Transferable Potentials for Phase Equilibria. 3. Explicit-Hydrogen Description of Normal Alkanes

Abstract

Motivated by shortcomings of the available united-atom models for alkanes, a new explicit-hydrogen model for n -alkanes (TraPPE-EH, transferable potentials for phase equilibria-explicit hydrogen) is developed from fitting to one-component fluid phase properties. In addition to Lennard−Jones sites on carbon atoms, this model utilizes Lennard−Jones sites on the centers of carbon−hydrogen bonds. Configurational-bias Monte Carlo simulations in the Gibbs and canonical ensembles were carried out to calculate the one-component vapor−liquid phase equilibria for methane to n -dodecane, to determine the phase diagram of supercritical ethane and n -heptane mixtures, to obtain the Gibbs free energies of transfer for n -pentane and n -hexane between helium vapor and n -heptane liquid phases, and to study the high-pressure region of the equation of state for n -pentane and n -decane. The explicit-hydrogen representation with its more faithful description of the molecular shape of alkanes allows us to find a set of Lennard−Jones parameters that yields significantly better agreement with experiment for one- and multicomponent phase equilibria than our united-atom alkane model, but the price is higher computational cost.