Contrasting Perspective on Asphaltene Molecular Weight. This Comment vs the Overview of A. A. Herod, K. D. Bartle, and R. Kandiyoti

Abstract

Asphaltene molecular weight (Asphaltenes, HeaVy Oils and Petroleomics; Springer: New York, 2007) continues to be the subject of a longstanding debate in the literature. A paper (Energy Fuels 2007, 21, 2176-2203 recently published (referred to as HBK) claims that asphaltene molecular weights are bimodal with one component in the roughly megadalton range and a second component in the roughly 5 kDa range. These claims are in sharp contrast to results published from a variety of measurements with the overall conclusions that asphaltene molecular weights are monomodal with a most probable 750 Da ((200) with a fwhm 500-1000 Da. In this report, we provide a summary of the four molecular diffusion techniques and seven mass spectral techniques from many groups around the world that are all in accord with the 750 Da most probable mass. Moreover, here we discuss why HBK reported anomalously large asphaltene molecular weights along with the unique claim of a bimodal distribution. In particular, the size exclusion chromatography (SEC) results that yield megadalton masses were performed with the solvent N-methylpyrrolidinone which is known to flocculate up to 50% of the asphaltenes. The megadalton mass is likely large asphaltene aggregates or flocs. In a previous referenced paper from the HBK labs, the better solvent for asphaltenes, tetrahydrofuran, did not give the megadalton peak in their SEC experiments as they stated; we suspect because the asphaltenes were suitably dissolved (although still with some aggregation). The corresponding discussion treats the known hierarchy of asphaltene aggregation at very low concentration in a good solvent, toluene. In addition, the mass spectral method used in HBK, laser desorption ionization, is shown herein and in the literature to yield anomalously large molecular weights for asphaltenes and polycyclic aromatic hydrocarbons due to gas phase aggregation if (1) the laser power is too high, (2) the surface concentration of asphaltenes is too high, or (3) if the ions are collected too quickly (i.e., from a dense plasma). Properly accounting for these potential pitfalls, one obtains the same most probable 750 Da molecular weight as from all of the other techniques. Finally, ESI MS is shown herein and in ample literature to be readily able to detect large masses (the primary reason ESI led to a Nobel Prize); the absence of large mass species in ESI MS of asphaltenes is because they are not present. The congruence of so many molecular diffusion techniques and mass spectral techniques is a powerful advance for asphaltene science.

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