Monte Carlo simulation of branched alkanes and long chain N-alkanes with anisotropic united atoms intermolecular potential

The anisotropic united atoms potential for linear alkanes proposed by Ungerer (J Chem. Phys., 112, 5499, 2000), called AUA4, has been used to predict several equilibrium properties (vapour pressure, vaporisation enthalpies, and liquid densities) of alkanes by Gibbs ensemble Monte Carlo simulation. In order to extend the potential to branched alkanes, potential parameters for the CH group have been determined by optimisation on the basis of equilibrium properties of isobutane, keeping the same parameters as AUA4 for the CH3 groups. The resulting CH parameters form a regular sequence with those previously determined for CH3 and CH2 groups, so that a physically consistent parameter set is obtained. Simulations have been performed at temperatures ranging from 450 to 800 K for long n-alkanes (C20, C25 and C30) and from 350 to 450 K for four heptane isomers (n-heptane, 2-methylhexane, 2,4-dimethylpentane and 2-ethylpentane). In order to achieve internal relaxation of long chains with a good efficiency, a specific Monte Carlo move was used in which a united atom is rotated around its nearest neighbours. Equilibrium properties of long chain alkanes are well predicted, and small differences between heptane isomers are represented with a good accuracy. It is concluded that the AUA4 potential shows an interesting degree of transferability.