Caracterização da agregação de asfaltenos por meio de fluorescência molecular

Abstract

The tendency of asphaltenes to aggregate and form clusters in solvents was investigated by fluorescence spectroscopy. This was done by evaluating the relative fluorescence quantum yield of asphaltenes diluted at several concentrations in toluene, and by studying the changes in the fluorescence spectra of asphaltene solutions as the composition of the solvent, toluene and cyclohexane, is changed. The asphaltene fraction (heptane insoluble) was collected from Marlim crude oil and solutions of this material varying from 0.016 g.L-1 up to 10.0 g.L-1 were prepared in toluene. Excitation wavelengths ranged from 300 nm to 500 nm, probing the excitation of molecules with aromatic chromophores of different sizes. The spectra were structureless and only a small bathochromic shift of the emission maximum with the concentration of asphaltenes was observed. Regardless the excitation wavelength, the total fluorescence intensity was maximum for concentrations of asphaltenes around 0.4 – 0.8 g L-1. This was attributed to fluorescence self-suppression of the asphaltenes. Stern-Volmer plots showed an unusual downward curvature, which was understood as a consequence of the formation of asphaltene aggregates. Solutions of same asphaltenes at concentration of 0.10 g.L-1 were prepared in toluene and cyclohexane in different proportions. It was observed a reduction in the fluorescence intensity as the cyclohexane content in the solutions grew. Two excitation wavelengths were chosen in this case: 290 and 390 nm. Since cyclohexane is more viscous than toluene at room temperature, the reverse tendency should occur if dynamic suppression were the main suppressing mechanism. Probably, the fluorescence suppression follows a static mechanism, which constitutes another evidence for the asphaltene aggregation process. The change in the emission intensity with concentration of the asphaltenes was modeled taking the aggregation process into account. Self-association equilibrium constants were estimated and aggregation parameters such as mass concentration of non-associated asphaltenes and number average degree of association were calculated. It was found that, at a concentration similar to which asphaltenes are found in petroleum, there are 3 to 6 asphaltene molecules per aggregate.