Data on the behavior of crude oil n-pentane- and n-heptane-insoluble fractions in aromatic solvent systems were obtained. Surface and interfacial tensions in systems formed by Brazilian crude oil n-pentane insolubles (C5I) and n-heptane insolubles (C7I) in the aromatic solvents toluene, pyridine, and nitrobenzene were measured at room temperature using the ring method and employing an automatic tensiometer. The results indicated possible asphaltene aggregation as well as the probable existence of critical micelle concentrations for both C5I and C7I fractions in each of the three solvents considered. Critical micelle concentrations measured for the C5I fraction in toluene and pyridine solutions were consistently higher than those in systems containing the C7I fraction, indicating a lower association tendency in the C5I fraction. Average molecular areas for asphaltenes adsorbed at different interfaces estimated from surface tension measurements were found to be in agreement with literature values and appeared to suggest a planewise surface adsorption of asphaltene molecules, which is consistent with currently accepted stacking aggregation mechanism of asphaltenes. The presence of a discontinuous point for the kinematic viscosity of asphaltene/toluene solutions as a function of asphaltene concentration suggested yet another type of aggregation of asphaltenes at concentrations higher than the observed cmc in toluene.

Petroleum could be seen as a system of asphaltenes dispersed in other petroleum components. The petroleum components excluding the asphaltenes are referred to in this paper as Petroleum Medium that Disperses the Asphaltenes – PMDA). This paper proposes a methodology to determine solubility parameter of the PMDA by identifying a model solvent which, when added to the petroleum, does not affect the asphaltenes phase behavior. In order to achieve such objective the asphaltenes precipitation onset (PO) of the petroleum was determined, followed by the identification of a solvent mixture which, when added to the petroleum, does not change the PO. The solubility parameter () of the PMDA was assumed equal to the solubility parameter of the identified solvent mixture.

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