ns. After all of these preliminary simulations adjustments, finally, MD simulations were extended for an additional 8 ns collecting statistical data at 450 K in NVT ensemble with the Nosé− Hoover thermostat.

Figure 1. Structure of 1-butyl-3-methyl-imidazolium cation with atom labeling.

hereafter) segregate and form domains at the proximity of [C4mim]PF6 ion pairs. The total charges we calculated by different methods for different groups of atoms are shown in Table 3. The partial charges of an ion pair determined for a single isolated ion pair, in neat bulk ILs and in IL/water binary systems, are visualized and compared in Figures 2 and 3. The trend of partial charge variation is almost the same in all cases. Each IL was divided into four different groups, as previously studied in literature:49,50 imidazolium ring, methyl group, butyl chain, and the anion; the total charge of atoms in each group is summed to determine the IL charge. (See Figures 4 and 5.) The partial charges obtained from ab initio calculations of single isolated ion pairs resemble closely the results of bulk CPMD simulations under the periodic boundary condition. (See Figure 2.) In line with the results of Delle Site and coworkers,30 we can conclude that the influence of the ‘‘bulk liquid system’’ on the electrostatic properties of a single ion pair appears to be rather negligible (Figure 2). Whereas the charge values both in gas phase (single ion pair) and in bulk phase (by CPMD simulation under periodic boundary condition) are almost the same, the absolute partial charges on anions and cations are influenced by the screening effect of neighboring molecules. The anion charge in the bulk phase is lower than that in the gas phase (Table 3). Interestingly, the CPMD simulation of IL/water binary systems reveals that the anion average charge for [C4mim]BF4 and [C4mim]PF6 reduced to −0.7464e and −0.7762e, respectively. This charge reduction occurs because of screening due to electrostatic interactions between the ions and the solvent water molecules. Methodologically, the depolarization of ILs molecules by water in IL/water binary system has a substantial effect on partial charges of IL molecules and hence on the derivation of a classical force field. Specifically, the presence of water molecules considerably modifies the partial charges on anions. The charge on BF4− anion is lowered more than that on PF6− anion when the IL is mixed with water. This can be attributed to the interaction of water molecules with ILs through anion effectively for the later anion, and through cation and anion for former anion. The gas-phase distance of water H molecules and the anion F molecules is longer in the case of PF6− anion than in BF4− (See Figure 6). This difference in the distance indicates that donor−acceptor interactions are stronger in the case of BF4− anion. The charges on F molecules in BF4− were found to be larger than those in PF6−. The average charge on each F atom in BF4− anion is −0.3261e, which is almost the same as the average charge on each H atom of water (0.3290e). Comparing this value to the average charge on each F atom in PF6− anion (−0.1820e) provides insight into the hydrophilic and hydrophobic nature of ILs containing these anions. (See Figure 6.) Strictly speaking, the charges in

3. RESULTS AND DISCUSSION Atom’s Charges. The partial charges for the single isolated [C4mim]BF4 and [C4mim]PF6 ILs were determined using CHELPG method. The absolute charges of cations and anions obtained in this work are considerably less than unity (Table 2), in agreement with the recent report.48 The net charge for a single ion pair is zero in all cases. (See Figure 1 for atom labeling of the two ILs.) For the CPMD simulation ensemble comprising 15 water molecules and 5 [C4mim]PF6 ([C4mim]BF4) ILs, the net charge on water molecules was found to be −0.0620e (−0.1790e). Therefore, the average electric field of IL ion pairs has an extensive polarization effect, leading to a net non-zero charge on water molecules. The larger induced charge on water molecules in the case of [C4mim]BF4/water binary mixture (shown by [C4mim]BF4/water hereafter) is due to the possibility of direct interaction with the neighboring water molecules. On the contrary, water molecules in the binary [C4mim]PF6/water mixture (shown by [C4mim]PF6/water 2069



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Table 3. Comparison of the Charges of Different Atom Groups [C4mim]BF4

