Retention of Alkane Compounds on Asphaltenes. Insights About the Nature of Asphaltene–Alkane Interactions

Retention of alkane compounds on asphaltenes was carried out in organic solutions. The distribution of retained and nonretained alkanes was assessed by GC/MS and interpreted from the perspective of reverse phase liquid chromatography (RPLC). Results showed that asphaltenes from unstable and stable Venezuelan crude oils behave like an alkyl-bonded phase to retain alkanes. The retention is influenced by the structural features of the studied asphaltenes, as well as by the own structure of the interacting alkanes. Asphaltenes from the unstable Furrial crude oil presented shorter but more abundant alkyl chains than asphaltenes from the stable Ayacucho crude oil and, consequently, a greater capacity to retain alkane compounds. Nevertheless, the preference for a particular group of alkanes was very similar in both asphaltene samples: n -alkanes were the strongest retained, followed by cyclic isoprenoids (steranes and terpanes). Acyclic isoprenoids, including pristane and phytane were the least retained of the three alkane families. The exclusion of methylene groups, a phenomenon observed in RPLC, was also observed in asphaltenes. This phenomenon could be responsible for the appearance of liquid-crystal-like structures in asphaltene–alkane composites and for the destabilization of asphaltene assemblies. The approach used in this work opens a new window to interpretation of the interactions of asphaltenes and alkanes at a molecular level and to understand the roles of both compound classes in the mechanisms of asphaltene precipitation and wax crystallization.