(735c) Spontaneous Crystallization and Rotation-Translation Decoupling in Model Bitumen Systems

The relaxation dynamics of multicomponent systems reflect a combination of the relaxation rates of individual molecules that comprise them. As part of an effort to understand how differences in crude oil chemistry affect mechanical properties, we have conducted molecular dynamics simulations of model bitumens comprised of asphaltenes with various size resin, naphthene aromatic, and saturate molecules. The results enable quantifying relationships between molecule size and relaxation rate within the mixtures, and several features have been found. Simultaneous analysis of rotational relaxation time via orientation time correlation functions and of diffusion coefficient revealed that rotation and diffusion decouple to different extents as a function of size and/or polarity. The product of rotation time and diffusion coefficient increases most at low temperature for asphaltenes, indicating dynamical decoupling such that rotations control viscosity and mechanical behavior. Spontaneous crystallization of n-C₂₂ chains at lower temperatures in some model systems ? over nanosecond MD time scales ? is consistent with experimental observations of wax crystals in some bitumens. Characteristics of the wax crystallization processes and upscaling to enable multiscale structural modeling will also be discussed.