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

Abstract

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∝ R G D ) and between scattered intensity and wave vector ( I( q)∝ 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)∝ M − τ ) with exponent τ=1.66. The fractal dimension values and the mass distributions of asphaltene aggregates are similar to those predicted by the RLCA model.