Over the past decade, room temperature ionic liquids (RTIL) - molten salts characterized by melting points below 100 celsius - have increasingly atracted interest as "green" solvents for many industrial applications, specially due to their particular physicochemical properties [1,2]. The main feature of these special molten salts is that they can be tailored by customizing their properties through varying the structures of cations or anions and their combinations [1,2]. Theoretically, RTIL have been mainly studied through simulation techniques of molecular models based on ab initio calculations. These simulation studies focused on testing molecular force fields for estimating dynamical properties, liquid structure, solvation, conductivity, gas solubility, etc [3]. To the best of our knowledge, up to date there is no theoretical work related to the phase transitions of these liquids. Nevertheless, recent experimental studies have probed that some families of ionic liquids can be distilled, and estimations of the critical parameters have been reported [4], which is very helpful to test new molecular models and force fields. Besides, there is strong experimental evidence that the RTIL show different liquid crystalline phases by changing the alkyl chain length of the cation [5].

The aim of the present work is to develop a simple model to describe the phase behavior of the RTIL. We have studied by computer simulations the liquid-vapor transition of a very rough approach of these liquids, where the cation is modeled as a rigid spherocylinder with a positive point charge in the center of one of the extremal hemispheres. On the other hand, the anion is described as a hard sphere with a negative point charge placed at the center. Specifically, we have analized the influence of the lenght of the spherocylinder in the critical parameters and the coexistence curve. The proposed model can be considered as an extension of the Restricted Primitive Model (RPM) of electrolytes. The techniques previously developed for the RPM have been applied and modified for this particular case [6]. The results show a shift of the coexistence curve towards lower temperatures and densities as the spherocylinder lenght is increased. This is in qualitative agreement with recent experimental findings [4]. The structural properties of the system have also been examined, revealing a highly structured vapor phase with an increasing tendency to form large chain and ringlike clusters as the spherocylinder is longer. This may have important implications in the physicochemical properties of RTIL.

References

[1] J. D. Holbrey and K. R. Seddon, Clean Products and Processes, vol 1, 223 (1999)

[2] H. Zhao, Chem. Eng. Comm., vol 193, 1660 (2006).

[3] C. G. Hanke, S. L. Price and R. M. Lynden-Bell, Molec. Phys, vol 99, 801 (2001); J. K. Shah and E. J. Maginn, Fluid Phase Equilibria, vol 222-223, 195 (2004).

[4] M. J. Earle et al, Nature, vol 439, 831 (2006); L. P. N. Rebelo et al, J. Phys. Chem. B, vol 109, 6040 (2005).

[5] K. Binnemans, Chem. Rev., vol 105, 4148 (2005).

[6] G. Orkoulas and A. Z. Panagiotopoulos, J. Chem. Phys., vol 101, 1452 (1994); A. Z. Panagiotopoulos and S. K. Kumar, Phys. Rev. Lett., vol 83, 2981 (1999); J. M. Romero-Enrique, G. Orkoulas, A. Z. Panagiotopoulos and M. E. Fisher, Phys. Rev. Lett., vol 85, 4558 (2000); J. M. Romero-Enrique, L. F. Rull and A. Z. Panagiotopoulos, Phys. Rev. E, vol 66, 041204 (2002); M. Martin-Betancourt, J. M. Romero-Enrique and L. F. Rull, to be submitted (2007).