STUDY ON THE INFLUENCE OF INTERNAL DEGREES OF FREEDOM IN MOLECULAR MODELS ON THE THERMO-PHYSICAL PROPERTIES

Abstract

With the increasing computer power nowadays it becomes possible to calculate and predict thermophysical properties by molecular simulations. Basis of these simulations are always the description of the interactions between particles on the molecular level via so called molecular models or force fields. Upto date at the Institute of Techni-cal Thermodynamics and Thermal Process Engineering (ITT) molecular models have been developed for a set of small molecules where the internal degrees of freedom were neglected. In the present work the influence of the internal degrees of freedom on the thermophys-ical properties of molecular models especially on the vapor-liquid equilibria is deter-mined. For this study the alkane series is chosen as example starting from butane up to decane. The interactions are described using the Transferable Potentials for Phase Equilibria-United Atom (TraPPE-UA) force field in the standard and two modified ver-sions regarding the internal degrees of freedom, to simulate a flexible model, a model where only torsion is allowed, and a fully rigid model, respectively. Gibbs ensemble Monte Carlo (GEMC) simulations including configurational-bias Monte Carlo (CBMC) moves are performed to calculate the vapor-liquid coexistence curves (VLCC) for butane, iso-butane, pentane, and hexane for a total of six to eight temper-atures for different molecules. The TraPPE-UA force field shows small, systematic de-viations from the experimental saturated vapor pressures and densities for all molecules and it slightly overpredicts the critical temperatures while only a neglectable depen-dence on the internal degrees of freedom is seen. For decane NpT and NVT ensemble simulations are performed for the liquid box only showing considerable deviations of the rigid model but no difference between the flexible and "just torsion" description of the molecule. A simulation of binary mixture iso-butane + n-butane gives no further information on the performance of the three different model descriptions due to difficulties in the simulation.