On the formation and properties of asphaltene nanoaggregates and clusters by DC-conductivity and centrifugation

The hierarchical nanocolloidal structure of asphaltenes in solvent and even in crude oil has recently been delineated by a variety of techniques. It is highly desirable to compare results for the same samples from various techniques dependent on different physics. Here, the critical nanoaggregate concentration (CNAC) of specific asphaltenes is determined using both DC-conductivity and centrifugation; excellent agreement is found. In addition, the role of the lightest asphaltenes (or heaviest resins) in nanoaggregation is now amenable to direct measurement; inclusion of this weaker binding fraction does not materially impact the CNAC except by a trivial effect of dilution of the aggregating asphaltenes. DC-conductivity shows there is no observable change in CNAC over a moderate temperature range indicating that the enthalpy of formation of the nanoaggregate is zero. A simple analysis reveals that the entropy of nanoaggregate formation is positive, in agreement with previous work. Finally, the critical clustering concentration is observed at higher concentrations, and as always the clusters of nanoaggreates are found to have a small aggregation numbers. As with the CNAC, the heaviest resins do not affect the clustering concentration except by a trivial dilution effect. (c) 2011 Elsevier Ltd. All rights reserved.