Molar masses of both n-pentane-extracted and n-heptane-extracted Athabasca asphaltenes were
measured in toluene or 1,2-dichlorobenzene with a vapor pressure osmometer (VPO). The initial
asphaltene molar mass, at concentrations below 0.5 kg/m3, is ≈1800 g/mol. The asphaltene molar
mass is found to increase with asphaltene concentration until a limiting value is reached at a
concentration between 10 and 20 kg/m3. The limiting value ranges from 4000 to 10 000 g/mol
and depends on the solvent, temperature, and asphaltene fraction. The results suggest that
asphaltenes form aggregates of 2−6 molecules in aromatic solvents. Interfacial tensions of
asphaltenes in toluene or 1,2-dichlorobenzene versus water were measured for asphaltene
concentrations from 0.3 to 100 kg/m3 using a drop volume tensiometer. The interfacial tension
decreases linearly with concentration, indicating that no micelles are formed. Hence, the
aggregation observed with VPO does not appear to be micellization. Similar results are obtained
for Cold Lake asphaltenes.
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