(220d) Reaction Ensemble Monte Carlo Simulations of Protic Ionic Liquid Formation

Mullen, R. G., University of California
Maginn, E., University of Notre Dame
A protic ionic liquid (PIL) is formed when an acid and base exhibit a high degree of ionization upon mixing

AH + B → [A—] [BH+]

and the mixture, by convention, has a melting point below 100 °C. The ionization of an acid-base pair is difficult to predict. The difference between the aqueous pKa of the acid and that of the base has shown some correlation with the boiling point of the mixture [1]. However, when paired with the same acid, bases with similar pKas may exhibit very different ionic conductivities [2].

Reaction ensemble Monte Carlo is one promising method to predict which acid-base pairs form PILs. The challenge lies in developing Monte Carlo moves that efficiently insert and delete charged species into an ionic liquid environment. Simple insertions, semi-grand insertions [3], and state-of-the-art continuous fraction component (CFC) insertions all fail at such a task. We present a combination semi-grand/CFC move that succeeds and demonstrate its effectiveness on ethylammonium nitrate and a series of bases paired with acetic acid.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Document release number LLNL-ABS-749853.


  1. Yoshizawa, W. Xu and C. A. Angell, “Ionic Liquids by Proton Transfer: Vapor Pressure, Conductivity, and the Relevance of ΔpKa from Aqueous Solutions”, J. Am. Chem. Soc., 2003, 15411-15419
  2. Stoimenovski, E. I. Izgorodina and D. R. MacFarlane, “Ionicity and Proton Transfer in Protic Ionic Liquids”, Phys. Chem. Chem. Phys., 2010, 12, 10341-10347
  3. G. Mullen and E. J. Maginn, “Reaction Ensemble Monte Carlo Simulations of Xylene Isomerization Under Confinement”, J. Chem. Theory Comput., 2017, 13, 4054-4062.