X-ray diffraction of subfractions of petroleum asphaltenes

Abstract

New data are reported from powder X-ray diffraction (XRD) investigations of fractions obtained from separation of the bulk heptane-asphaltenes into subfractions using toluene-heptane mixtures. The latter indicate that when removing large parts of the heptane-asphaltene fraction by extraction with solvent mixtures having an increased toluene content, the stacking distance (or intensity of the 002 peak) or other parameters derived are not particularly changed. Hence, this infers that no increase in molecular interaction takes place by removal of more soluble species. This is confirmed as the stack diameter is insensitive to subfractionation, as observed from the 100 band. The stacking of asphaltenes has previously been inferred from powder X-ray diffraction of solid asphaltenes. The XRD patterns usually reveal three different broad peaks where the stacking of the asphaltene constituents is determined from the 002 peak that sometimes occurs as a shoulder (rather than as a distinct peak) on a peak related to alkyl ordering. The possibility of stacking as a major interaction is discussed in terms of structural data, as is the quantitative information and sensitivity of this when the necessary curve resolution and fitting routines are used. © 2005 American Chemical Society.