IntroductionAmong several intricate physical, physico-chemical, and biological factors, natural light plays an important role [1][2][3][4][5][6] in the mitigation of oil spills at sea or ground surface. Nevertheless, little is known about the contribution of photooxidation to the lengthy degradation of asphaltenes [7][8][9][10][11], the most recalcitrant fraction to weathering processes. Their extremely complex composition probably accounts for this lack of data, by making gradual changes outstandingly difficult to investigate.The chemical nature of asphaltenes is poorly defined. In fact, they correspond to a solubility class of constituents: these precipitated from crude oils by adding at least 40 volumes of normal alkanes (nC 5 to nC 7 ). Asphaltenes contain lamellar macromolecules resulting from the condensation of polyaromatic, naphthenic and heterocyclic nuclei, bearing alkyl side chains and sometimes functionalized. Their molecular weight varies from several hundreds to several thousands [12,13]. Cross linked by alkyl, sulphur and oxygen bridges, the polycyclic sheets constitute a matrix that traps various aliphatic hydrocarbons, and form micelles spread among the other petroleum constituents (hydrocarbons and resins).Addressing chemical changes in such structures is a technical challenge, and hence separating samples into different fractions is an unescapable step before analysis. To investigate the effects of light in asphaltenes isolated from Blend Arabian Light crude oil, samples were fractionated by sequential elution solvent chromatography [14]. This technique, suitable for liquid coals [15][16], and adapted to crude oil asphaltenes [17] was selected, for it separates compounds according to functionalities. Subsequent analyses involved gravimetry, elemental analysis, NMR, UV-fluorescence and chiefly FTIR spectroscopy.
Materials and methodsThe effects of light exposure were studied in asphaltenes separated from Blend Arabian Light topped at 150°C (BAL 150). In addition, a number of asphaltenes from several other crude oils (Rospo Mare, Ekofisk, Oman, Handil, Irak, Saoudian (PBS), Kuwait, Shengli, Bach Ho, Zarzaïtine, Blend Arabian Medium and Bunker C) were fractionated and investigated to elucidate some FTIR data.
Precipitation of asphaltene fractionsAsphaltenes were precipitated in compliance with the NFT 60-115 AFNOR standard [18]. Crude oil (5 g) was mixed to n-heptane (200 mL) in a 250 mL round bottom flask, heated to reflux (90°C) under agitation during 2.5 hr, and then allowed to settle for 12 hr at room temperature. This solution was filtered on a glass microfiber filter (Whatman GF/A, porosity 0.45 µm). Warm heptane was used to drain out the precipitate coating the flask wall, and to thoroughly rinse the solid collected on the filter. The solid was dissolved with dichloromethane; the solution was then concentrated in a rotary evaporator and dried under nitrogen flow up to complete solvent elimination. In the case of BAL 150 crude oil, this technique yielded an asphaltene fraction of about 3%.
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