(239b) Computing the Melting Point of Ionic Liquids from Atomistic Simulations

Ionic liquids are distinguished from conventional molten salts by their melting point ? generally agreed to be below 100^oC. There are currently no reliable methods available to predict melting points of ionic liquids, nor do we have a good understanding of how chemical and structural features affect melting points. Thermodynamic integration based methods have been a useful tool to calculate melting points using molecular simulations, but these methods have not been applied to complex systems such as ionic liquids. We extended the methods developed by Eike and Maginn[1,2] to calculate the melting temperature of the ionic liquid 1-n-butyl-3-methylimidazolium chloride. This ionic liquid exhibits two crystal polymorphs, an orthorhombic and a monoclinic structure. There is disagreement in the literature as to the relative stability of these two structures. We computed the melting point for the ionic liquid using two different force fields, and found that parameterization of a particular dihedral potential affects the relative stabilities of the two polymorphs.

The calculated melting point of the orthorhombic polymorph of the ionic liquid was 92±6 ^oC, which is slightly higher than the experimentally determined melting point of 66^oC. We calculated a melting point of 100 ±4 ^oC for the monoclinic crystal phase, which compares with the experimentally predicted melting point of 45-65 ^oC. Our calculations suggest that the monoclinic phase has a slightly lower free energy than the orthorhombic phase, but the magnitude of this difference is highly dependent on the force field parameterization. We have also studied the effect of various parameters involved in the thermodynamic integration, and recommend values that give the most reliable melting point.

[1] D. M. Eike, J. F. Brennecke, and E. J. Maginn, J.Chem. Phys. 122, 014115 (2005).

[2] D. M. Eike and E. J. Maginn, J. Chem. Phys. 124,164503 (2006).