Calorimetric Evidence about the Application of the Concept of CMC to Asphaltene Self‐Association

Abstract

For many years, the concept of critical micellar concentration (CMC) has been projected from surfactant science into asphaltene science. There are several similarities between these two species, such as the stabilization of water‐in‐oil emulsions and surface activity, which suggested that asphaltenes may also have a concentration at which self‐association occurs (CMC). This article presents evidence found by calorimetry and spectroscopic techniques, that suggest that this concept may not be adequate for asphaltene self‐association in toluene solutions. Isothermal titration calorimetry has been widely used in surfactant science to determine both the CMC and the enthalpy of micellation of many surfactants. The concentration interval could be divided into three regions: monomer region, micellation region, and micelle region. The absence of the first region (monomer) in the concentration range usually found in the literature as the CMC region of asphaltenes indicates that this concept is not appropriate for asphaltene self‐association. Tests were performed down to concentrations of 34 ppm without any sign of a critical micellization or aggregation concentration. Based on the various techniques applied, which also include IR and fluorescence spectroscopy, it is concluded that asphaltenes do not exhibit CMC behavior. Instead, the association of asphaltenes is believed to occur step wise. This is not in disagreement with the fact that the aggregates may end up having a definite size.

Keywords:

• Asphaltenes
• isothermal titration calorimetry
• self‐association

Acknowledgments

The authors acknowledge the Danish Technical Research Council (STVF) for financial support, Mr. Zacarias Tecle for his great help in the laboratory, and Juan Murgich for fruitful discussions.