[C4mim]PF6

group

q/e B3LYP CHELPG

q/e CPMD ESP pure


Im-ring methyl butyl cation anion

0.4878 0.1730 0.2075 0.8683 −0.8684

0.5652 0.0916 0.1696 0.8264 −0.8264

0.2068 0.1662 0.3752 0.7482 −0.7464

group

q/e B3LYP CHELPG

q/e CPMD ESP pure


Im-ring methyl butyl cation anion

0.4944 0.1789 0.1976 0.8709 −0.8709

0.5118 0.0884 0.2162 0.8164 −0.8164

0.4710 0.0450 0.2608 0.7768 −0.7762

Figure 4. Comparison of partial charges of different atoms group for single ion pair, neat, and water mixture of [C4mim]PF6 IL. Figure 2. Comparison of CHELPG partial charges for a single isolated ion pair of [C4mim]PF6 obtained at B3LYP/6-311++G(d,p) level with simulated electrostatic potential (ESP) charges for pure bulk of this IL and its mixture with water by using CPMD.

Figure 5. Same as Figure 4 but for [C4mim]BF4.

and water system (EIL‑W) and the sum of the energies of the single ion pair (EIL) and water (EW), can be calculated by: E inter(kJ/mol) = 2625.50[E IL ‐ W (au) − E IL(au) − E W (au)]

(1)

Each complex and corresponding water and anion were optimized for structure and energy at the B3LYP/6-311+ +G(d,p) level of theory by the above computational procedure. Boys−Bernardi’s counterpoise procedure (CP) to correct for the basis set superposition error (BSSE) was used.51 The calculated Einter values of a [C4mim]PF6 and [C4mim]BF4 with a water molecule are −17.90 and −31.32 kJ/mol, respectively, which shows that the water molecule interact more strongly with [C4mim]BF4 than [C4mim]PF6. For comparison, we have calculated the interaction energy of [C2mim]Cl (−52.28 kJ/ mol) calculated at the same level of theory and using the above procedure. Figure 7 shows the optimized structures of ion-pair/ water systems. Clearly, water molecules interact through both

Figure 3. Same as Figure 2 but for [C4mim]BF4.

[C4mim]BF4/water binary system are arranged such that there is no abrupt change in the force acting at the interface of a water molecule interacting with a BF4−, whereas an abrupt change exists in the case of [C4mim]PF6/water. This explains the hydrophobicity of the latter over the hydrophilicity of the former IL. Table 4 allows the comparison between the charges obtained. The ion-pair/water interaction energy (Einter), which is defined as the difference between the energy of the ion pair 2070



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Figure 6. Optimized gas-phase structure of one water molecule adjacent to PF6 or BF4 anions at the B3LYP/6-311++G(d,p) level of theory. The partial charges are by CPMD.

Table 4. Comparison between Average Atomic Charges of the Anion the Water Molecule so Obtained [C4mim]BF4 atom B F OW HW

[C4mim]PF6

q/e B3LYP CHELPG

q/e CPMD ESP pure


atom

1.0894 −0.4894

1.0514 −0.4694

0.5582 −0.3261 −0.6700 0.3290

P F OW HW

q/e B3LYP CHELPG

q/e CPMD ESP pure


1.3982 −0.3782

1.3480 −0.3607

0.3160 −0.1820 −0.6760 0.3360

Figure 7. Structure of minimum energy found for the [C4mim]PF6, [C4mim]BF4, and [C2mim]Cl ionic liquid pairs with the water molecule calculated at the B3LYP/6-311++G(d,p) level. The distances are in angstroms.

