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capacity consistent with that of an ordered solid, while at ca. –30°C
they undergo a phase transition to an amorphous (glassy) phase,
structurally controlled by interactions between polar alkane side
chains, and dominant up to 25–30°C (denoted as α-phase in Fig. 5).
In a following phase transition asphaltenes acquire more dense
structures, which are fairly stable up to ca. 100°C, and are con-
trolled by bonding to pericondensed aromatic segments, (β-phase in
Fig. 5). In 100–180°C temperature range there appear yet another
asphaltene phase with some crystalline order (γ-phase in Fig. 5).
At higher temperatures, amorphous asphaltenes soften and liq-
uefy, while crystalline domains melt at ~220–240°C. Finally, above
ca. 350°C, asphaltenes decompose and form liquid crystalline
mesophase, precursor of coke.
For asphaltene-containing free-flowing fluids, including native
crudes, the best documented specific temperatures fall onto the α–
β phase boundary in the range of 25–35°C (line A in Fig. 5). E.g.,
a transition to a more dense (β) phase was manifested by notice-
able shrinking of complex asphaltene aggregates,
38
by a decrease of
surface tension
44
and by an increase of deposition from asphaltene
solutions.
14
In support of the above discussed demixing phenom-
ena, this boundary has been interpreted as “upper critical solution
temperature” (UCST) both in bitumen
45
and in asphaltene solu-
tions.
48
Comparatively less investigated are the β–γ phase
boundary (line B in Fig. 5) and the upper γ-phase boundary
(line C in Fig. 5). At the “closed loop” domain the latter boundary
may be identified with “lower critical solution temperature”
(LCST).
The data of Fig. 5 show that temperature-driven transitions
between α, β and γ phases are observed at all asphaltene concen-
trations above the demixing boundary (line 2). Hence, apparently,
these phases are inherent already to the primary asphaltene
nanoparticles and, most probably, their inner structures are con-
trolled by different types of possible bonding of asphaltene mono-
mers, as discussed above. In view of thermally-induced variations
of structural order, earlier proposed models of primary aggregates
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