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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 .

The proposition of the hypothesis-relating structure with properties of asphaltenes is a very important endeavor. In view of their complexity, being an intricate mixture of high-molecular-mass compounds, experimental and theoretical procedures must be designed to check whether the structures proposed are consistent with expected and found behavior. During recent years, a structural characteristic of asphaltene, whereby the several polycyclic systems present in them are joined by flexible aliphatic chains, has been proposed by several authors. For simplicity, we will refer to this as the rosary-type structure. In this paper, using both reported as well as new experimental and theoretical results, an attempt is made to show how asphaltene properties, such as molecular-mass distribution, molecular fragmentation, solubility, adsorption, trapping of molecules, solvent swelling, and aging, could be accounted for in terms of the above rosary-type structures.

The Sphere method developed by Hansen (Hansen, C. M. Hansen Solubility Parameters: A User's Handbook; CRC Press: Boca Raton, FL, 1999) to calculate components of solubility parameters (SP), δ D , δ P , and δ H , where D, P, and H stand for dispersion, polar, and hydrogen bonding respectively, have been applied to resins, to asphaltenes, and fractions A1 and A2. Mean values for these compounds in MPa 0.5 were asphaltenes (19.5 ( 0.1, 4.7 ( 0.2, 4.2 ( 0.1); A2 (19.6 ( 0.1, 5.8 ( 0.1, 4.4 ( 0.2); A1 (20.9 ( 0.2, 5.6 ( 0.3, 6.8 ( 0.2); resins (18.6 ( 0.2, 3.6 ( 0.3, 3.2 ( 0.3). Also, the SP components of the asphaltene sample (AsH-NS), denuded from acidic natural surfactant (NS), were determined affording values equal to A2. These values were obtained after using 57 solvents, and the method is consistent with all known solubility properties of asphaltenes and confirm the expected solubility behavior of A1 and A2, with A1 being the less soluble material in all solvent examined. Excellent affinity between asphaltene with resins was predicted as well as affinity between samples examined. These results are coherent with a colloidal model whereby fraction A1 is in not contact with the solvent, being within a core shielded from the media by a periphery composed by fraction A2 and NS. Flocculation results measured in n-C 7 -1-methylnaphthalene mixtures correlate with the above SP showing the following order of flocculation tendency: A1 . A2 > asphaltenes (AsH). We also observed that flocculation points of A2 and AsH-NS were very close, and this strongly supports the colloidal model above. SP were helpful in analyzing other properties such as adsorption, vaporization, affinity, and molecular mass and paraffin's properties.

The precipitation and deposition of asphaltenes is a primary problem related to the processing, transportation and production of oil. Flocculation of asphaltene aggregates is likely to occur during the production and processing of crude oil. Recently, it has been shown that nanotechnology in the form of nanoparticles is useful for the inhibition or prevention of asphaltene formation damage. Although it is well known that the adsorption of asphaltenes on the nanoparticle surface would reduce the capacity of these asphaltic compounds to interact with each other, limited studies have been performed regarding the processes and the mechanisms associated with the effect of nanoparticles on the inhibition of the formation damage due to asphaltenes. To better understand this phenomenon from a mathematical approach, a population balance model (PBM) is proposed to describe the kinetics of asphaltene flocculation-fragmentation in the presence of nanoparticles. The model assumes that asphaltenes in the presence of a shear rate are related to the aggregation and fragmentation phenomena and includes a term related to the asphaltene adsorption on nanoparticles. An adsorption kinetic term was introduced into the model using the double exponential model. Experimental data of the kinetics of asphaltene aggregation were obtained by Dynamic Light Scattering (DLS) measurements at a fixed initial asphaltene concentration of 1000 mg/L and with different Heptol mixtures. In this study, commercial silica, γ-alumina and magnetite nanoparticles were used as adsorbents to study the effect of the chemical nature of the nanoparticles on the inhibition of the asphaltene growth and for model validation. Additionally, to demonstrate the versatility of the proposed model, the effect of asphaltene was also evaluated. The obtained results from the proposed population balance model agree well with the experimental data, within an % RSME < 9%.

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.

Adsorption of asphaltenes on glass surface has been studied by using photothermal surface deformation spectroscopy. This technique was adapted for direct measurement of the amount of asphaltene adsorbed on glass plate surfaces. Solid-solution adsorption isotherms for asphaltenes were obtained at room temperature in toluene. Stepwise or multilayer adsorption was detected in the three cases examined (Furrial, Jobo, and Hamaca) after long contact periods. After the first layer due to glass surface saturation one (Jobo and Hamaca) or two (Furrial) further layers were measured. Analysis of results suggests that this adsorption behavior could be related to the sudden increase of aggregate concentration typical of micelle formation. For instance, below 4000 mg/L Furrial asphaltenes behave like a mixture containing two surfactants with cmc of 1500 and 3000 mg/L, respectively. A slow increase in molecular weight when measured in toluene was observed for Hamaca (from 4230 to 17420) and Cerro Negro (from 5350 to 15170) over a 3 day period, suggesting a slow formation of aggregates. Accordingly, the above stepwise adsorption takes a long time to appear (more than 3 days). Isotherms measured at shorter times only showed a continuous but not stepwise increase in adsorption with concentration.

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