Molecular Modeling and Experimental Studies of the Thermodynamic and Transport Properties of Pyridinium-Based Ionic Liquids

A combined experimental and molecular dynamics study has been performed on the following pyridinium-based ionic liquids:  1-n-hexyl-3-methylpyridinium bis(trifluoromethanesulfonyl)imide ([hmpy][Tf₂N]), 1-n-octyl-3-methylpyridinium bis(trifluoromethanesulfonyl)imide ([ompy][Tf₂N]), and 1-n-hexyl-3,5-dimethylpyridinium bis(trifluoromethanesulfonyl)imide ([hdmpy][Tf₂N]). Pulsed field gradient nuclear magnetic resonance spectroscopy was used to determine the self-diffusivities of the individual cations and anions as a function of temperature. Experimental self-diffusivities range from 10^-11 to 10^-10 m²/s. Activation energies for diffusion are 44−49 kJ/mol. A classical force field was developed for these compounds, and molecular dynamics simulations were performed to compute dynamic as well as thermodynamic properties. Evidence of glassy dynamics was found, preventing accurate determination of self-diffusivities over molecular dynamics time scales. Volumetric properties such as density, isothermal compressibility, and volumetric expansivity agree well with experiment. Simulated heat capacities are within 2% of experimental values.