[C4mim]PF6. This can be attributed to the strong induction and hence polarization effect by PF6− anion on the ring than BF4−, which accordingly is an indication of strong interaction between the [C4mim]+ cation ring and the PF6− anion. The charge on the C2 atoms of imidazolium ring is negative in all cases and becomes more negative upon the addition of water to the system. Therefore, the presence of water leads to the weaker interactions between the cation and the anion of both hydrophobic and hydrophilic ILs. Pair Distribution Function. The properties of mixtures are influenced drastically by its composition. Hanke and LyndenBell52 have shown that at low concentrations water molecules are isolated from IL or exist in small independent clusters. As the molar proportion of water molecules reaches 75%, a percolating network of waters is formed as well as some isolated molecules and small clusters, which change the structure and properties of the mixture dramatically. The water molecule in

anion and cation in the case of hydrophilic IL but just through the anion in the case of hydrophobic one. These results are consistent with the above charge analysis of anions and water in the ILs/water binary mixture simulated by CPMD. The sum of F atoms charges in PF6− anion is −2.2691e (by gas-phase DFT calculation, Table 2) and is −2.1644e (by bulk CPMD simulation, Table 2). The corresponding values for BF4− anion in [C4mim]BF4 IL are −1.9578e and −1.8778e, respectively. When mixed with water, the sum of F’s charge in BF4− and in PF6− anions drastically drops to −1.3046e and −1.0922e, respectively. The charge per F atom in PF6− anion is lower (both in the gas phase and in the bulk phase) than in the BF4− anion. This charge difference could lead to a stronger hydrogen bond formation between water molecules and BF4− anion than with PF6−. For the IL/water mixture, whereas the positive charge of the cation resides mostly on the butyl chain in [C4mim]BF4, it resides largely on the imidazolium ring in 2071



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the imidazolium-based ILs/water binary mixture is more strongly bound to the anion than to the cation via hydrogen bonding.53,54 Pair distribution functions (g(r)) of OW···OW and OW···F obtained from CPMD simulations of [C4mim]BF4/water and [C4mim]PF6/water mixtures are shown in Figures 8−11. These

Figure 10. Comparison between the OW···F g(r) values of CPMD and Classical MD in [C4mim]BF4/water mixture. Long distances omitted for clarity.

Figure 8. Comparison between the OW···OW g(r) values of CPMD and Classical MD in [C4mim]BF4/water mixture. Long distances omitted for clarity.

are compared, in the same Figures, with the results of classical MD simulations for the ensemble of 512 ILs and 1536 water molecules using DL_POLY package. As can be seen in Figure 8, the position and the intensity of the first peak from CPMD are very close to those from classical MD. The first CPMD peak positions of OW···OW in [C4mim]BF4/water and [C4mim]PF6/water mixtures are 2.63 (Figure 8) and 2.55 Å (Figure 9),

Figure 11. Same as Figure 10 but for [C4mim]PF6/water mixture.

Figure 9. Same as Figure 8 but for [C4mim]PF6/water mixture.

respectively. This indicates that water molecules tend to segregate and form clusters more extensively in the later mixture than in the former. In the same way, the mean distance of OW···F is shorter in [C4mim]BF4/water (2.65 Å, Figure 10) than in [C4mim]PF6/water (2.85 Å, Figure 11). It should be mentioned that the higher fluctuation seen for g(r) of CPMDs are due to small ensemble size. The first peak positions in g(r) of hydrogen bond donor− acceptor atoms (HW···F) are 1.64 and 1.90 Å for [C4mim]BF4/ water and [C4mim]PF6/water systems, respectively. (See Figure 12.) To some extent, these distances are within the domain of corresponding classical hydrogen bond length. Tian et al.55 have reported a length of 1.85 Å matching the classical

Figure 12. Comparison between g(r) of HW···F for [C4mim]PF6/ water mixture and [C4mim]BF4/water mixture by classical MD. Long distances omitted for clarity.

hydrogen bond in a system of NaBF4/water mixture. The HW···F interaction in [C4mim]BF4/water is of high probability and of low dynamics at the short-range distances but is structureless and unimportant at long ranges. This is quite in sharp contrast with the HW···F interaction in [C4mim]PF6/ water, which is of low probability, featureless, and of high dynamics at both short and long ranges. 2072



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In addition, the height of the first OW···OW peak in [C4mim]PF6/water shows enhancement as compared with [C4mim]BF4/water. A comparison with the results for pure water indicates that water molecules are more structured in both mixtures. (See Figure 13.)

