(441b) Molecular Simulations of Ammonium-Based Protic Ionic Liquids

The structure and transport properties of a series of trialkylammonium triflate protic ionic liquids (PILs) and associated solutions involving neutral species were studied using molecular dynamics (MD). It is well know that the use of classical force fields where the ions carry net unit charges can lead to overstructured liquids and poor estimation of dynamical and transport properties, in particular too-high viscosities and too-low diffusion constants and ionic conductivities. The application of a uniform charge scaling factor can effectively address this issue, and is now commonly used. In this work we have determined the optimal scaling factors for several trialkylammonium triflate PILs simulated with the force field of Lopes and Padua (J. Phys. Chem. B 2004, 108, 16893) by comparison with recent experimental data from Yasuda et al. (J. Chem. Eng. Data 2013, 58, 2724). Simulations performed with optimized charge scaling showed generally good agreement with experimental data, and in all cases were much improved over the original force field. While diffusion constants could be very well-described over a large temperature range, even with optimized charge scaling the viscosity tended to be overestimated at low temperatures, and the ionic conductivity underestimated at most temperatures. We hypothesize that this is due to poor reproduction of the temperature dependence of the density; while the density near room temperature is well-reproduced, the thermal expansion coefficients of the simulated PILs are systematically low, leading to over-high densities at higher temperatures. The optimal scaling factors were found to vary significantly even between PILs where the cation differed only by a single methyl group. In the second part of this work, the solvation and dynamics of the neutral products of proton transfer reactions (trialkylamines and triflic acid) were studied in their parent PILs, in order to better understand how acid-base equilibria in PILs affects their properties.