Thermodynamic properties of the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate from Monte Carlo simulations

Abstract

We report results from the first molecular simulation study of 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF 6 ], a widely studied ionic liquid. Monte Carlo simulations are carried out in the isothermal-isobaric (NPT) ensemble to calculate the molar volume, cohesive energy density, isothermal compressibility, cubic expansion coefficient and liquid structure as a function of temperature and pressure. A united atom forcefield is developed using a combination of ab initio calculations and literature parameter values. Calculated molar volumes are within 5% of experimental values, and a reasonable agreement is obtained between calculated and experimental values of the isothermal compressibility and cubic expansion coefficient. PF 6 anions are found to preferentially cluster in two favorable regions near the cation.

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…They obtained the radial distribution curves of dimethylimidazolium chloride and hexafluoro- [108] phosphate liquids using neutron diffraction and argued for charge ordering of ions in RTILs resembling what is found in the solid state. Charge ordering has also been discussed in a number of molecular dynamics computer simulation studies on 1-alkyl-3-methylimidazolium-based RTILs in recent few years [47,[142][143][144][149][150][151][152][153][154][155][156][157][158][159]. The radial distribution functions calculated in these papers all have suggested long-range charge ordering, giving support to the idea that RTILs are unique in having more structure ordering than do conventional molecular liquids.…”

“…Wide-angle X-ray scattering results on the [C 4 mim][I] room temperature ionic liquids show prominent peaks in the residual radial distribution curve [118], indicating certain periodical arrangements of the iodide anions. The existence of different rotamers and local structures has also been found by interpretation of results from optical heterodynedetected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [139], from neutron scattering and diffraction experiments versus temperature [111][112][113]140], from Coherent anti-Stokes Raman Scattering (CARS) [141] and by theoretical molecular dynamics calculations [43,47,[142][143][144]. The local structures may also lead to other unique properties of ionic liquids; for example, if magnetic anions are aligned in RTILs, novel magnetic liquids will be created [145,146].…”

“…Early studies probed the nature of interactions in socalled first-generation chloroaluminate ionic liquids [40], and now also information becomes available for second generation, air-stable systems, such as the imidazolium mixtures. Many RTIL studies have been directed towards possible applications [8, 11, 41]: a number of reports have been published covering theoretical aspects [42][43][44][45][46][47], X-ray crystallography of the frozen melts [48][49][50][51][52], NMR relaxation, conductivity, viscosity, diffusion [4,28,[53][54][55][56][57][58][59], and gas solubilities in ionic liquids [22,60]. Hydrogen-bonding is known to occur between the cations and anions in most ionic liquids, as demonstrated by several investigators on e.g., systems containing 1-alkyl-3-methylimidazolium.…”

The simulation of imidazolium-based ionic liquids†

“…[13] By 2004 a range of potentials based upon each of the main force field codes had appeared, some also offering improved flexibility and a more detailed representation of the cation and anion. [11,12,15,17,21, 34] At the same time "experiments" in modelling charge transfer [24,25], and including polarization effects [73] had been carried out. In some cases simplified potentials have been strategically chosen to allow for long simulation times of large volumes (864 ion pairs for 16ns).…”

“…The partial charges used in the simulation of imidazolium based ionic liquids has varied particularly widely. [12,33] Specifically the charges associated with the nitrogen atoms of the imidazolium ring vary from postive [11,21,30, 34] through neutral [15,17] to negative [12,13,33] depending on the parametrisation. Urahata and Ribeiro have commented upon this inconsistency, but they also note, however, that the dipole moments obtained from the calculations.…”