The average number of neighboring atoms n within a sphere of radius R was obtained from the simulated radial distribution functions of OW atoms n(R ) = 4πρ

∫0

R

g (r )r 2 d r

(2)

In our case, R is the first minimum of the corresponding g(r), ρ is the particle density, and r is the interatomic distance. The coordination number of water molecules in [C4mim]PF6/water and in [C4mim]BF4/water is 2.69 and 2.17, respectively. The smaller water coordination number in the [C4mim]BF4/water system relative to the [C4mim]PF6/water provides criteria, in addition to g(r) values and spatial distribution, to conclude that water molecules segregate to form larger clusters when [C4mim]PF6 IL is placed in water. These results were achieved by CPMD simulation (of 15 H2O + 5 IL). Cluster formation can also be seen from the results of the classical and CPMD simulations using the spatial distribution functions. (See Figure 15.) Clearly, the water molecules are localized around the anion of [C4mim]BF4 but are absent around [C4mim]PF6 anion and tend to make domains in larger distances. Dipole Moment and Dielectric Constant. The presence of water has been known to influence the nanostructural organization of RTILs.59 The electronic structure of water molecules changes significantly in the presence of ILs. In fact, if a water molecule enters into a liquid matrix, it redistributes its electronic charge due to the direct interaction with neighboring molecules and the mean fields produced by the bulk. In turn, this molecule induces electron charge redistribution on the neighboring molecules according to the mutual spatial arrangement and perturbs locally the field produced by the bulk. When ILs dissolve in water, the local charge distribution of water molecules will be significantly modified due to the strong ionic character of ILs. The dipole moment is sensitive to both electronic polarization and structural relaxation and is used very often as a molecular descriptor and the physical indicator of the electronic and atomic structure. In fact, the dipole moment is an indicator for charge displacement as well as (being a vector) of local molecular packing. It sums up electronic and steric influences and describes the interplay of charge and shape. The distributions of dipole moment of neat water and of the water relaxed in the presence of [C4mim]PF6 and [C4mim]BF4 ILs are shown in Figure 16. The dielectric constant (a key parameter for modeling solvent behavior) is a state-point-dependent property of a substance and incorporates both short- and long-range spatial and orientational correlations. The dielectric constant of water is high; when an ionic compound is dissolved in it, the columbic interactions between ions are greatly reduced. The value of dipole moment plays a crucial role in determining the dielectric properties of liquid water. The average value of water dipole moment simulated (by CPMD) for the pure water, [C4mim]PF6/water, and [C4mim]BF4/water are 2.85, 2.45, and 2.32 D, respectively. It is well-established that the experimental dipole moment of water molecule in the gas phase is 1.86 D.60 Silvestrelli and Parrinello61,62 have found the average value of μ = 2.1D in the water dimer and μ = 2.4D in the water trimer. They also have found that the average dipole moment of a water molecule simulated in the bulk liquid of 64 water molecule is μ ≈ 3D, with large fluctuations around this value using the maximally localized Wannier function formalism.63,64 They have noted that as a consequence of charge transfer due

Figure 13. Comparison between g(r) of OW···OW in pure water and IL/water mixture by classical MD. Long distances omitted for clarity.

It was shown that the distance between atoms participating in a hydrogen bond can be employed to assign the hydrogen bond strength.56−58 Thus, the present study shows that the anion BF4− is capable to make a stronger hydrogen bond with water than the anion PF6−. This difference in the interaction strength can root from the difference in the charge on F’s atoms discussed previously. The g(r) of OW···HW could provide useful information about the number of hydrogen-bonded molecules. The positions of the first peak in this g(r) are 1.64 and 1.63 Å for [C4mim]BF4 and [C4mim]PF6/water mixture, respectively. (See Figure 14.) The corresponding value is 1.72 Å for pure

Figure 14. Comparison between g(r) of OW···HW in pure water and IL/water mixture by classical MD. Long distances omitted for clarity.

water. In particular, the first OW···HW peak, which is a signature of the hydrogen bonding, reveals that this bonding in [C4mim]PF6/water is sharper than in [C4mim]BF4/water and in pure water. The same conclusion can be drawn for the second neighboring shell. 2073



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Figure 15. Spatial distribution of anion and water molecule around the cation. (a) [C4mim]BF4/water: left, classical MD; right, CPMD. (b) [C4mim]PF6/water: left, classical MD; right, CPMD. Red, H2O; yellow, BF4−; green, PF6−; orange, C; white, H; blue, N.

