The structure of liquid heptane at wax surfaces

Abstract

The properties of the molecules in a liquid heptane film on an octacosane crystal surface have been studied using molecular dynamics techniques. Molecule energies, conformational properties and dynamic properties were calculated and correlated with height above the crystal surface. This analysis indicated that the liquid heptane is strongly ordered in the vicinity of the wax surface, showing at least four layers in which the heptane tended to align parallel to the surface. Substantially more structure was found in the interfacial heptane layer, with the heptane also being aligned parallel to the paraffin molecules in the wax surface, and showing a strongly anisotropic diffusive behaviour that reflected the underlying topology of the wax surface. Simulations were also performed to study the effect of an adsorbed comb-shaped polymer (poly(octadecyl acrylate)) on the liquid properties. The polymer was found to mix with the heptane to produce an additional, strongly ordered layer that would provide a kinetic barrier to any subsequent wax deposition.