Investigation of Asphaltene Association by Front-Face Fluorescence Spectroscopy

Abstract

The tendency of asphaltenes to aggregate and form clusters in solvents was studied 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 a Brazilian heavy crude oil, and solutions of this material varying from 0.016 g/L up to 10 g/L were prepared in toluene. Front-face emission spectra were obtained in two wavelength ranges, from 310 to 710 nm, excited at 300 nm (short range), and from 410 to 710 nm, excited at 400 nm (long range). Severe quenching was observed at concentrations above about 0.1 g/L. Stern-Volmer plots (reciprocal of quantum yield against concentration) exhibited nonlinear, downward-curved behavior, indicating that a more complex suppression mechanism, probably influenced by the association of the asphaltene molecules, is taking place. The same asphaltenes were dissolved (0.1 g/L) in binary mixtures of toluene and cyclohexane, and emission spectra in both the short range and long range were obtained. Fluorescence was progressively quenched at longer wavelengths of the spectra as the proportion of cyclohexane in the solvent grew. Cyclohexane, a poor asphaltene solvent, is probably inducing static quenching through association of asphaltenes.

Evaluating steady-state and time-resolved fluorescence as a tool to study the behavior of asphaltene in toluene

“…This re-absorption effect was recognized in previous fluorescence studies with crude oils and asphaltenes where front-face sample holders were used or appropriate corrections were applied. 9,10,14,27 However, in other studies this artifact was not recognized 11,28,29 and, in these cases, conclusions on the aggregation of asphaltene based on spectral shifts are not warranted. The control experiment shown in Fig.…”

“…Changes in the shape of the emission spectra with the asphaltene concentration have been correlated to the aggregation of asphaltene. 10,11,13 However, it is important to differentiate between an aggregate induced shift of the emission spectra and distortions due to artifacts, such as self-absorption. Such an artifact became apparent at relatively low asphaltene concentrations when the 90-degree arrangement between the excitation and emission optics was used (Fig.…”

“…The emission spectra from asphaltene solutions are broad due to the presence of different fluorophores [5][6][7][8][9][10][11][12] and shifts in the spectra and decreases in intensities at high asphaltene concentrations were related to aggregation of asphaltenes. 10,11,13 Different fluorophores in crude oils or asphaltene samples have different lifetimes and shorter lifetimes were observed when the concentrations of crude oil or asphaltenes were raised. 7,12,[14][15][16][17] These observations were attributed to energy transfer between separate asphaltene molecules or between asphaltene molecules within aggregates.…”

“…4 Fluorescence, due to its high sensitivity, has also been used to characterize asphaltenes. The emission spectra from asphaltene solutions are broad due to the presence of different fluorophores [5][6][7][8][9][10][11][12] and shifts in the spectra and decreases in intensities at high asphaltene concentrations were related to aggregation of asphaltenes. 10,11,13 Different fluorophores in crude oils or asphaltene samples have different lifetimes and shorter lifetimes were observed when the concentrations of crude oil or asphaltenes were raised.…”

Asfalteno: um desafio para indústria de petróleo e a busca de soluções pela nanociência

“…16,21,32,138 Sua ocorrência é o principal motivo para as precipitações e entupimentos durante o processo de refino e transporte do óleo. 138 32 irá depender da técnica e do solvente utilizados na medida 16,18,32,140,141 Como pode ser visto no espectros de fluorescência da Figura Partindo-se de uma concentração inicial de asfaltenos a 20 mg. 10, 20, 30, 40, 45, 50, 60, 80, 100 e 200 mg.L -1 ). águas adsorvida fisicamente e coordenada respectivamente.…”