Viscosimetric and Neutron Scattering Study of Asphaltene Aggregates in Mixed Toluene/Heptane Solvents

TLDR

Asphaltene aggregates are highly polydisperse, and recent colloidal aggregation models provide the framework for interpreting their behavior. The study aims to characterize the size, molecular weight, and internal structure of asphaltene aggregates in mixed toluene/heptane solvents. Viscosimetry and small‑angle neutron scattering were used to measure intrinsic viscosities and scattering profiles, from which aggregate volume, radius of gyration, and molecular weight were derived. Intrinsic viscosities first decrease then increase with heptane fraction, mirroring a Flory‑Fox‑like dependence of aggregate volume on mass, while aggregate molecular weight scales with radius of gyration as a power law with exponent ≈ 2, indicating a self‑similar internal structure.

Abstract

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.