Probing Asphaltenes Aggregation with Fluorescence Techniques

Date

2014-10-16

Authors

Zhang, Hui Ting

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Abstract

Asphaltenes correspond to the fraction of oil that is insoluble in heptane but is soluble in toluene. The aggregates of asphaltene are of interest because they cause serious problems in the production of oil. Asphaltenes contain fluorescent moieties, and as such they can be studied by fluorescence techniques. The first objective of this work was to develop methodologies to study the fluorescence of asphaltenes, and to investigate the fluorescence of asphaltenes at various concentrations. Time-resolved fluorescence studies indicate that asphaltenes have different chromophores with different lifetimes. The average lifetime of the asphaltene emission decreased when the asphaltene concentration was increased because of quenching processes occurring within the aggregates. The measurement of lifetimes at different excitation and emission wavelengths demonstrated that different components of asphaltene aggregate at different concentrations. The second objective of this work was to investigate how accessible the asphaltene aggregate is to small molecules by fluorescence quenching experiments. Nitromethane was the quencher used in the fluorescence of asphaltenes. The quenching efficiencies were found to be independent of the concentration of asphaltenes. However, the quenching efficiencies differed for different chromophores, suggesting a selective quenching for nitromethane of the excited states for the different chromophores of asphaltenes. The third objective of this thesis was to investigate the fluorescence of externally added probes that might be incorporated in asphaltene aggregates through π−π stacking. Pyrene was chosen as the probe because its fluorescence properties are strongly affected by its surroundings. The pyrene emission was quenched by nitromethane. The quenching efficiencies determined for pyrene in the absence or the presence of asphaltene aggregates were the same. This suggests that pyrene is located in an open environment, where the asphaltene aggregates do not offer any protection for pyrene from nitromethane.

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Keywords

oil, aggregates, quenching, pyrene

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