Handbook of Size Exclusion Chromatography and Related ...

1008F, but polar aromatics and asphaltene mixtures are Newtonian (99). It has long
been proposed (111,112) that asphalt exists as asphaltene micelles or clusters
solubilized by polar aromatics.
Yen and associates (113–116), based on x-ray analysis, proposed that
asphaltenes and resins (polar aromatics) existed as flat, condensed aromatic disks
to which alkyl and naphthenic side chains were attached, forming a unit sheet.
Through p bonding between aromatic sheets, and no doubt hydrogen bonding
between heteroatom groups, the unit sheets arrange themselves in stacks, forming
a particle or cluster. Unless two sheets are connected by a side chain, the unit sheet
weight is approximately the molecular weight. In asphalt, polar aromatic sheets
can combine in a stack with asphaltene sheets and, being less condensed, help to
solubilize the asphaltenes in the remaining, less miscible fractions. When an
asphalt is dissolved in a solvent, the polar aromatics may be extracted from the
stack, causing the depleted asphaltene particles to clump, increasing apparent
molecular weight and perhaps causing precipitation. Although Yen’s work
involves a number of structural assumptions, his unit sheet weights are similar to
those obtained by FIMS and, like FIMS, yield higher molecular weights for resins
than for asphaltenes.
Others (70,71,117–120), using nuclear magnetic resonance and elemental
analysis with certain structural assumptions, have obtained very similar results for
unit sheet weights. Several researchers have applied this procedure to asphalt
fractions produced by preparative SEC. The unit sheet weights are always less than
SEC- or VPO-determined molecular weights. Kiet et al. (71) found nearly constant
sheet weights for his large molecular size fractions, which exhibited an over fourfold
change in VPO molecular weights that he attributed to an increasing number
of sheets per stack in the heavier fractions. Haley (26) hardened preparative SEC
fractions by air blowing: VPO molecular weights showed a considerable increase.
The unit sheet weights increased for the heavier fractions, reflecting an increase in
aromaticity and some crosslinking, but considerably less than the VPO molecular
weights. There are a number of studies indicating that these asphaltene
conglomerates exist in disclike structures. This is discussed in some detail by
Baltus (121) and Lin et al. (122). Ravey et al. (123) separated asphaltenes into a
number of fractions by SEC and used small angle neutron scattering to obtain
particle dimensions. In dilute THF the dimensions were roughly 13 nm diameter
and 0.5 nm thickness. The diameter increased in polar solvents. Lin et al. (122)
developed a suspension viscosity model for asphaltenes in asphalt which predicted
a disc-shaped particle with an aspect ratio that varied from about 18 to 24.
Acevedo (124) predicted a disc shape for octylated asphaltenes based on viscosity
measurements and SEC data.
Domke et al. (125,126) showed that oxidation kinetics of asphalt was
affected by oxygen diffusion into the asphaltene particle and its associated
material. The results were also affected by the nature of the solvating material.
© 2004 by Marcel Dekker, Inc.