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.
The densities of binary mixtures of n-hexadecane and cyclohexane at high pressures were measured in the range
of (6.895 to 62.053) MPa at six different temperatures varying from (318.15 to 413.15) K and for eight compositions.
The measurements were made by a high-pressure Anton Paar DMA 512 P densimeter integrated with the Ruska
2370 mercury Free PVT System. The densimeter was calibrated using analytical grade toluene, cyclohexane, and
n-heptane as calibration fluids. The experimental error of density measurements is estimated as 0.5 kg·m-3. The
measured densities at 348.15 K agree well with the available literature values at different pressures. The excess
volumes, thermal expansion, and isothermal compressibility coefficients were obtained from measured densities.
All data were correlated successfully with a modified Peng−Robinson equation of state.
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