The more the water molecules are isolated from each other, the more the value of the dipole moment tends to the gas-phase value. We calculated the value of the dielectric constant of water molecules from the simulation information involving the fluctuation of the total dipole moment of ensemble M = ∑pμp by εr = 1 +

4π (⟨M2⟩ − ⟨M ⟩2 ) 3VkBT

(3)

where V is the volume of simulation cell and kBT is the Boltzmann constant times the absolute temperature. The local dipole moments from the results of CPMD simulations are derived based on maximally localized Wannier centers (MLWCs).67 We analyzed 80 snapshots obtained from a 40 ps long CPMD run. First, the MLWCs denoting the center of charge of a two electron orbital are assigned to the molecules. Subsequently, the dipole moment |μp| of the pth molecule is calculated according to:

Figure 16. Simulated (CPMD) dipole moment distribution of neat water (dark blue solid line), water in [C4mim]PF6/water (red dotted line), and water in [C4mim]BF4/water (light blue dashed line).

to hydrogen bonding in the liquid phase the average OH intramolecular bond length is ∼2% larger than that in the isolated molecule, and thus the dipole moment is considerably larger than that in the gas phase. Maginn et al.65 have suggested that in the presence of low dielectric constant medium such as an IL66 the actual dipole moment of water should be lowered. Here we can relate this phenomenon to the formation of water clusters in ILs/water mixtures (see Figures 5 and consider coordination numbers). The size of the clusters is larger in the case of [C4mim]PF6 compared with the [C4mim]BF4 system.

3

|μp | =

n

m

∑ (∑ 2rij + ∑ qkrik)2 i=1

j=1

k=1

(4)

where the index i runs over all components (x, y, z) of the molecule’s dipole vector with n MLWCs and m nuclei of the charge qk. The coordinate components rij of the MLWCs and rik of the atoms are the corresponding components of the distance 2074



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vector between the geometric center of a molecule and the MLWC or nucleus, respectively. The simulated dielectric constant for pure water εr = 71.3 at 298.15 K, which could be considered as reference system. The results for water in [C4mim]PF6/water and in [C4mim]BF4/ water are εr = 63 and 46.5, respectively. Surface Reorientation of Hydrophobic ILs in the Presence of Water. The parameters used to simulate the pure ILs were found to exaggerate the IL/water mixing. Wipff et al.68 have modified these parameters by scaling down the partial charges to [C4mim]+0.9[PF6]−0.9 compared with the standard model of [C4mim]+1[PF6]−1, leading to a better agreement with the experiment. In previous sections, we showed that the CPMD-simulated partial charges on atoms constituting IL for an IL/water mixture are lower than those of pure IL due to screening effect of neighboring molecules. These charges are to some extent different from those of pure bulk CPMD simulation and the gas-phase ab initio calculation. We applied these charges to simulate the density profiles at hydrophobic/water and hydrophilic/water interfaces by classical MD. The application of these screened charges leads to faster dynamic of IL ion pairs and facilitates the equilibration. The atoms density profiles obtained from MD simulation were used to determine spatial positioning and average orientational ordering of the molecules at the liquid surface qualitatively. Whereas [C4mim]BF4 mixes readily at the start of simulation, the hydrophobic IL [C4mim]PF6 makes a distinct boundary with water. It should be mentioned that some degree of aggregation between water molecules is indeed in one phase region, yet this neither represents a two-phase system nor it does exhibit a clear phase separation due to finite size effects induced by the small box size in the CPMD simulations. The final configurations of these simulations are shown in Figures 17 and 18, and the relevant density profiles are shown in

Figure 18. Snapshot of classical MD cell of [C4mim]BF4 at the end of simulation.

