Vapor–liquid equilibria of vibrating square well chains

Abstract

Vibrating square well (SW) 2-mer, 4-mer, and 8-mer with average reduced bond lengths of 0.97±0.03, 0.60±0.03, and 0.40±0.03 were studied by discontinuous molecular dynamics (DMD) simulation in the NVE ensemble. Average bond angles for the reduced bond length of 0.4 were constrained to 127±16° while the longer bond lengths were freely jointed. Vapor–liquid equilibria of the vibrating SW fluids were determined based on DMD simulation by isochoric integration and compared to that of rigid SW chains from Gibbs ensemble Monte Carlo (MC) simulation. The binodals of vibrating chains show a shift to higher temperatures relative to rigid chains, reflecting their less repulsive (more attractive) nature. Vapor pressures of the vibrating chains were computed through isochoric integration with Clausius–Clapeyron consistency to 5% or better. Vapor pressure behavior for each chain model was characterized in terms of critical temperature, critical pressure, and acentric factor. The trend in acentric factor vs. chain length showed that shorter bond lengths gave improved agreement with the experimental trend for n-alkanes. Nevertheless, the trends in acentric factor did not support any molecular model for alkanes which represented methylene segments as individual SW interaction sites. If SW chains are to be applied as models of alkanes, each interaction site must be assigned more than one methylene segment.