Asphaltenes: Absorbers and Scatterers at Near-Ultraviolet-Visible-Near-Infrared Wavelengths

This study examined the dominant mechanisms of near-ultraviolet-visible-near-infrared (NUV-vis-NIR) light attenuation in toluene solutions with asphaltene concentrations (C-A) in the wide range of 0.20-1880 mg/L. The analysis of experimental results indicates that in the near-ultraviolet range (270-430 nm) the main mechanism at all C-A is electronic absorption by 1-4 ring aromatic chromophores and by vanadyl porphyrins. In the visible range (430-730 nm), no distinct peaks of electronic absorption by larger aromatic chromophores were found and all light attenuation spectra followed a lambda(-4) dependence, consistent with the dominance of Rayleigh scattering mechanisms. In the near-infrared range (730-1100 nm), a significant mechanism of light attenuation by more dilute solutions (C-A < 30-40 mg/L) is photon absorption via excitation of C-H stretching vibrations in aromatic chromophores. However, for more concentrated solutions (C-A > 120-130 mg/L), the most important light attenuation mechanism in the NIR range also becomes Rayleigh scattering.