Currently, in most publications on
asphaltenes (including textbooks) it is stated that asphaltenes, both
in crude oils and in simple solvents, have a "critical micelle
concentration" ("CMC") of several grams (or even tens of grams) per
litre. Accordingly, a common belief is that experiments at
concentrations somewhat lower than "CMC" provide information on the
properties of individual molecules (monomers) of asphaltenes. One of
the conclusions of such experiments has been that asphaltenes have an
extremely high molecular weight (MW) - in excess of tens of thousands
Dalton. In petroleum engineering, the knowledge of asphaltene MW is
essential for development of effective technologies for
removal/prevention of deposits, tertiary oil recovery, heavy oil
processing, etc.
Our studies (with an active contribution of EEE
students - D.Yu. Eliseev, B.R. Akhmetov, S.A. Losev and
others) for the first time provided direct experimental evidence of
asphaltene aggregation at concentrations three orders of
magnitude lower that believed before. Namely, the simplest molecular
aggregates (asphaltene dimers) are formed at asphaltene concentrations,
not exceeding 3-5 MILLIGRAMS per litre.
By analysis of the newly discovered aggregation
stages we suggested that true MW of asphaltene molecules does not
exceed 1000 Dalton. (More recent direct experiments by Mullins and
others in USA provide MW <750
Dalton for asphaltenes).
This new information on asphaltene agregation stages has
been obtained in studies of Russian crude oils from various
georgaphical locations (measurements of viscosity, density,
UV/Vis absorption, refractive index, static and dynamic light
scattering, NMR spin-spin relaxation, etc)
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Principally different correlations of NMR
relaxation time and dynamic viscosity in dilute crude oil solutions,
containing asphaltene monomers and simple oligomers (А-С) and in
concentrated solutions, containing asphaltene molecular nanoclusters
and more complex aggregates (D-E)
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