Relaxation time, diffusion, and viscosity analysis of model asphalt systems using molecular simulation.

Molecular dynamics simulation was used to calculate rotational relaxation time, diffusion coefficient, and zero-shear viscosity for a pure aromatic compound (naphthalene) and for aromatic and aliphatic components in model asphalt systems over a temperature range of 298-443 K. The model asphalt systems were chosen previously to represent real asphalt. Green-Kubo and Einstein methods were used to estimate viscosity at high temperature (443.15 K). Rotational relaxation times were calculated by nonlinear regression of orientation correlation functions to a modified Kohlrausch-Williams-Watts function. The Vogel-Fulcher-Tammann equation was used to analyze the temperature dependences of relaxation time, viscosity, and diffusion coefficient. The temperature dependences of viscosity and relaxation time were related using the Debye-Stokes-Einstein equation, enabling viscosity at low temperatures of two model asphalt systems to be estimated from high temperature (443.15 K) viscosity and temperature-dependent relaxation time results. Semiquantitative accuracy of such an equivalent temperature dependence was found for naphthalene. Diffusion coefficient showed a much smaller temperature dependence for all components in the model asphalt systems. Dimethylnaphthalene diffused the fastest while asphaltene molecules diffused the slowest. Neat naphthalene diffused faster than any component in model asphalts.