Experimental measurement of the mass distribution of petroleum asphaltene aggregates using ultracentrifugation and small-angle X-ray scattering

Fractionation of petroleum asphaltene in toluene has been carried out using ultracentrifugation. The separated asphaltenes are examined by small-angle X-ray scattering (SAXS). The radii of gyration and molar mass of the separated asphaltenes range over a wide scale that provides a good indication of the self-similar structure of asphaltene aggregates. The relations between mass and radius of gyration (M proportional to R(G)(D)) and between scattered intensity and wave vector (l(q) proportional to q(-d)) were studied. Values of both D (= 1.99) and d (= 2.06) show a fractal dimension D(f) of the asphaltene structure close to 2. The mass distribution of these aggregates can be calculated from the knowledge of both the quantity of separated fractions and the mass M of every fraction. We found experimentally a mass distribution with a power-law shape (N(M) proportional to M(-tau)) with exponent tau = 1.66. The fractal dimension values and the mass distributions of asphaltene aggregates are similar to those predicted by the RLCA model. (C) 2000 Elsevier Science Ltd. All rights reserved.