Figure 19. Total density profile of [C4mim]PF6/water interface.

Figure 20. Total density profile of [C4mim]BF4/water interface.

Figure 17. Snapshot of classical MD cell of [C4mim]PF6 at the end of simulation.

formation of domains by IL and water. The average pressure tensor components (Pxx, Pyy, Pzz) for [C4mim]BF4/water are negligibly small ((3.11, 8.28, and 6.25) × 10−4, respectively), indicating that the system is homogeneous in three dimensions, whereas the tensors are appreciable for [C4mim]PF6/water ((5.28, 5.32, and 5.9) × 10−1, respectively), an indication of a definite phase separation (compare Figure 17 with 18). Understanding the distinction of cation orientation at the IL/ vapor and IL/water interfaces is important. The atoms density profiles obtained from MD simulations were used to determine spatial positioning and average orientational ordering of the molecules at the liquid surface qualitatively.

Figures 19 and 20. These densities were calculated from the sum of densities of each atom weighted by its atomic number. The density profile of [C4mim]PF6 (Figure 19) exhibits oscillations in the middle of the liquid bulk propagating to the surface and does not show appreciable enhancement in the subsurface region characteristics of most IL/vapor interfaces.69 This is evidence of tendency of [C4mim]PF6 molecules toward the bulk. For hydrophilic [C4mim]BF4 molecules, the density profile (Figure 20) shows a roughly a uniform distribution without any distinct interface. Extensive fluctuation of the water density profile concerted with that of IL can be attributed to the 2075



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constant of water (εr = 71.3 at 298.15 K) is very close to the experiments. A local depolarization of the water molecules mix with the ILs indicates a rather complete water miscibility of the [C4mim]BF4, whereas no appreciable miscibility can be detected for [C4mim]PF6. The dipole moment of water reduces to 2.32 D when mixed with [C4mim]BF4, whereas it reduces to 2.45 D when it is mixed with [C4mim]PF6. Overall, the extent of interaction of water with the anion can be used as criteria for the water miscibility of an In[C4mim]BF4, the water molecule is mainly around the imidazolium ring, where the anion resides contrary to [C4mim]PF6, in which water demonstrates an unfavorable interaction with the imidazolium ring.

The atoms density profile for [C4mim]PF6/vapor and [C4mim]PF6/water interfaces is shown in Figures 21 and 22, respectively. From these plots, it is clear that the butyl group tends to be on the vapor side but reorients in the presence of water preferentially toward the IL bulk.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +98 711 613 7353. Fax: +98 711 228 6008. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are indebted to the research council of the Shiraz University for the financial supports. Financial support from Enhanced Oil Recovery (EOR) Center of the College of Engineering is greatly acknowledged.

Figure 21. Atom’s density profiles of [C4mim]PF6 at equilibrated [C4mim]PF6/vapor at 450 K.

■

REFERENCES

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Figure 22. Atom’s density profiles of [C4mim]PF6 at equilibrated [C4mim]PF6/water at 450 K.

4. CONCLUSIONS We have performed ab initio MD simulation using Car− Parrinello approach for the pure bulk of [C4mim]PF6 and [C4mim]BF4 ILs and their mixtures with water to analyze charge distribution and molecular polarization. These results indicate that the main interaction of water is mainly with the anion of the ILs. The charge analysis shows that the BF4− anion can interact with the hydrogen of water molecule more effectively than with the PF6−, as justified by the similarity of charge values of F (in the former anion) and H molecules of water. These can shed light on the hydrophobic and hydrophilic nature of these ILs. The fluctuation in molecular dipole moments in the CPMD ensemble of IL/water mixture has been used to monitor the change in average dipole moment of water. The simulated dipole moment of pure water 2.85 D is close to the reported in literature (∼3D). In the same way, the simulated dielectric 2076



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Local Depolarization in Hydrophobic and Hydrophilic Ionic Liquids

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