Polar molecules, heavy fractions

The heavy-end portions (usually called heavy fractions) of bitumen (e.g. asphaltenes, preasphaltenes) can exist both in a random oriented particle aggregate form or in an ordered micelle form, peptized with resin molecules (16.17). In their natural state, asphaltenes exists in an oil-external (Winsor s terminology) or reversed micelle. The polar groups are oriented toward the center, which can be water, silica (or clay), or metals (V, Ni, Fe, etc.). The driving force of the polar groups... 

It is essential to state that the heavy fractions such as asphaltene and preasphaltene do contain large numbers of polar molecules (23.24). These polar molecules behave exactly as surfactants or amphiphiles (asphaltene usually contains a long-chain substituent (25)). We again have to emphasize that it is almost not possible to create a colloidal micelle from pure hydrocarbon and water without any surfactant. Hence, we conclude to say that asphaltene or asphaltene-like molecules (as-phaltics) will participate in a manner according to membrane-mimetic chemistry. 

A crude oil can be separated in two main components asphaltenes and maltenes. The heavy fractions of petroleum can be defined as molecules possessing more than 25 atoms of carbon distributed in polar and heavy compounds, such as asphaltenes and resins, having high boiling points (Merdrignac and Espinat, 2007). Since asphaltenes are the main constituents of heavy crude oils and their structure and composition directly affect the whole composition of the petroleum, they deserve special attention. 

The high viscosity of heavy crude oils is essentially due to the high levels of asphaltene content. Asphaltene is the highest MW component of crude oil, is a friable, amorphous dark solid, which is colloidally dispersed, in the oily portion of the crude. Asphaltenes are considered to be heavily condensed aromatic molecules with aliphatic side chains and with high heteroatom content (S, N, and O) as well as high-metal content. The asphaltene fraction is physically defined as that fraction insoluble in n-alkanes, but soluble in toluene and is the most polar fraction of oil. 

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