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The asphaltenes are defined as the fraction of oil insoluble in solvents of low molecular weight aliphatic, such as n-pentane and n-heptane, but soluble in toluene.

Another definition considers that the asphaltenes are flat molecules, poly and poly cyclic aromatic containing heteroatoms and metals, which exist in a state of aggregation in suspension and are surrounded, stabilized by resins (gum inhibitors agents) are not pure, nor are they identical molecules , which has an electric charge and are poly dispersed.

The asphaltenes are polydisperse compounds on functional groups, molecular weight and structure. The content of aromatic compounds in the asphaltenes is between 40-60%, with an atomic H / C of 1-1.2. A high percentage of aromatic rings are connected to the intermolecular structure, and therefore the asphaltenes have flattened shapes.

Recently, it was felt that the asphaltenes were macromolecules of molecular weight of tens of thousands and possessed a structure composed of highly condensed aromatic matrix, surrounded by aliphatic chains and naphthenic rings, incorporating functional groups such as alcohols , primary and secondary amines, acids, ethers, esters, ketones and most complex joint such as porphyrins.

Recent studies, which presents analysis techniques tend to present the asphaltenes with a much smaller molecular weight between 800 and 3000 and a less condensed matrix, limited to four or five benzene rings linked by naphthenes and extended by gruposalquil.

Infrared spectrophotometry, nuclear magnetic resonance and X-ray diffraction, have been used to determine the chemical details of heteroatoms, functional grouping, and types of carbon and hydrogen in the asphaltenes.

The hetero mostly found in asphaltenes from different crude oils are sulfur (S), nitrogen (N) and oxygen (O). Nitrogen is more like part of the aromatic groups, while the oxygen and sulfur form bridges between them in a cyclic or linear structure. Sulfur heterocycles exists predominantly as teofénicos (typically 65-85%) with the rest in groups sulfide. Nitrogen is related to the groups pyrrole, pyridine and quinoline, still the dominant group pyrrole. The oxygen-related groups are hydroxyl, carbonyl, carboxylic acid and ether.

In some raw metal atoms are determined mainly nickel (N) and vanadium (V). The structure of the asphaltenes shows that relatively polar hydrocarbons with parts provided by the aromatic rings and functional groups and parties formed by the nonpolar aliphatic chains, which can present a duality giving polar apolar interfacial properties.