Asphaltene

Asphaltenes are defined generally as the insoluble part of bitumen in n-heptane and soluble in methylbenzene (toluene). Other definitions exist, like the insolubility in n-pentane or n-hexane, which leads to different weight percentages – the lower the carbon number, the higher the content obtained from bitumen. By the definition of insoluble in n-heptane and soluble in toluene also long-chained hydrocarbons (with more than about 40 carbons) will count as asphaltenes, due to their solubility. Also asphaltene extraction methods, especially the used filter, and extraction temperature have an impact on the amount of asphaltenes and their properties.

asphaltenes after bitumen filtration

At room temperature asphaltenes form a black powder and constitute 5 to 31wt% of bitumen. The density of asphaltenes is about 1,15 g/cm³. Isolated dry asphaltenes do not melt in oxygen atmosphere, but in inert atmosphere they form a liquid-like product with no clear melting point and start to decompose at about 350 °C, leaving carbonaceous residue (coke).

Asphaltenes have a H/C ratio between 0,98 and 1,56 and have a higher content of hetero elements (nitrogen and oxygen). An average molecule consists of 4 to 10 fused aromatic rings and aliphatic chains. Some aliphatic chains are assumed to link multiple groups of rings (archipelago type). Asphaltenes contain more condensed rings and polar groups compared to the fractions from maltenes (n-heptane soluble part of bitumen. The condensed rings form almost planar sections of the asphaltene molecule, which can associate through pi-pi bonding to stacks. These stacks (nanoaggregates) are assumed to be the reason for structural formation and for short-range order. Additional almost all metals from bitumen are present in the asphaltenes. Usually the total amount of metals is below 0,1wt%, which leads to a content of metalorganic molecules (like porphyrines) of about 1 to 2wt% of asphaltenes. Three classes of compounds can be identified in asphaltenes: polyaromatics with relatively few saturated substituents, porphyrines (insoluble in n-heptane) and other metal organic compounds and n-alkanes with more than about 40 carbons.

porphyrine

possible molecular asphaltene structure

Asphaltenes have a large contribute on all physical properties, for example with increasing asphaltene content viscosity and density increases.