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Direct Observation of Asphaltene Nanoparticles on Model Mineral Substrates
journal contribution
posted on 2017-05-29, 00:00 authored by Gijo Raj, Alain Lesimple, Jamie Whelan, Panče NaumovThe propensity for
adherence to solid surfaces of asphaltenes,
a complex solubility class of heteropolycyclic aromatic compounds
from the heavy fraction of crude oil, has long been the root cause
of scale deposition and remains an intractable problem in the petroleum
industry. Although the adhesion is essential to understanding the
process of asphaltene deposition, the relationship between the conformation
of asphaltene molecules on mineral substrates and its impact on adhesion
and mechanical properties of the deposits is not completely understood.
To rationalize the primary processes in the process of organic scale
deposition, here we use atomic force microscopy (AFM) to visualize
the morphology of petroleum asphaltenes deposited on model mineral
substrates. High imaging contrast was achieved by the differential
adhesion of the tip between asphaltenes and the mineral substrate.
While asphaltenes form smooth continuous films on all substrates at
higher concentrations, they deposit as individual nanoparticles at
lower concentrations. The size, shape, and spatial distribution of
the nanoaggregates are strongly affected by the nature of the substrate;
while uniformly distributed spherical particles are formed on highly
polar and hydrophilic substrates (mica), irregular islands and thicker
patches are observed with substrates of lower polarity (silica and
calcite). Asphaltene nanoparticles flatten when adsorbed on highly
oriented pyrolytic graphite due to π–π interactions
with the polycyclic core. Force–distance profiles provide direct
evidence of the conformational changes of asphaltene molecules on
hydrophilic/hydrophobic substrates that result in dramatic changes
in adhesion and mechanical properties of asphaltene deposits. Such
an understanding of the nature of adhesion and mechanical properties
tuned by surface properties, on the level of asphaltene nanoaggregates,
would contribute to the design of efficient asphaltene inhibitors
for preventing asphaltene fouling on targeted surfaces. Unlike flat
surfaces, the AFM phase contrast images of defected calcite surfaces
show that asphaltenes form continuous deposits to fill the recesses,
and this process could trigger the onset for asphaltene deposition.