VISCOSIMETRIC AND NEUTRON-SCATTERING STUDY OF ASPHALTENE AGGREGATES IN MIXED TOLUENE HEPTANE SOLVENTS/

A series of viscosimetric and small-angle neutron scattering experimen ts on asphaltenes diluted in mixed toluene/heptane solvents has been c onducted, with the purpose of characterizing the size, molecular weigh t, and internal structure of asphaltene aggregates as a function of so lvent conditions. With increasing flocculant (i.e., heptane) content i n the solvent, the intrinsic viscosities of asphaltene aggregates firs t decreased, went through a minimum for heptane fractions approximate to 10-20%, and then increased at the approach of flocculation. These v ariations paralleled those of the volume of aggregate occupied per uni t mass of asphaltene, a behavior reminiscent of the Flory-Fox relation ship for polymers in a solvent. This volume, proportional to the cubed radius of gyration of the aggregates divided by their molecular weigh t, was determined from the neutron scattering data. For increasing hep tane 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 du e care to the possible systematic effects of the strong polydispersity of these aggregates, these results are discussed in light of recent m odels of colloidal aggregation.