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

Abstract

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. 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.

Waxes in asphaltenes of crude oils and wax deposits

“…The mean molecular weight for ''island'' asphaltenes is 750 with a distribution 500-1000, and the mean size is 2 nm (Mullins et al 2012). The ''archipelago'' model (Acevedo et al 2007;Strausz et al 2008) represents several fused ring systems that are interconnected by alkyl chains. These two molecular structures probably coexist, and the debate is now centered on their proportions and specific roles in an asphaltene mixture.…”

Structural Comparison of Asphaltenes of Different Origins Using Multi-stage Tandem Mass Spectrometry

“…8,9 The molecular weight distribution of asphaltenes remains controversial. [10][11][12] Vapor pressure osmometry and size-exclusion chromatography have yielded molecular weights of several thousands of Daltons. [13][14][15] However, fluorescence depolarization and mass spectrometry have indicated an average molecular weight of only 450-850 Da.…”

An Investigation on the Feasibility of Simulating the Distribution of the Boiling Point and Molecular Weight of Heavy Oils

“…Some researchers state that asphaltene molecular weight is about 750 g/mol, that is between 500 and 1000 g/mol (Groenzin and Mullins, 2000;Mullins et al, 2007;Mullins et al, 2008;Herod et al, 2007;Mullins et al, 2012;Dong et al, 2014). Others do not agree with the statement that asphaltene molecular weight is about 750 g/mol (Strausz et al, 2008;Wiehe, 2012). Wiehe (2012), for example ascribes to the opinion that the average molecular weight of asphaltenes is of the order of 3000.…”

“…Wiehe (2012), for example ascribes to the opinion that the average molecular weight of asphaltenes is of the order of 3000. Strausz et al (2008) mentioned about the highest molecular weight asphaltene fractions having molecular weight >17,000 g/mol. In other words there is still no full consensus about the magnitude of the molecular weight of the heavy end residual petroleum fractions.…”