New techniques and methods for the study of aggregation, adsorption, and solubility of asphaltenes. Impact of these properties on colloidal structureand flocculation

The solubility of Furrial asphaltene in toluene was 57g L-1. However, using a new technique, based on the precipitation of this sample by the phenol P NP, we found that a fraction (2), comprising 47% of the asphaltene, is of l ow solubility. This suggested that this material constitutes the colloidal phase, and the rest acts as the dispersing fraction. This technique allowed the fractionation of asphaltenes in fractions A(1), A(2), and A(3) accordi ng to solubility, going from practically insoluble (A(1)) to low (A(2), 1 g L-1) to high (A(3), around 57 g L-1). The adsorption isotherms of asphalte nes on glass and silica in toluene consist of a sequence of steps or step-w ise adsorption. The first layer or first step is formed by the adsorption o f free asphaltene molecules and by small aggregates (aggregation number bet ween 3 and 6) which saturate the glass or silica surface in the usual manne r (L-type or H-type isotherms). However, we suggest that the second, third, and other asphaltene layers adsorb sequentially according to the above dif ferences in solubility. The very slow changes with time and the negligible desorption from the surface measured for the above isotherms were interpret ed as the effect of packing or the building up of a well packed layer. This would be achieved by the slow formation and rupture of bonds between neigh boring molecules at the surface. Thus, molecules with difficulties to pack, adsorbed by a kinetically controlled process, are either rejected or reloc ated in a thermodynamic controlled process. The above results and ideas wer e used to improve the models for asphaltene and petroleum colloids and to u nderscore the importance of surfaces and colloid dispersants in asphaltene precipitation during the production of crude oils. For instance, the result s described below suggest that colloids are constituted by a well pac ked a nd insoluble asphaltene core, impervious to the solvent, and by a loose pac ked periphery which, by allowing solvent penetration, keep the colloid in s olution. According to this model, desorption of compounds in the above loos ely packed periphery, such as the one promoted by a surface, would be the m ain cause of asphaltene precipitation from crude oils. In this case, solubi lity reductions caused by pressure drops during oil production would have a minor effect. Also, preliminary number average molecular weights M-n for f our asphaltenes, obtained using a new procedure, are presented here. The M- n values obtained ranged from 780 to 1150 g/mol.