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.

determining ageing by luminescence spectroscopy

Luminescence is the umbrella term for phosphorescence and fluorescence.

Each bitumen fraction has a unique features regarding its spectra. The main part of saturates exhibit less luminescence. Aromatics exhibit the highest intensity, slowly decreasing forwards resins. The decreasing intensity of bitumen over ageing is likely owned to the change in the amount of aromatics. Resins and solid asphaltenes have two characteristic peaks at about 536 nm and 605 nm, which too can be found for saturates, but at a lower intensity. Asphaltenes in a solid state exhibit nearly no luminescence. In solution asphaltenes displays a spectrum similar to aromatics, but with lowest intensity and due to the phenomenon that aspahltenes form stacks and agglomerate in solution, it is assumed that asphaltenes dissolved in maltenes emit a spectrum similar to solid asphaltenes.

normalized emission spectra (excitation at 280 nm)

The figure shows the changes in the spectrum over the three ageing stages. Non-aged bitumen (B) lacks the peaks typically for resins and asphaltenes, but short-term aged (B_LRTFOT) and long-term aged (B_PAV and B_F282) exhibit these peaks. Unfortunately there is not a correlation between peak intensity and ageing stage. The different curve slopes, starting at about 340 nm, are caused partially by normalizing of the spectra. Not normalized B, B_LRTFOT and B_PAV spectra have the same starting slope, but differ in intensity maxima. Due to normalization the maxima are the equal, yet the slope changes. So the maxima or the slope can provide a hint to the ageing progress.

Separation into fractions

fractions of a chromatographic column separation

Bitumen can be separated into fractions with same physical and chemical behavior. The first separation is by distinguish soluble and insoluble parts. The solvent is often n-heptane but other solvents are also used, which lead to other results. The soluble part is called maltenes and the insoluble is called asphaltenes. Maltenes can be further separated into 3 fractions with a chromatographic column. The separation is based on adsorption to the stationary phase and redissolve potential of the mobile phase.
The cut points between the fractions is optical and done manually, even coupling with spectral devices is possible.

asphaltenes