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Fig.3C show the results for TF=45°C, i.e. above the critical
formation temperatures. The effects of such a strong thermal pre-
load are rather small. For all flow temperatures the activation en-
ergies are reduced approximately to the level, observed at 20–30°C
in case of RT formation (Fig.3A).
The most pronounced changes of activation energies were reg-
istered (Fig.3B) after formation of emulsions at TF=36°C, close to
the critical temperature in the matrix crude oil. In this case activa-
tion energies for the W/O emulsion and the crude sharply increase
from 10–12 kcal/mol (42–50 kJ/mol) to 70–100 kcal/mol
(290–420 kJ/mol) with flow temperatures decreasing from 30°C to
10–12°C. In contrast, activation energies for bitumen emulsion
sharply decrease at low flow temperatures, down to 1.5 kcal/mol
(6.3 kJ/mol) at 13°C. The observed large variations of activation
energies are indicative of notable modifications of the emulsion’
molecular structures induced by formation in the critical TF range.
These structural modifications we attribute primarily to the spe-
cific molecular processes involving asphaltenes.
4. Discussion
4.1. The role of asphaltenes
It is a well-known fact that the presence of asphaltene frac-
tions serves to stabilise the emulsions formed during crude oil pro-
duction (3–6) and that asphaltenes also play a dominant role in the
stabilisation of water droplets in diluted bitumen emulsions (7, 8).
It is mostly the aggregation state of the asphaltenes and not
their amount that controls the stability of an emulsion. When as-
phaltenes start to form large aggregates, their impact on emulsion
stability is the greatest (9, 10). For example, it was observed (8)
that in diluted bitumen emulsions, the emulsified water droplets
are stabilised by steric repulsion between adsorbed “monolayers”
(8–10 nm thick) of asphaltene aggregates on the droplet surfaces.
Similarly, it was concluded from rheological measurements of as-
phaltene-containing crude oil - water emulsions (9), that the water
droplets are stabilised by adsorbed 10 nm asphaltene layers, con-
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