Optical characterization of asphaltenes at the air-water interface.

Asphaltenes extracted from Arabian light crude oil have been investigated at the air-water interface of a Langmuir trough by in situ optical techniques. Brewster angle microscopy (BAM) and reflection spectroscopy have been used to extract new information about the microscopic organization of the asphaltene films in terms of association phenomena and chromophore orientation, respectively. The use of different spreading concentrations in the range 0.1-15 mg mL-1 reveals significant changes in the behavior at the interface with more condensed isotherms above 2 mg mL-1. This break point is related to the nanoaggregate-to-cluster association threshold in organic solution widely accepted in the recent asphaltene literature. BAM images support this observation with very different morphologies for the two spreading concentrations employed, 0.1 and 4 mg mL-1, respectively. The study of intensity changes in the corresponding normalized reflection spectra also confirms the transition in the asphaltene interfacial behavior between these two spreading concentrations. Finally, this technique helps with understanding the changes observed in the asphaltene films during a set of compression-decompression cycles.