Xinli You, Mangesh I. Chaudhari, Lawrence R. Pratt, Noshir Pesika, Kalika M. Aritakula, Steven W. Rick
Reported here are experimental and molecular dynamics simulation results for propylene carbonate as a solvent for electrochemical double-layer capacitors based on carbon nanotube forests. Propylene carbonate (PC) wets graphite with a contact angle of 31{\deg}. Molecular dynamics results agree with this result after reduction of the strength of dispersion attractions to the graphite C atoms by 40%, relative to the models used initially. A simulated nano-scale PC droplet on graphite displays a pronounced layering tendency and an Aztex pyramid structure for the droplet. Computed surface tensions of the PC liquid-vapor interface permit an extrapolative estimate of the critical temperature of PC that is accurate to about 3%. Average PC molecule binding energies, and their variances, are evaluated, and the distribution of binding energies is closely Gaussian. Evaluation of the density of the coexisting vapor then permits estimation of the excluded volume contribution to the PC chemical potential, and that contribution is about 2/3rds of the magnitude of the contributions due to attractive interactions, with opposite sign. PC molecules lie down flat on the PC liquid-vapor surface, and tend to project the propyl carbon toward the vapor phase. For close PC neighbors in liquid PC, an important packing motif stacks carbonate planes with the outer oxygen of one molecule snuggled into the positively charged propyl end of another molecule so that neighboring molecule dipole moments are approximately antiparallel. Calculated dielectric constants for liquid PC are in satisfactory agreement with experiment.
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http://arxiv.org/abs/1212.5304
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