Packing effects on the conformational equilibrium of alkanes

Abstract

The equation of state and the conformational equilibrium of repulsive models of n-hexane, n-heptane and n-octane are investigated by the method of molecular dynamics simulation. For repulsive models the simulation results show that the conformational population at liquid densities differs from that of the gas. Packing effects enhance the population of chains in more spherical conformations, which are in general those presenting a large fraction of bonds in the gauche state. Good agreement is found when comparing the simulation data with a recently proposed equation of state for hard n-alkanes models. Furthermore, the theory predicts correctly the shift in the conformational equilibrium due to packing effects. The role of attractive forces on the conformational equilibrium is also discussed. It is found that attractive and repulsive forces favour different kinds of conformers at high densities. This explains why the conformational equilibrium of n-alkanes at liquid densities does not differ significantly from that of the gas. The present simulation data may be useful for testing different theoretical treatments recently proposed for hard n-alkane models.