A Critique of Asphaltene Fluorescence Decay and Depolarization-Based Claims about Molecular Weight and Molecular Architecture

摘要

Relying on experimental and theoretical data available from the literature, it is shown that the conclusions derived from measurements of fluorescence decay and depolarization kinetic times as reported in a series of papers over the past decade (Ralston, et al. Energy Fuels 1996, 10, 623-630; Groenzin, et al. J. Phys. Chem. A 1999, 103, 11237-11245; Groenzin, et al. Energy Fuels 2000, 14, 677-684; Buenzostro-Gonzales, et al. Energy Fuels 2001, 75, 972-979; Groenzin, H., et al. Energy Fuels 2003,17, 498-503; Badre, S., et al. Fuel 2005, 85, 1-11 and references therein) are egregiously wrong. To start with, the decay time measurements were done with inappropriate instrumentation which resulted in misleading results. Misinterpretation of the results led to the mistaken conclusion that bichromophoric type molecules are absent from petroleum asphaltene and therefore the architecture of the asphaltene molecule features a single condensed cyclic core spiked with some alkyl chains, in spite of irrefutable chemical evidence to the contrary. It was further concluded that if the asphaltene core is a single condensed ring, then the fluorescence depolarization with rotational correlation time method is applicable for the molecular weight determination of asphaltene. This is definitely not so, since, regardless of any other considerations, asphaltene is a mixture of a plethora of different, unknown components, with unknown concentrations along with innumerable different, unknown and some known chromophores portraying widely different absorption coefficients, fluorescence quantum yields, and kinetic decay times. Consequently, asphaltene fluorescence is a highly complex function of the above attributes and as such it is a totally unsuitable property for its molecular weight determination. The injection of an incorrect, single condensed ring core architecture for asphaltene has caused some confusion in asphaltene chemistry that has now hopefully been settled.