Interfaces of propylene carbonate

Abstract

Propylene carbonate (PC) wets graphite with a contact angle of 31• at ambient conditions. Molecular dynamics simulations agree with this contact angle after 40% reduction of the strength of graphite-C atom Lennard-Jones interactions with the solvent, 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. Extrapolation of the computed tensions of PC liquid-vapor interface estimates the critical temperature of PC accurately to about 3%. PC molecules lie 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. The calculated thermal expansion coefficient and the dielectric constants for liquid PC agree well with experiment. The distribution of PC molecule binding energies is closely Gaussian. Evaluation of the density of the coexisting vapor then permits estimation of the packing 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.

Assessment of Simple Models for Molecular Simulation of Ethylene Carbonate and Propylene Carbonate as Solvents for Electrolyte Solutions

“…The liquid density of standardly simulated PC at p = 1 atm is several percent denser [44] than the experimental value near T = 300 K. The thermal expansion Fig. 2 (left) For PC molecules in the liquid-vapor interfacial layer at T = 300 K, the probability density for projection of the unit vector normal to the carbonate plane onto the axis, perpendicular to the interface.…”

“…This feature is likely to be specifically important for electrolytesinvolving free ions-but we emphasize that dispersive van der Waals interactions are modeled separately in these forces fields. We note in passing that establishment of saturated solution conditions, perhaps involving ion pairing as a mechanism for phase separation, is primarily sensitive to (attractive) dispersive van der Waals interactions [43][44][45]. As with the common empirical force fields for liquid water simulation [3][4][5], the non-polarizable force field parameters should be recognized as effective values that approximate the outcomes obtained with more complicated force fields.…”

“…We will collect and discuss molecular simulation results on the EC and PC liquids and on solutions with electrolytes relevant to EDLC capacitors based on CNT forests [44,[46][47][48]. We will emphasize macroscopic characteristics that are often considered in discussions of such applications, particularly interfacial structure, dielectric responses, and molecular mobilities.…”

“…The vapor pressures of EC and PC are low in regimes of practical use [44,66], and thus they are strongly bound liquids. We further characterize [46] this 'strongly bound' quality by the ratio T c ∕T t of their critical temperatures to their triplepoint temperatures.…”

“…2 (left) For PC molecules in the liquid-vapor interfacial layer at T = 300 K, the probability density for projection of the unit vector normal to the carbonate plane onto the axis, perpendicular to the interface. The inset in upper-right corner of that panel indicates the slab geometry used for these calculations [44,65]. The 'outer' (vapor) direction corresponds to projections near 1.0.…”