Investigation of Asphaltene Association with Vapor Pressure Osmometry and Interfacial Tension Measurements

TLDR

The study used vapor pressure osmometry to measure asphaltene molar masses in toluene or 1,2‑dichlorobenzene and drop‑volume tensiometry to assess interfacial tensions of asphaltenes versus water across a range of concentrations. Asphaltene molar mass starts at ~1800 g mol⁻¹ below 0.5 kg m⁻³, rises with concentration to a solvent‑, temperature‑, and fraction‑dependent limit of 4000–10 000 g mol⁻¹, implying 2–6‑molecule aggregates, while interfacial tension drops linearly with concentration, showing no micellization and yielding similar behavior for Cold Lake asphaltenes.

Abstract

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.