The critical micelle concentration of asphaltenes as measured by calorimetry

Micellization of asphaltenes in solution has been investigated using a micro calorimetric titration procedure (Andersen, S. I.; Birdi, K. S. J Colloid Interface Sci. 1991, 142, 497). The method uses the analysis of heat of dissociation and dilution of asphaltene micelles when a pure solvent (or solvent mixture) is titrated with a solution of asphaltene in the same solvent. The asphaltene concentration of the injected solution is at a level above the critical micelle concentration (CMC). In the present paper the procedure is applied in investigation of asphaltenes as well as subfractions of asphaltenes obtained by solvent extraction with toluene/heptane mixtures. These subfractions get more complex in structure as the toluene content of the extraction solvent increases and at the same time the CMC of the asphaltenes in pure toluene decreases. There is an indication that the presence of "lower" asphaltene species leads to less association in solution due to the interaction between these and the heavier asphaltenes. Asphaltenes of different origin have also been analyzed as a function of toluene/heptane solvent composition. It confirms the early finding that CMC decreases upon heptane addition. However, the relative decrease depends on the nature of the asphaltenes. Results from n-heptane asphaltenes from eight different locations are presented.