New Answers to the Optical Interrogation of Asphaltenes: Monomers and Primary Aggregates from Steady-State Fluorescence Studies

Significance Statement

Asphaltene constituents of petroleum impact on all aspects of crude oil production and utilization. If asphaltenes precipitate it can block pipelines and stop oil production. Therefore has been debate in the past over the structure of individual asphaltene molecules. From literature, asphaltenes are known to form nanosized colloidal particles nanocrystallites, nanocolloids or nanoaggregates in good solvents and native crude oils at concentrations of 100 mg/L and above. However, there is not much of proof that to suggest that asphaltenes can aggregate into smaller oligomeric species at much lower concentrations.

Researchers from Gubkin Russian State University of Oil and Gas in Russia tried to determine the size and structure of aggregating asphaltene molecules using steady-state fluorescence emission techniques. The work is now published in the peer-reviewed journal, Energy & Fuels.

The research team most said, the popular description of petroleum asphaltenes using Modified Yen Model MYM were improperly deduced from measurements of steady-state fluorescence emission SSFE spectra, this led the team to carry out study on optical interrogation of asphaltenes.

The authors presented SSFE data that were obtained from benzene solution with an asphaltene concentration of 0.34 mg/L. A comparison with the standard spectra shows that the emission in the first range may be attributed to fluorophores with a single aromatic ring like in substituted benzenes. The wavelength range of 315 −335 nm is attributed to emission from fluorophores in nonsubstituted or substituted molecules with two fused aromatic rings.

In contrast to the MYM description, the team found out that from the new SSFE experiments that aggregating asphaltenes appear to be of smaller molecular species, typically with 1−3 ring aromatic fluorophores.

Only a low-intensity characteristic feature at the tail of the spectrum may be attributed to emission from fluorophores with 3 fused aromatic rings. According to the research team, the measured relative intensities of characteristic fluorescence peaks are inversely proportional to the relative abundances of the molecules with respective fluorophores in the studied samples, they explained, it was due to the differences in individual extinction coefficients and quantum yields. The researcher affirmed that, the size distribution of asphaltenes cannot be derived solely from fluorescence measurements, as assumed, using the Modified Yen Model.

The positions of characteristic peaks indicate only a number of aromatic rings in individual asphaltene fluorophores, the leading author said, any conclusions regarding the nature of particular substituents would be merely speculation. The SSFE data shows that larger asphaltene molecules are apparently inactive during aggregation over the studied concentration range. Likewise, the SSFE data show that larger (>4 ring) asphaltene molecules are inactive during aggregation over a studied concentration range.

This study determined the size and structure of aggregating asphaltene molecules, and provided evidence that proved the conventional Modified Yen Model has serious flaws. Results of this study should improve oil production and utilization.

Journal Reference

Igor N. Evdokimov, Aleksey A. Fesan, and Aleksandr P. Losev, New Answers to the Optical Interrogation of Asphaltenes: Monomers and Primary Aggregates from Steady-State Fluorescence Studies, Energy Fuels 2016, 30, 4494 −4503.

Department of Physics, Gubkin Russian State University of Oil and Gas, Leninsky Prospekt, 65, Moscow B-296, GSP-1, 119991, Russia

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