Molecular Size Determination of Coal-Derived Asphaltene by Fluorescence Correlation Spectroscopy

Abstract

The molecular properties of asphaltenes have been the subject of uncertainty in the literature; in particular the molecular architecture is still a matter of debate. Some literature reports provide evidence that the contrast of petroleum asphaltenes versus coal-derived asphaltenes is useful for understanding the governing principles of asphaltene identity. Here, we employ fluorescence correlation spectroscopy to measure the diffusion constants of asphaltenes obtained from the distillation resid from coal liquefaction fluids. Concentrations employed herein are ∼10−8 molar, precluding asphaltene aggregation. These are compared with the same measurements on petroleum asphaltenes. These results confirm that the molecular sizes of these coal-derived asphaltenes are much smaller than virgin petroleum asphaltenes. Coal-derived asphaltenes are simpler than petroleum asphaltenes and provide correspondingly tighter constraints for understanding asphaltene molecular architecture. The small size, small alkane fraction, and large PAH of coal-derived asphaltenes are consistent with an “island” molecular architecture.

Model molecules mimicking asphaltenes

“…For instance Small-Angle X-ray and Neutron Scattering (SAXS and SANS) experiments were generally performed at concentrations close to 10 g.L -1 for asphaltenes dissolved in model solvents 17,18,[48][49][50][51] . It was not until 1999, using techniques such as fluorescence depolarization technique [52][53][54][55][56] , fluorescence correlation spectroscopy [57][58][59] and mass spectrometry 60,61 that reliable results, accounting for the molecular aggregation at higher concentrations, became available. These results have shown that typical mean molecular weights of asphaltenes were ∼750 g.mol -1 with a factor of 2 in the width of the molecular weight distribution.…”

Probing structural features of self-assembled violanthrone-79 using two dimensional infrared spectroscopy

“…The asphaltene-like PAHs have physical attributes that are similar to asphaltenes: their molecular diffusion behaviors are similar, they have similarly sized PAH cores consisting of seven to ten fused aromatic rings that are pericondensed, and they have heteroatom contents similar to those found in natural asphaltenes. 19 The use of the asphaltene-like compounds is prevalent because it is convenient to modify their peripheral chains via synthetic routes and these compounds provide a route to exploring the consequences of the peripheral side chains being symmetrically versus asymmetrically distributed about the PAH core. In this work, the investigation is focused on gaining insights into the nanoaggregate structures formed by the asphaltene-like molecule violanthrone-79 in chloroform.…”

“…The nanoaggregate structures of asphaltene-like molecules have been probed previously with proton nuclear magnetic resonance (H-NMR) spectroscopy, 18 scanning and transmission electron microscopies (SEM and TEM), 20,21 time-resolved fluorescence depolarization (TRFD) spectroscopy, 22 fluorescence correlation spectroscopy (FCS), 19,23 and molecular dynamics (MD) simulations. 16,24 The SEM and TEM experiments were used to visualize the morphology of the nanoaggregates and to determine the spacing between individual molecules within a nanoaggregate.…”

Multiscale Approach Linking Self-Aggregation and Surface Interactions of Synthesized Foulants to Fouling Mitigation Strategies

“…Other than DBE/carbon number, other boundary conditions imposed were the relevant range of molecular weights, the number of fused aromatic rings, the type and chemistry of heteroatoms and a propensity for selfassociation. These are considered the fundamental properties of the organic foulants that have been debated in literature over the past 50 years 8,27,28 . An example of a model organic foulant that is consistent with currently-accepted guidelines is the 1,6-bis(pyren-1-ylthio)hexane (BPH) molecule whose X-ray structure and coarse-grained model are shown in Fig.…”

“…This group of molecules is defined as a solubility class 6 and constitutes the most polar fraction of crude oil that precipitates upon the addition of n-alkanes. Asphaltenes make up from 0 to 20 % by volume of oil and they consist of the highest boiling point material that will remain in the distillation residue 7,8 .…”