About the Colloidal Nature of Asphaltenes and the MW of Covalent Monomeric Units

Abstract

Problems surrounding molecular aggregation, covalent molecular weight, and their experimental investigation in asphaltene chemistry are reviewed. Chromatographic, fluorescence spectroscopic, and mass spectroscopic (MS) methods for the investigation of these problems are surveyed and their merits and limitations discussed. Dissociation of asphaltene in dilute solution can be followed in time by monitoring the gel permeation retention time variation with the age of the solution. This way, Athabasca asphaltene was reported to dissociate from several thousand to less than about 1000 g mol-1 molecular weight (MW) species in CH2Cl2 solution to an extent of at least 80% in 14 days' time. The dissociation products represent the monomeric covalent molecules of asphaltene, and the remaining undissociated 20% could be slowly dissociating aggregates or high-MW covalent asphaltenes. The vapor pressure osmometry (VPO) determined number average MW of the same asphaltene was of the order of 4000 g mol-1, manifesting the aggregated state of the asphaltene at the higher concentrations used for VPO measurements. Of the MS methods, the most thoroughly investigated and proposed to be the best suited to asphaltene studies are the laser desorption ionization/matrix-assisted laser desorption ionization (LDI, MALDI) time-of-flight (TOF) MS. However, results obtained from various laboratories do not compare well; in some cases the bulk of the m/z lies below 1000, and in others it lies well above m/z 1000. 252Cf plasma desorption MS data are more self-consistent in the sense that the bulk of m/z values always lie below m/z 1000. The upper m/z limit in most cases is around a few thousand m/z but may extend up to tens of thousands m/z. The problems affecting these methods for the determination of covalent, monomeric asphaltene MW distributions are fragmentation of covalent bonds, multiple ionization, and the production of cluster ions. Fluorescence-based methods are not suitable for MW measurements in asphaltene; the reasons for this are discussed in detail.