Составители:
16
cal nanoaggregate concentration).
43
Fig. 5 shows that the “former
CMC” boundaries reflect phase transformations not in primary
systems of asphaltene molecules, but in secondary systems of com-
plex nanocolloids formed at the demixing boundary. Moreover, as
indicated above, at least one of “former CMC” lines may appear to
be just a continuation of a demixing (liquid-liquid separation)
closed loop.
Highest-concentration boundaries (lines 4 and 5 in
Fig. 5). In studies of “CMC” by viscosity measurements
44
additional
strong effects were observed at ~20–35 g/l (line 4 in Fig. 5) and
were ascribed to a “second aggregation concentration”. As a result
of detailed SANS studies, phase behavior in the highest-
concentration range was interpreted as follows.
38
In the “dilute re-
gime” (between boundaries 3 and 4 in Fig. 5) asphaltene aggre-
gates are independent entities with radii of a few nanometers. In
the “semidilute regime” (above boundary 4 in Fig. 5) the internal
structure of aggregates remains unchanged, but these aggregates
interpenetrate and form soft fractal objects, imparting high fluid
viscosities. The “concentrated regime”, apparently above ~70–90 g/l
(boundary 5 in Fig. 5), is characterized by the appearance of a
phase consisting of large (>0.1 μm) flocculated asphaltene do-
mains, which may form “spatially-organized two-phase textures”
(gel-like structures) and sedimentation of this phase may also oc-
cur. Hence, in simpler fluids, boundary 5 may be regarded as a
“free-flowing” limit. Higher asphaltene concentrations apparently
are encountered only in multicomponent highly viscous/gelled sys-
tems (bitumen).
Temperature-Defined Phase Boundaries
The majority of experimental data on “specific temperatures” has
been obtained for asphaltene-rich bitumen and for corresponding
neat (solid) asphaltene fractions.
35,45-47
In short, these experiments
revealed the presence of several temperature-controlled phases of
aggregated asphaltenes (cf. the right-hand part of Fig. 5). At freez-
ing temperatures (not shown in Fig. 5) asphaltenes exhibit a heat
Страницы
- « первая
- ‹ предыдущая
- …
- 14
- 15
- 16
- 17
- 18
- …
- следующая ›
- последняя »
