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.

The physical chemistry of the heavyweight fraction of crude oil is still a subject of vivid discussions due to the complications which arise from the tendency of this fraction to aggregate and finally hinder the refining processes. Asphaltene molecules, that compose the largest part of this fraction, can be separated into two subfractions, A1 and A2, through a treatment with p-nitrophenol. In this paper, starting from the molecule models suggested by Acevedo et al. (Energy Fuels20183266696677.), we screened the chemical structure and composition of the two subfractions and investigated their aggregation mechanism using classical molecular dynamics simulations. The results show that oxygen atoms, present as hydroxyl or carboxylic groups, are a key factor in the formation of large aggregate. From the analysis of the simulations, we estimated the size of the aggregates and showed how the flexibility of the molecules may affect the size of the aggregates. Finally we showed that in addition to the structural differences (such as the H/C ratio and DBE) that distinguish the A1 and A2 subfractions, the solubility of the subfractions is also strongly dependent on the ability of the molecules to bind through hydrogen bonds.