Molecular dynamics computer simulation of chain molecule liquids

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Abstract
Two model liquids each composed of linear chain molecules containing six interaction sites have been investigated using molecular dynamics computer simulation. One of the liquids has features which resemble those of n-hexane. For the other liquid the interaction parameters are identical except that the dihedral angle potential restricting the torsional motions is set to zero. This flexible form of hexane is called flexane. Simulations have been performed at constant density at temperatures of 300 K and 200 K, the latter being close to the normal freezing temperature of real n-hexane. The molecular conformational distributions for the two liquids are quite different although this has little effect on the intermolecular configurational properties; it is shown that the two liquids are in closely corresponding thermodynamic states at both temperatures. The unrestricted torsional motions in flexane result, however, in a 70 per cent enhancement of the self diffusion coefficient at 200 K as compared to the more rigid molecule. The mechanism of diffusive motion is discussed in terms of motions parallel and perpendicular to a ‘long axis’ of the molecules. Resolution of the centre-of-mass velocity correlation function into its site-site components reveals some specific features of the coupling between diffusive motion and intramolecular degrees of freedom in these model chain molecules.