ABSTRACT
Asphaltenes are alkylated polycyclic aromatic species containing heteroatoms such as sulfur and nitrogen, as well as metals such as vanadium and nickel [1]. They are the heaviest and most polar fraction of a crude oil. Mass spectral and diffusion measurements of asphaltenes point to a molecular weight between 500 and 1000 Da, which is compatible with an alkylated polycyclic structure with roughly seven rings [2-4]. This structure is called the “island” architecture. There is, however, another school of thought that advocates the “archipelago” architecture. In that picture, asphaltenes are composed of alkyl-bridged aromatic and cycloalkyl groups connected with alkyl chains [5,6]. Regardless of their exact molecular structure, asphaltenes can form aggregates primarily through π-π stacking of aromatic rings and other interactions. They commonly form nanoaggregates with an average size between 2 and 20 nm [1,7]. These aggregates are porous; thus, smaller molecules and other mobile species can enter their fractal structure. Asphaltenes tend to deposit onto surfaces. Silica and alumina are the prime examples of almost irreversible adhesion of asphaltenes to solid surfaces, which is why conventional chromatography of asphaltenes is difficult. Metal surfaces such as the inner surface of pipelines are also not immune to the adhesion of asphaltenes. They also adsorb to the oil-water interface, which results in stabilization of emulsion films [8,9]. Asphaltenes are generally considered a nuisance in the industry, with limited commercial value as, e.g., road pavement material. However, they are important in understanding the geochemical background of reservoir, the reservoir connectivity, and the migration of the fluid [3,10].
