Asphaltenes in Bitumen, What They Are and What They Are Not

Asphaltenes in bitumen and crude oil are generally defined as the fraction which is insoluble in n-heptane. There is also a perceived definition which describes asphaltenes as a very polar, high molecular weight fraction in bitumen. The standard definition and the perceived definition are not identical and thus a lot of confusion has been created. In this paper we conclude that the asphaltenes consist of a very heterogeneous fraction with molecular weights from about 250 g/mol up to 2000 g/mol, with an average of about 750 g/mol. The amount of the hetero atoms nitrogen and oxygen is not enough to make asphaltenes particular polar. A careful examination of the chemistry of asphaltenes reveals that they consist basically of three types of molecules, polycyclic aromatic compounds, n-alkanes with a chain length larger than 40 carbons and metal containing porphyrines.