(305e) Asphaltene Nanoaggregate and Cluster Dissociation

Small-angle X-ray scattering (SAXS) can provide detailed size and structural analysis of asphaltenes in dilute solvent mixtures. Such results can be used to investigate asphaltene structural changes or possible dissociation processes that have been reported in the literature to occur at the critical nanoaggregate concentration (CNAC) of approximately 100 mg/L [1,2]. SAXS experiments were conducted at the Advanced Photon Source synchrotron at Argonne National Lab using asphaltenes dispersed in toluene, THF, and 1-methylnapthalene. Asphaltene concentrations ranged from 5 vol. % down to 0.00125 vol. % (15 mg/L). The most dilute concentrations were sufficiently low to observe asphaltene nanoaggregate dissociation, which was detected by a decrease in concentration-normalized scattering intensity. Nanoaggregate dissociation occurred gradually as a function of concentration and no critical value was observed. In addition, coherent scattering was observed at the most dilute concentrations, indicating that nanoaggregates dissociation may only partially occur down to 15 mg/L. The fraction of asphaltene in the molecular vs. aggregate state was measured and fit to a two-state aggregation model. The size of asphaltene clusters decreased as a function of dilution and mass fractal scaling was observed for this size decrease. This finding reveals that asphaltene associations are fractal for any degree of aggregation or clustering, even at dilute concentrations. Finally, first of its kind wide-angle X-ray scattering results will also be discussed, revealing significant differences in local molecular ordering between asphaltenes dispersed in different solvents.  

[1]       Zeng, H., et al, Energy & Fuels, 23(3), 2009, pp. 1201-1208.

[2]       Lisitza, N. V., et al, Energy & Fuels, 23(3), 2009, pp. 1189-1193.