Reports: ND655150-ND6: Thermodynamics of Acid-Base Equilibria in Protic Ionic Liquids

Lev D. Gelb, University of Texas, Dallas

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. PILs are formed by proton transfer between acidic and basic species, and are promising as “green” solvents, as catalysts, in fuel cells and electrochemical energy storage, and in carbon capture and other applications. Acid-base equilibria in PILs are critical to their successful application, but are complex and incompletely understood.

In the first year of the project, we completed a study concerning the properties and quality of available PIL force fields for the proposed calculations, including investigation of how to choose the optimal charge scaling factor to best reproduce experimental results. This research was presented at the AIChE 2016 Annual Meeting and ACS 2016 Southwestern Regional Meeting, and described fully in the paper “Structural and Transport Properties of Tertiary Ammonium Triflate Ionic Liquids: A Molecular Dynamics Study,” A. Taghavi Nasrabadi and L. D. Gelb, J. Phys. Chem. B 121 (2017) 1908-1921.

We have since completed two studies; a paper on the first is nearly ready for submission and the second is also being written up; both were also described in A. Taghavi Nasrabadi's PhD thesis, which was successfully defended in August 2017.

The first study is an in-depth look into how the properties of selected alkylammonium acetate PILs vary with the degree of proton transfer, denoted χ. 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. Three PILs were considered: two primary ammonium acetate PILs, N-propylammonium acetate [N3][Ac] and N-butylammonium acetate [N4][Ac], and one tertiary ammonium acetate, N-dimethylbutylammonium acetate [N114][Ac]. Isothermal-isobaric simulations were used to determine PIL properties under conditions ranging from no proton transfer χ=0 (a neutral solution of acid and base molecules) to complete proton transfer χ=1 (a fully ionic solution). The density and viscosity increase with increasing χ, due to stronger electrostatic interactions in more ionic solutions. The self-diffusion coefficients of all species likewise decrease with χ. Ionic conductivity shows a complex profile, increasing at low χ but passing through a maximum and decreasing as the solution become even more ionic. This is due to competition between the increasing number of charge carrier (ions) as χ is increased and their decreasing mobility. The diffusion constants of cations and anions are nearly the same in all of these PILs, indicating that correlated motion of ions is significant. These correlations are stronger at lower χ which may indicate the presence of ion pairing; this is consistent with the results of Walden plots, which show that the ionicities of these PILs increase with increasing χ. Finally, simulations results were used to estimate χ in experimental systems through an interpolation procedure, by attempting to determine the χ at which simulated values for viscosity, conductivity, etc., match experimental ones. This gave χ~1.0 for both [N3][Ac] and [N4][Ac] and χ~0.17 for [N114][Ac] suggesting that proton transfer is nearly complete in the primary PILs but not in the tertiary PIL, consistent with available (qualitative) experimental observations.

In the second study we examined proton transfer equilibria in trimethylammonium acetate, [N111][Ac]. As originally proposed, we used a thermodynamic cycle to determine the reaction free energy in the PIL by reference to aqueous solution, where the reaction free energy is known. The reaction free energies in water and in the PIL are connected by the “transfer free energies” for each participating species between the two media, which requires calculation of four separate transfer free energies. These are obtained using two-phase simulations in which a test molecule is moved across the water/PIL interface and the mean force normal to the interface measured as a function of position. A rigorous thermodynamic approach was used in which the reaction free energy in the PIL solution was itself determined as a function of the degree of proton transfer χ, from which the equilibrium χ value for this PIL was then determined to be χ=0.24 at room temperature and standard pressure. In partial confirmation of this result, we also found that the experimental viscosity of [N111][Ac] (which depends strongly on χ) is reproduced to within statistical uncertainty at this χ, and that the similar [N114][Ac] PIL (studied above) is also only partially proton-transferred. This study was quite computationally intensive; even for this relatively simple PIL approximately 13 microseconds of simulation time were required, which used up 13,000 node-hours (or about 300,000 core-hours) on the Lonestar5 supercomputer at the Texas Advanced Computing Center. Nonetheless, this is the first rigorous determination of the equilibrium state of the proton-transfer reaction in a protic ionic liquid, and the methods used should be effective for other PILs.

We are presently working to extend this study to other ionic liquids and to optimize our computational protocols so as to reduce the time requirements for these calculations; these efforts will likely consume most of the current (extended) year.