Changes of Diameter Distribution with Temperature Measured for Asphaltenes and Their Fractions A1 and A2. Impact of These Measurements in Colloidal and Solubility Issues of Asphaltenes

Asphaltenes and their fractions A1 (lower solubility) and A2 (higher solubility) were dissolved (3%) in resins and studied at three temperatures (T = 150, 200, and 250 degrees C), using the combined TEM FF (transmission electron microscopy freeze fracture) technique. In this study, resins coprecipitated with asphaltenes during separation from crude oil were used as solvents. In all solutions, asphaltene particles or nanoaggregates were observed in the range from about 2 to about 12 nm. While for the A2 sample, all calculated average diameters (d(n), d(w), d(z)) decrease with temperature, for asphaltenes and A1, the d(z)(T) curve have a minimum. For A1 such a minimum was also observed for d(w). These results are coherent with the view in which asphaltene colloids are porous bodies or fractals constituted by molecules of different structure and solubility. Most soluble molecules (A2TM) and molecular aggregates (A2 and A1-A2 type) would prevail both in solution and at the porous colloidal periphery at any T. Solvent held at the periphery would hinder flocculation. When T is raised, a fraction of molecules at the asphaltene colloidal periphery dissolves, thus reducing d(z); however, further increase in T thins the solvent layer, promoting flocculation and thus increasing d(z). Such flocculation is mainly promoted by A1TM. These results, as well as others previously reported, underline the importance of considering asphaltenes as a mixture of compounds with very significant solubility differences, such as the one determined for A1 and A2 fractions. For the temperatures examined, using TEM FF particle density for A1 sample was the highest, which is consistent with its low solubility when compared to asphaltenes and A2.