asphaltenes

Asphaltenes are a broad, complex fraction of crude oil and bitumen defined by their solubility properties: they are soluble in aromatic solvents such as toluene or xylene but precipitate when a paraffinic solvent such as n-heptane is added. They are high molecular weight, polar components composed of condensed polycyclic aromatic rings with heteroatoms (nitrogen, sulfur, and oxygen) and trace metals such as nickel and vanadium. Because of their structural heterogeneity, asphaltenes do not correspond to a single molecule but to a fraction that forms nano-aggregates in solution, stabilized by inter- and intramolecular interactions.

In crude oil, asphaltenes are part of the resin-plus-asphaltene fraction and play a key role in viscosity, stability of emulsions, and potential for deposition and clogging. Content varies with oil type and maturation, typically a few percent to tens of percent by weight. Asphaltenes can self-associate to form colloidal networks; changes in temperature, pressure, or composition (for example, dilution with lighter alkanes) can trigger precipitation, leading to deposits in wells, pipelines, and refining equipment. Analytical approaches separate crude oil into saturates, aromatics, resins, and asphaltenes; onset precipitation tests quantify the amount of n-alkane required to induce precipitation.

Industrial relevance includes removal of asphaltenes via deasphalting, which uses paraffinic solvents to produce lighter fractions and asphalt-free feeds. Controlling asphaltene stability is important to reduce fouling, emulsions, and coke formation during processing. The exact molecular structure is not fully defined; instead, researchers describe typical architectures and aggregation behavior, along with the effect of metals and heteroatoms on stability and reactivity.