The Role of Asphaltene Solubility and Chemistry on Asphaltene Aggregation

摘要

Asphaltenes (n-heptane insolubles and toluene solubles) from a variety of crude oils were fractionated in mixtures of heptane and toluene and analyzed chemically, by vapor pressure osmometry (VPO), and by small angle neutron scattering (SANS). Solubility profiles of the asphaltenes and their sub-fractions indicated strong cooperative asphaltene interactions of a particular sub-fraction that is polar and hydrogenh bonding. This sub-fraction had lower H/C ratios and modestly higher N, V, Ni, and Fe contents than the less polar and more soluble sub-fraction of asphaltenes. Number average molar masses by VPO in toluene indicated that the most polar, least soluble sub-fraction was aggregated (>10, 000 g/mol) while the more soluble fractions approached monomer dimensions (approx 2, 000 g/mol) at high dilution. Neutron scattering studies showed that the most polar, least soluble sub-fractions formed aggregates considerably larger than both the whole asphaltenes and their more soluble subfractions. Aggregate sizes were calculated using the Guinier method and by fitting the scattering data to Lorentzian lineshapes. The less soluble samples were fit to a Lorentzian lineshape with an additional power law term to account for intense scattering due to flocs in the low wave vector (Q) range. Correlatin lengths obtained from the Lorentzian models are quantitatively proportional to the radius of gyration obtained from Guinier analysis by a scale factor of 1.62. Aggregate size of the asphaltenes and their subfractions increased with decreasing solvent aromaticity up to the solubility limit, beyond which the aggregate size decreased with heptane addition. The presence of the lwo Q feature in the scattering curves at 25 deg C indicated that the individual aggregates were flocculating. The intensity of the low Q feature was diminished upon heating of the samples to 80 deg C. The asphaltenes were dissociated into even smaller aggregates with the addition of a more polar solvent, such as 1-methylnaphthalene. A detailed picture of SANS information on distributions of asphaltenes in a variety of solvents will be presented.