Evidence of the aggregation behaviour of asphaltenes according to the Yen-Mullins model by direct visualization of their Langmuir-Blodgett films

Abstract

Within this work we provide an extensive experimental approach comprised by pressure-area isotherms and optical studies at interfaces (by means of Brewster angle microscopy and reflection spectroscopy) combined with electronic microscopies and mass spectrometry examination of Langmuir-Blodgett films transferred from spreading concentrations above and below the nanoaggregate-to-cluster aggregation threshold. It will be shown that the associated structures already present in the bulk solutions are preserved upon transference to a large extent and primarily determine the subsequent asphaltene film structure at the interface. The different morphologies derived from each aggregation state reveals the intermolecular cohesion forces that are of short-range character in the nanoaggregate range, while the elongated rod-like structures, mainly observed for high spreading concentrations, indicates a longer order interaction responsible for the clusters formation at high molecular packing. These features provide new evidence of a two-step aggregation process at interfaces that, for this particular asphaltene sample, matches with that predicted by the modified Yen model.