P. K. Kilpatrick and Keith L. Gawrys*
Department of Chemical and Biomolecular Engineering
North Carolina State University, Raleigh, NC USA 27695-7905
peter-k@ncsu.edu; 919-515-7121
Small-angle neutron scattering (SANS) has proven to be very useful for deducing the sizes and morphologies of asphaltenic aggregates in solution. Selection of an appropriate form factor model – i.e. a geometric model of the shape of the aggregates -- is important for determining the volumetric properties of asphaltenic aggregates, such as the radius of gyration (RG), molar mass, and apparent fractal dimension. We show here that the polydisperse oblate cylinder model best approximates the shape of asphaltenic aggregates. We also have performed fits to scattering curves using the Guinier approximation, and a comprehensive mass-fractal model. The mass-fractal model provided apparent fractal dimensions (2.2 to 3) for the aggregates that generally decreased with increasing aggregate size, indicating increased surface roughness for larger aggregates. The polydisperse oblate cylinder model provided typical values of the aggregate thicknesses from 5 to 32 Å, the average aggregate radius from 25 to 125 Å, and ~ 30 % radius polydispersity. Subsequent calculation of average aggregate molar masses indicated a range of solvent entrainment from 30 to 50 % (v/v) within the aggregates that were consistent with previous viscosity measurements. Inclusion of entrained solvent in the analysis of the scattering data is essential in order to obtain self-consistent volumetric and molar mass information. Additional calculations were performed to estimate the proportion of microscale flocs to nanoscale aggregates in the solutions. The inclusion of solvation effects is also essential for the accurate determination of fractal dimensions. We apply the model to the estimation of virial coefficients to gauge the strength of inter-aggregate interactions. We also describe recent experiments to probe the rate of solvent exchange into and out of aggregates.