Phase diagrams for a multistep potential model of n-alkanes by discontinuous molecular dynamics and thermodynamic perturbation theory

Discontinuous molecular dynamics simulations and thermodynamic perturbation theory have been used to study thermodynamic properties for chain molecules. A multistep potential model is considered in this work through characterization of the vapor pressure and saturated liquid density for the n-alkanes from ethane to octane. The multistep potential model provides similar to4% accuracy for the entire molecular weight range. By contrast, previous studies of the square-well potential showed that the range of the square-well needed to vary with molecular weight to provide comparable accuracy. Therefore, having multiple steps with different depths for each functional group appears to be an essential part of making the potential model transferable. The depths of the wells vary such that the CH2 potential appears flatter than the CH3 potential, in an apparent attempt to moderate the overlaps implicit in the interaction site model. (C) 2002 American Institute of Physics.