Detailed Molecular Dynamics Simulation of the Structure and Self-Diffusion of Linear and Cyclic n-Alkanes in Melt and Blends

Results are presented for the density, free volume, self-diffusion, structure, and conformation of short linear and cyclic n-alkanes in their own melt and in blends at equal carbon number from detailed atomistic molecular dynamics (MD) simulations in the isothermal-isobaric (NPT) statistical ensemble using the explicit-atom optimized potentials for liquid simulations (OPLS-AA) force-field. In agreement with experimental data reported in an earlier study by von Meerwall et al. (2003), cyclic alkanes are characterized by higher densities and diffuse more slowly than their equivalent linear alkanes. Their configurations are also dominated by certain conformers whose exact shape depends on the molecular length n of the cyclic alkane. The smaller the value of n the more symmetric the shape of these conformers. The MD results support the findings of von Meerwall et al. (2003) that the overall (single average) diffusion coefficient of linear and cyclic alkanes in their blend is equal to the weight-average of the diffusion coefficients of the neat species at the same temperature. Simulation results are also presented for the average size, individual diffusivities, and intermolecular C-C pair distribution function of the two components (linear and cyclic) as a function of molecular weight and blend concentration in cyclic molecules.