Mechanisms of Asphaltene Aggregation: Puzzles and a New Hypothesis

TLDR

Asphaltenes, the most refractory fraction of crude oil, have long posed challenges in production, transportation, and refining due to their complex, diverse composition and conflicting theories of bonding and aggregation. This review seeks to rationalize the relationship between asphaltene structure and the thermal stability of its aggregates. The authors examine the chemical and physical bonding properties of asphaltenes, including a proposed free‑radical pancake‑bonding mechanism supported by FT‑ICR MS and AFM data, to explain aggregate formation and stability. They identify polycyclic aromatic hydrocarbons that stabilize free radicals as key contributors to aggregation and outline directions for future research.

Abstract

In petroleum science, asphaltenes are well-known as the most refractory fraction of crude oil and remain infamous for problems in production, transportation, and refining processes. Hence, they have been continuously analyzed and modeled over more than half a century. Defined only by solubility and characterized by an extremely complex and diverse molecular composition, asphaltenes lack discriminative molecular descriptions and definitive chemical characteristics. Most of the historic research has focused on representative structures in an attempt to yield physical and chemical models of asphaltenes, and thus, conflicting theories of bonding structures and aggregation interactions remain. This review focuses on the aggregation behavior of asphaltenes and attempts to rationalize a structure–property relationship to asphaltene aggregate thermal stability. Herein, the chemical and physical bonding properties of asphaltenes and the contradictory interpretations of their nature and composition are examined, particularly as they impact formation and stability of asphaltene aggregates. We propose a new hypothesis involving free radical interactions, i.e., pancake bonding, as an additional significant contributor to the bonding structure and formation of aggregates in asphaltenes that is consistent with the latest findings based on Fourier transform ion cyclotron resonance mass spectrometry and non-contact atomic force microscopy techniques. Various structural features of polycyclic aromatic hydrocarbons contributing to this interaction involving a stable free radical are highlighted, and suggestions for future research are presented.