Viscosimetric and neutron scattering study of asphaltene aggregates in mixed toluene/heptane solvents

A series of viscosimetric and small-angle neutron scattering experiments on asphaltenes diluted in mixed toluene/heptane solvents has been conducted, with the purpose of characterizing the size, molecular weight, and internal structure of asphaltene aggregates as a function of solvent conditions. With increasing flocculant (i.e., heptane) content in the solvent, the intrinsic viscosities of asphaltene aggregates first decreased, went through a minimum for heptane fractions ≈ 10-20%, and then increased at the approach of flocculation. These variations paralleled those of the volume of aggregate occupied per unit mass of asphaltene, a behavior reminiscent of the Flory-Fox relationship for polymers in a solvent. This volume, proportional to the cubed radius of gyration of the aggregates divided by their molecular weight, was determined from the neutron scattering data. For increasing heptane fractions in the solvent, the molecular weight of the aggregates increased with their radius of gyration according to a power law, the exponent being in the range of 2. This exponent also characterized the self-similar internal structure of the asphaltene aggregates. With due care to the possible systematic effects of the strong polydispersity of these aggregates, these results are discussed in light of recent models of colloidal aggregation.