Molecular models and reactive Monte Carlo simulations of protic ionic liquids

N. C. FORERO MARTÍNEZ AND M. G. DEL PÓPOLO

Dublin, Republic of Ireland

Congreso; CECAM meeting: Computational models of room temperature ionic liquids; 2009

The liquid-vapor  transition in Coulomb fluids is a subject of long standing interest in condensed matter physics. Unfortunately, its experimental investigation in solvent-free electrolytes has been seriously hampered  by the extreme conditions under which the transition normally occurs. A new series of organic molten salts, collectively known as protic  ionic liquids (PILs), offer a testing ground to investigate condensation in ionic systems at relatively low temperatures and pressures. PILs form via  proton transfer between acid and base molecules and the resulting ions  condense into a fluid  whose composition can be tuned by adjusting thermodynamic parameters.  Experiments show that most PILs boil before the onset of thermal decomposition, and that vaporisation is accompanied by the reversal of the ionic/molecular equilibrium. The phase change is thus concomitant  with a significant change in bonding character of the components as the liquid phase is mainly ionic while the vapor is molecular.  We have designed a molecular model and a Monte algorithm to simulate the gas/liquid transition in Ethylamonium Nitrate (EAN),  a prototypical and very well known PIL.  Ab-intio computations (DFT and MP2) have been performed to obtain ground state energies and structures of the EAN crystal, clusters,  isolated ions and acid/base molecules . We  discuss force-field details, its parameterisation and testing, as well as the the implementation of the  reactive Monte Carlo simulations. Our results indicate that  the single EAN pair is unstable and spontaneously transforms into nitric-acid  and ethylamine, the molecular constituents of the vapor phase. Iconicity appears as soon as additional molecules are added to the smallest molecular cluster, pointing to the internal field as the factor stabilising the ionic phase.