The critical micelle concentration of asphaltenes as measured by calorimetry

Micellization of asphaltenes in solution has been investigated using a micr o calorimetric titration procedure (Andersen, S. I.; Birdi, K. S. J Colloid Interface Sci. 1991, 142, 497). The method uses the analysis of heat of di ssociation and dilution of asphaltene micelles when a pure solvent (or solv ent 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 procedur e is applied in investigation of asphaltenes as well as subfractions of asp haltenes obtained by solvent extraction with toluene/heptane mixtures. Thes e subfractions get more complex in structure as the toluene content of the extraction solvent increases and at the same time the CMC of the asphaltene s in pure toluene decreases. There is an indication that the presence of "l ower" asphaltene species leads to less association in solution due to the i nteraction between these and the heavier asphaltenes. Asphaltenes of differ ent 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 asph altenes. Results from n-heptane asphaltenes from eight different locations are presented.