Cumene solutions of asphaltenes (8 g/L) were treated with p-nitrophenol (PNP) and a precipitated solid was collected after 3 days. For the asphaltene samples examined, between 48 and 70% of the sample could be precipitated by this treatment. PNP, known to form chargetransfer complexes with aromatic compounds, probably forms this type of compounds with asphaltenes, leading to precipitation. The sample precipitated was dissolved in chloroform, and the PNP was removed by extraction with aqueous sodium hydroxide. Although the initial asphaltene sample has the usual high solubility in toluene (higher than 50 g/L, room temperature), the precipitated asphaltene sample obtained in this way (A 1 ), has a very low solubility in toluene (≈90 mg/L, room temperature). These results show that asphaltenes are a mixture of compounds with vast differences in solubility and suggest that their solutions in aromatic solvents are constituted by a colloidal phase, formed by the low-soluble fraction A 1 dispersed by a soluble asphaltene fraction A 2 .

Seven asphaltene samples and six octylated asphaltene (OA) derivatives were analyzed using laser desorption ionization−time-of-flight (LDI−TOF) mass spectrometry (abbreviated as MS) and vapor pressure osmometry (VPO) techniques. Molecular weight distributions (MWDs) that were determined using MS spanned, for all asphaltenes samples and their octylated counterparts, a similar range, from ∼100 Da to ∼10 000 Da. For asphaltenes, the number-average molecular weight (M n ) and weight-average molecular weight (M w ) afforded values in the 1900 ± 200 and 3200 ± 400 intervals, respectively. Consistently heavier values were observed for the OA derivatives (M n ≈ 2300 ± 200 and M w ≈ 3600 ± 200). To select the adequate laser power for these measurements, experiments at different laser powers were performed, to increase volatility and reduce fragmentation to a minimum. Several other experiments were performed to validate these results. First, good agreement between the measured and calculated M n values was observed for OA materials (calculated from asphaltene M n and n, the number of octyl groups introduced, as determined from elemental analysis); second, M n values, as measured by VPO and MS, were determined to be equal, within an average standard deviation of ±27.0%. These results and calculations strongly suggest that the MWD, the molecular weight range, and the molecular weight averages determined using the present MS technique are reasonable estimates of the molecular weight properties of asphaltenes and not the result of artifacts such as fragmentation, polymerization, incomplete volatilization, etc., which may be occurring during the MS experiment.

Both, the toluene-insoluble (A1) and toluene-soluble (A2) asphaltene fractions, isolated using the PNP method, have been characterized by elemental analysis, molecular weight (VPO, SEC, and LDMS), and NMR (1H and 13C). The most prominent results of the analysis were the differences in hydrogen aromaticity f H, high content in both fractions of hydrogen bonded to aliphatic carbons joined to aromatics (f α), and differences in carbon aromaticity f C. Thus, low f H and high f α in A1 were consistent with a single, rigid, and flat core formed by fusion of polycyclic aromatic and naphthenic units (a single and large PANU), whereas for A2, high f H and high f αwere consistent with a more flexible structure where several smaller PANU are joined by aliphatic chains. Using a MM program, models for A1 and A2 were built and the solubility parameters calculated were found in keeping with solubility difference suggesting that the above structural differences account for the solubility difference. Similar molecular weight and heteroatom content found for these fractions suggest that these play a minor or insignificant role in solubility. A dispersion mechanism of A1 by A2, relevant to solubility of asphaltene in organic solvents, is proposed.

Two acidic samples, A and B, were isolated from Cerro Negro extraheavy crude oil. Sample A has a relatively low number average molecular weight (M n = 480 Da), low aromaticity (f a = 15.2%), and low γ (6.8 m Nm-1, toluene−0.1 N HCl), whereas for B, these values were M n = 1200 Da, f a = 20%, and γ = 14.8 m Nm-1. These values could be compared to those for Cerro Negro asphaltenes under the same conditions (M n = 5000, in toluene by VPO, f a = 47%, γ = 20.9 m Nm-1).

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