Electronic absorption edge of crude oils and asphaltenes analyzed by molecular orbital calculations with optical spectroscopy

Previous molecular orbital calculations have shown that polycyclic aromatic hydrocarbons (PAHs) with 4-10 fused rings account for the center electronic band positions for bulk asphaltenes. Here, this work is extended to cover low-energy electronic transitions of crude oils and asphaltenes. The primary determinants in optical absorption band location are shown to be the size and geometry of PAHs. Large PAHs are shown to exhibit optical properties exhibited by asphaltenes and crude oils. Furthermore, these results are consistent with the observed Urbach spectral profiles. The rapidly declining electronic absorption at wavelengths exceeding 600 nm is shown to be consistent with the presence of a few large ring systems. Measurements of concentration and temperature independence of crude oil and asphaltene optical spectra imply that potential contributions to their coloration from charge-transfer and potential free radicals are not significant. Nevertheless, the very small electronic absorbances for very low energy are found to fall outside of the absorption profile of large hydrocarbon PAHs. Future work is indicated to account for addressing these very low absorbances for asphaltenes.