Dispersion of Asphaltene Nanoaggregates and the Role of Rayleigh Scattering in the Absorption of Visible Electromagnetic Radiation by These Nanoaggregates

摘要

Asphaltenes exist in the form of a colloidal dispersion in crude oils and solvents. Even in a good solvent such as toluene, asphaltene aggregates persist at the nanoscale. In this study, the impact of Rayleigh scattering on the apparent absorption of visible radiation by asphaltene aggregates in solution was assessed. Recent work with a stirred diaphragm diffusion cell indicates that membranes with pore sizes less than 5 nm are capable of removing the species responsible for the absorption of visible light with wavelengths >550 nm. A further analysis of the spectra of the whole asphaltene samples in toluene indicates that the absorbance of visible light with wavelengths >600 nm follows a λ~(-4) dependence for asphaltenes from a range of sources over a wide range of concentration. This functional dependence is consistent with Rayleigh scattering, rather than a mixture of colored components or chromophores. Rayleigh scattering equations were combined with experimental visible spectra to estimate the average nanoaggregate sizes, which were in a very good agreement with the sizes reported in the literature by other methods. Various additives, solvents, and ultrasound and heat treatments were employed in an attempt to completely disaggregate the asphaltene nanoaggregates in solution at room temperature. None of the treatments eliminated nanoaggregration, but some treatments increased absorption due to formation of larger aggregates, as confirmed by acoustic spectroscopy.