as solvents in interaction-rich systems. It focuses on the solvation of aromatic compounds of varying polarity in ionic liquids. Experimental information, including solubility, phase diagrams, and structural data, is now available for the entire series of fluorinated benzenes in the ionic liquid 1-ethyl- 3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide, [C2mim][NTf2]. This aromatic series is composed of 13 molecules ranging from benzene to perfluorobenzene and includes all possible partially fluorinated species. Such a rich data set provides a unique test ground for studying the roles of the dipole and quadrupole moments on the interactions of aromatic molecules with an ionic liquid, and thus improves our fundamental knowledge of these sophisticated solvents. Ionic liquids are not simple fluids: they are composed of asymmetric, flexible ions with delocalized electrostatic charges and a presence of both charged and nonpolar groups in their molecular structures. Their behavior as solvents is a consequence of a balance between Coulomb and van der Waals forces, a fact that can lead to structurally heterogeneous liquids.1 The solubility of benzene and other arenes in [C2mim][NTf2] was first reported by Lachwa et al.2 Very recently, Shiflett and Yokozeki3 measured the solubility of benzene and of its 12 fluorinated derivatives in the same ionic liquid. In this comprehensive study, the authors were able to fit the experimental liquid-liquid equilibrium data to the NRTL (nonrandom two liquids) solution model and also to establish a general empirical correlation between the dipole moment of the aromatic solutes and the immiscibility gaps in the corresponding mixtures with ionic liquids. However, that general trend did not hold for many of the aromatic compounds with null or small dipole moments and the authors admitted that it would be necessary to consider additional intermolecular interactions. They stated that “This poses a unique and interesting challenge for theoretical modelers to explain these measurements.” In this work, we have decided to accept that challenge. We have used ab initio calculations and molecular dynamics simulations to explain from a molecular perspective the rather complex behavior of the binary mixtures of the ionic liquid [C2mim][NTf2] with benzene and its 12 fluorinated derivatives, and we believe that the insights obtained in the present work are general and applicable to interpret results of other solutions and mixtures involving ionic liquids. Simulations of mixtures of ionic liquids with benzene and some of its fluorinated derivatives were first reported by Lynden-Bell and co-workers.4 In recent studies, we also discussed the liquid-liquid and solid-liquid phase behavior of binary mixtures of different ionic liquids with benzene and two of its fluorinated derivatives, hexafluorobenzene and 1,3,5-trifluorobenzene.5 The corresponding phase diagrams have shown liquid-liquid immiscibility windows completely shifted toward aromatic-rich compositions; i.e., the three studied aromatic compounds are quite soluble in the pure ionic liquids, but no measurable amount of the latter can be dissolved in any of the pure aromatic compounds. The phase diagrams also exhibited different kinds of interesting solid-liquid behavior, ranging from the occurrence of eutectic points, the existence of congruent melting points and the corresponding formation of inclusion crystals, or the observation of different ionic liquid crystalline phases (polymorphism). The different types of phase behavior were probed at a molecular level using X-ray diffraction techniques and by molecular dynamics simulations.5 The structural data thus obtained * Corresponding author. E-mail: jnlopes@ist.utl.pt. † Universidade Nova de Lisboa. ‡ Instituto Superior Te´cnico. § CNRS/Universite´ Blaise Pascal. J. Phys. Chem. B XXXX, xxx, 000 A 10.1021/jp903556q CCC: $40.75 XXXX American Chemical Society Downloaded by AUSTRIA CONSORTIA on July 6, 2009 Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/jp903556q revealed the interactions of ionic liquids with aromatic compounds, and how these are affected by the nature of the aromatic compounds (namely, the reversal of the molecular quadrupole moment when switching from benzene to hexafluorobenzene), the aromatic or nonaromatic character of the cation of the ionic liquid, and the size of the corresponding anion. 2. Computational Details Ab Initio Calculations. Multipole moments and electrostatic charge distributions were calculated using Gaussian 036 at the MP2/cc-pVTZ(-f)//HF/6-31G(d) level of theory, thus using the same basis set as in the OPLS-AA model for perfluoroalkanes7 and for ionic liquids.8,9 The cc-pVTZ(-f) basis set10 was used for single-point energy calculations in geometries optimized at the HF/6-31G(d) level, as is current practice in the development of force field parameters for molecular simulation.11,12 For the C and N atoms, the cc-pVTZ-(-f) basis set is created by removing the f functions from the definition of the triple-ccpVTZ basis set of Dunning.10 The combination of the levels of theory and basis sets used here has been tested on a large collection of molecules (Halgren test) and was demonstrated to yield accurate conformational energetics.13 It must be stressed that no constraints were placed in any of the molecules during geometry optimization or point charge calculation in order to reproduce a specific molecular moment. The point is that, although the theoretical results give slightly overestimated dipole and quadrupole moments (see the Results and Discussion section below), they form a consistent set for the 13 aromatic molecules under discussion. Moreover, and given the empirical nature of the correlation that will follow (see Results and Discussion), any differences between the experimental and theoretical electrical moments will be absorbed by the fitting of the empirical parameters. Molecular Dynamics Simulations. The molecular force field used to represent the ionic liquids is based on the OPLS-AA model14 but with parameters specifically tailored for the ions in question.8,9 Following the spirit of OPLS-AA, intramolecular terms related to covalent bonds and angles are taken from the AMBER force field,11 and efforts are concentrated on carefully describing conformational and intermolecular terms. Benzene and its derivatives were represented also within the OPLS framework using the parametrization proposed by Jorgensen14 and the CHelpG charges obtained in this work (see previous section). The full set of parameters is given as Supporting Information. All simulations were performed using molecular dynamics algorithms, implemented in the DL_POLY program.15 In the case of the benzene and hexafluorobenzene mixtures, we started from low-density initial configurations, composed of 192 ion pairs and 64 solute moleculessan ionic liquid mole fraction of 0.75. These were equilibrated at constant NpT for 500 ps at 300 K using a Nose´-Hoover thermostat and barostat with time constants of 0.5 and 2 ps, respectively. Electrostatic interactions were treated using the Ewald summation method considering six reciprocal-space vectors, and repulsive-dispersive interactions were explicitly cut off at 16 Å (long-range corrections were applied assuming the system had a uniform density beyond this cutoff radius). The final configurations of these preequilibrated systems, namely, the one containing hexafluorobenzene, were used to generate the initial configurations of all other systems containing the partially fluorinated benzene solutes, also with ionic liquid mole fractions of 0.75. Further simulation runs of 100 ps were used to produce re-equilibrated systems at the studied temperatures. Finally, 1000 configurations were stored from production runs of 300 ps for each one of the possible 13 systems. Successive 300 ps runs of each system showed no drift in the corresponding equilibrium properties at this stage. The stored configurations for each system were used to generate the spatial distribution functions (SDFs) presented.
