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.

Fractions A1 and A2 have been isolated from asphaltenes using the para-nitrophenol (PNP) method, 20,21 with fraction A2 being characterized as one with a tendency for aggregation smaller than asphaltenes and the opposite being observed for fraction A1. Hence, measuring molecular mass (MM) properties of fraction A2 would suggest MM properties of asphaltenes where aggregation is either smaller or insignificant. In this work, using several samples of asphaltenes and different solvents and temperatures, we investigated the aggregation tendency of fraction A2 using the vapor pressure osmometry (VPO) technique. When measurement of the number average molecular mass (M n ) was performed in toluene and o-dichlorobenzene (ODB), M n ≈ 1000 g mol -1 was found. Evidence for fraction A2 aggregation was detected in chloroform and nitrobenzene. We found that ODB was the best solvent for VPO, where a constant value close to 1000 g mol -1 was measured for fraction A2 when the temperature was changed from 80 to 120 °C. In all cases studied, the M n ratio of fractions A1/A2 was greater than the one suggesting that aggregation properties of asphaltenes are mainly due to fraction A1.

Super high-molecular-weight (SHMW) asphaltene aggregates, hereafter called asphaltene clusters, have been observed for the first-time using gel permeation chromatography with inductively coupled plasma mass spectrometry (GPC-ICP HR MS). Presumably these clusters are composed by nanoaggregates joined by physical interactions such as polar and dispersion forces. These clusters were observed for three asphaltenes samples (Hamaca, Cerro Negro and Boscan) and in their corresponding subfractions, A1 (insoluble in toluene) and A2 (soluble in toluene). Under the present conditions, dilution experiments showed no significant change of these profiles suggesting that these clusters, as well as the other lower molecular weight (LMW) aggregates and molecules are not in equilibrium with each other and behave as independent units. This is supported by the presence of trapped compounds within asphaltenes (TC) which are released after the p-nitrophenol treatment. A temperature cycle was implemented to monitor possible changes of GPC-ICP HR MS profiles with temperature. Here Boscan samples were heated from 25 to 200 °C, left standing by for 24 h and then cooled back to 25 °C before measurement. Whereas we found small changes for Boscan asphaltenes (As-Bo), vast changes in the profiles were detected for A1-Bo and A2-Bo subfractions, and these disclose interchange of material between the cluster and the other components of the solution. These results are discussed in terms of different arrays that may result when asphaltenes are separated in the two subfractions, and nanoaggregates are relocated within the cluster before and after heating. Preliminary molecular dynamics calculation afforded intensity-size distribution coherent with the usual experimental GPC chromatography profiles.