A combined liquid chromatography coupled to a mass spectrometer with an ICP detector (μSEC-HR ICP MS; μSEC ICP for brevity) technique was used to analyze the metals in four asphaltenes and their corresponding A1 (toluene insoluble), A2 (toluene soluble), and trapped compound (TC, heptane soluble) fractions. For three of the asphaltene samples, the normalized μSEC ICP profiles for both nickel and sulfur were very similar, showing that nickel porphyrins were distributed in almost all types of asphaltene aggregates. Extensive overlapping with sulfur profiles was observed for all vanadium and nickel profiles at retention times below the maximum bands. This suggests that large amounts of nickel and other organometallic or metal-porphyrin-type (MP) compounds are interlocked with asphaltene molecules, forming aggregates in solution. The separation of MP compounds using common separation techniques is very difficult as extraction would require dissociation into several molecules. The presence of TCs (e.g., compounds other than asphaltenes that are soluble in n-heptane) in asphaltene aggregates was related to the fractal structure of asphaltene aggregates in which voids are filled with components coming from the surrounding media. Apparently, complete trapping of TCs is achieved by performing aggregate rearrangement after penetration, leading to an aggregate structure in which the TCs remain trapped. A similar trapping mechanism is proposed herein for the MP compounds. Accordingly, no covalent bonds or specific interactions appear to be required to account for the presence of MPs within asphaltene aggregates.

Asphaltene adsorption from a toluene solution on a glass surface was studied as a function of concentration, at ambient temperature and atmospheric pressure. The thickness of the asphaltene film measured by ellipsometry was found to be in the 20−298 nm range. The film thickness was found to increase by 38−70 nm after 24 h. Solutions containing asphaltene mixtures from different crude oils result in significant variations in film thickness when compared with single asphaltene cases. For CN-Ceuta and DM153-Ceuta mixtures, a diminution was observed, whereas an increase was found with the DM153-Furrial mixture. Finally, the results attained with asphaltenes in which low molecular weight compounds have been removed suggest a film swelling effect due to the alkyl-type resins which are found to modify the colloidal properties of the aggregated asphaltenes in agreement with the literature.

An electron microscopy study of four Venezuelan crude oils and their corresponding maltenes has been performed, using the combine freeze fracture-transmission electron microscopy (FFTEM) technique. All samples showed the presence of spherical primary colloidal particles, with average diameters in the range of 7-9 nm and an apparent Gaussian distribution. Large aggregates of primary particles, with the longest length being ∼200 nm or more were observed in some crude oils. These aggregates could be broken down to smaller aggregates or to primary particles by stirring and/or heating the crude oil. This indicates that the binding energy of these aggregates is weak, on the order of kT. Dilution with benzene up to 80 times does not dissolve the colloids completely but does reduce its diameter. On the other hand, the addition of heptane (up to 3 volumes) leads to an increase in diameter, which suggests the adsorption of resins upon dilution. In agreement with previous findings, 3 these results were observed to be consistent with the model for the colloidal particle, where low-solubility fractions are mainly located at the core and soluble fractions prevail at the periphery.

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