Limitations of Size-Exclusion Chromatography in Analyzing Petroleum Asphaltenes: A Proof by Atomic Force Microscopy

Abstract

The analysis of a Maya asphaltene using size-exclusion chromatography (SEC) and 1-methyl-2-pyrrolidinone (NMP), tetrahydrofuran (THF), and toluene as the eluent (separately used) showed radically different chromatograms. The SEC of fossil fuel hydrocarbons, e.g., asphaltenes, using NMP has shown a two-peak structure, which has been the source of debate, whether the peak excluded from the column porosity (appearing at the shorter retention times) contained asphaltenic aggregates or very large molecular masses of thousands of mass units. The SEC of asphaltenes using THF and toluene showed retained peaks at the longer retention times. In this work, atomic force microscopy (AFM), a technique capable of reliable and accurate detection of the interaction forces between various molecules and in different media, was exploited to investigate the interaction forces between polystyrene (PS) particles (SEC column packing) and asphaltenes in air and in NMP. AFM showed a strong interaction between the PS particles and asphaltenes, the outcome of which can be the aggregation of asphaltenes in the SEC columns. The use of solvents, such as toluene, NMP, and THF, which either damage the column packing or do not dissolve asphaltenes completely, the interaction between asphaltenes, and the SEC column packing and aggregation of asphaltenes in the SEC columns are the major limitations of SEC in analyzing asphaltenes. Furthermore, PSs, currently used to calibrate SEC columns, are significantly different from asphaltenes in their structure and solubility behavior. In addition, the quantitative results of the SEC are considerably different from the reports from other techniques, e.g., LD, MALDI, FT-ICR MS, and molecular diffusivity methods. Thus, SEC results cannot be used quantitatively to determine mass distributions of asphaltenes.

Catalogue of Plausible Molecular Models for the Molecular Dynamics of Asphaltenes and Resins Obtained from Quantitative Molecular Representation

“…Discussions focusing on quantifying apparently simple general properties, such as the average molecular weight of an asphaltene, have become open, and sometimes bitter debates. 5,6,7 The efforts to experimentally determine the nature of the "prototypical" asphaltene molecule have been metaphorized 8 by using the Hindu tale of a group blind men attempting to agree on the description of an elephant after each of them only had the experience of each touching a different part of the animal; proclaiming that the elephant is like a tree (by whom had touched its legs), or like a snake (by whom had touched its nose), a wall (by touching its side), and so forth.…”

Molecular mass ranges of coal tar pitch fractions by mass spectrometry and size‐exclusion chromatography

“…Such data are relevant for the processing of these materials and their behaviour in refinery plant as well as being of academic interest. There has been vigorous discussion about how best to achieve the characterization of such materials, mainly focused on petroleum‐derived asphaltene 1–7. One part of the debate has focused on comparing petroleum asphaltenes with coal‐derived asphaltenes.…”

Petroleum Size Exclusion Chromatography: Mechanisms Explaining the NMP Front Peak

“…The prevailing mechanisms cannot explain the occurrence of these early peaks, which were also the center of much discussions in the literature regarding especially analysis of asphaltenes using the SEC technique. (1)(2)(3)(4).…”

“…These monodisperse uniform sized beads are commonly made up from polystyrene-divinylbenzene copolymers that form a network giving high mechanical stability. The process enabled can basically be exemplified as follows under ideal conditions: If a sample consists of three species of very large size (1), intermediate size (2) and small size (3) these will exit the column in that order. Large molecules (if larger than the biggest pore mouth) will not enter the beads and will exit first at the column outlet, while as size gets smaller the molecules will diffuse and penetrate deeper and deeper into the pack.…”

“…The selection of solvent as indicated above affects both the volumetric property of the column packing, and the adsorption of solutes to the column but it will also affect the conformation and the aggregation state of the solute being examined. While toluene and THF and even heptane-toluene mixtures with more than 50 vol % toluene may be decent solvents for petroleum asphaltenes the partial dissolution of asphaltenes and petroleum fractions in NMP has been reported 2,9,33 . NMP has a specificity for aromatic molecules over naphthenic structures.…”