Molecular Dynamics Simulations of Protic Ionic Liquids

This dissertation concerns the study of protic ionic liquids (PILs) by means of molecular dynamics (MD) simulations. PILs are a subset of ionic liquids in which cations possess an acidic proton. They have been a focus of intense research in the past decade mainly due to their promising properties. In this dissertation, we first begin with an introduction to PILs, and briefly review their properties and applications in Chapter 1. In Chapter 2, the main features of MD simulations are explained. Empirical force fields used in simulation studies of ILs have often failed to correctly describe their dynamics and transport properties. Chapter 3 describes how to improve the ability of well-known empirical force fields to describe the dynamical properties of tertiary ammonium triflate PILs, by scaling the atomic partial charges of ions using an optimal scaling factor derived from experimental data. Our results show that this method successfully enhances the dynamics of the simulated PILs, and improves the computed transport coefficients without increasing the computational cost. The degree of proton transfer in PILs, which indicates the percentage of ions formed from the reactant acid and base molecules, is a key quantity in this field. However, the effects of this quantity on PIL properties are still not well-understood. In Chapter 4, we try to understand these effects by simulating a family of alkylammonium acetate PILs over the entire range of the degree of proton transfer. Our results show that properties of PILs change dramatically with varying degree of proton transfer. We also use the data obtained to estimate the degree of proton transfer in experimental PILs by comparison with simulation. In Chapter 5, we introduce a rigorous thermodynamic approach by which to calculate the degree of proton transfer and equilibrium constants in PIL media. Our approach is based on a thermodynamic cycle and uses constrained MD simulations to obtain the free energy change associated with the proton transfer reaction. We apply this scheme to trimethylammonium acetate, a tertiary ammonium acetate PIL known for their low degree of proton transfer. Our results show that the proton transfer takes place only partially in this PIL, which is in good agreement with experiment. This approach can effectively be used to predict the degree of proton transfer and equilibrium constant in PILs with variety degrees of proton transfer, which is difficult to assess experimentally. In Chapter 6, we briefly summarize our findings and elaborate on possible future applications of the computational approaches used in this dissertation.