Asphaltene Self-Association and Precipitation in Solvents—AC Conductivity Measurements

Techniques used for investigating asphaltene self-association are reviewed. The principles, fundamental differences, and limits of each technique are briefly discussed. A new approach using AC conductivity measurement for detecting asphaltene self-association is proposed and demonstrated using Alberta bitumenderived asphaltene as a model system. Preliminary results showthat the ACconductivity measurement is sensitive to subtle capacitance change arising from asphaltene self-association but only within a certain frequency range. Apercolation model with parallel capacitor–resistor circuit is adopted to establish the theoretical basis for this approach. This model predicts the functional behavior of the AC conductivity and exhibits phase transition-like behavior upon asphaltene self-association. The conductivity measurements show functional forms similar to the predicted ones and exhibit discontinuity near 120 mg/L in toluene where self-association is believed to occur. This value agrees (on the same order of magnitude) with earlier surface tension,1 laser thermal lensing,2 and ultrasonic3 measurements. In addition to detecting asphaltene self-association, AC conductivity is also applicable to characterization of asphaltene precipitation in toluene upon addition of nonsolvent such as heptane. The sensitivity is high and the method is simple. These two experiments suggest that AC conductivity method can be a good option for measuring flocculation, precipitation, and phase separation of petroleum complex fluids, provided the right frequency range is chosen. Further validation of this method is needed for other complex fluids.