61st Annual Report on Research 2016

In this Project we are studying the thermodynamics of acid-base equilibria in protic ionic liquids (PILs) by molecular simulation. Ionic liquids are excellent solvents with many desirable properties and are a focus of intense research. Protic ionic liquids are used as “green” solvents, as promoters or catalysts in chemical synthesis, in fuel cells, in biochemistry, and in chemical analysis. Their ability to donate and accept protons is critical in many of these applications, but is not well understood.

In this first year, we have been primarily concerned with evaluating the properties and quality of available PIL force fields for the proposed calculations. As such, 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. 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 show generally good agreement with experimental data, and in all cases are much improved over the original force field. We have found that, 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. A manuscript describing this work is nearly ready for submission, and presentations describing it will be given at the AIChE 2016 Annual Meeting and ACS 2016 Southwestern Regional Meeting.

In subsequent work we have examined the properties, in particular ionic conductivity and viscosity, of selected alkylammonium acetate PILs. Unlike the triflates in the previous study, in these PILs experimental evidence shows that proton transfer between acid and base components is incomplete, so that there is a significant concentration of neutral species present. In these simulations (which are not yet complete) we measure various properties as a function of the degree of proton transfer. We have found that the viscosity is significant reduced by the presence of the neutral (that is, protonated acid and deprotonated base) species, which leads to an interesting maximum in the conductivity near 50% proton-transfer. That is, even though the fully-proton transferred solution has much greater concentration of charge carriers, the increased viscosity they produce lowers the ionic conducivity. This study should be completed in Fall 2016, after which work will begin on free-energy calculations for predicting the degree of proton transfer a priori.