Articles Solvent Entrainment in and Flocculation of Asphaltenic Aggregates Probed by Small-Angle Neutron Scattering Keith L. Gawrys, † George A. Blankenship, and Peter K. Kilpatrick* Department of Chemical and Biomolecular Engineering, North Carolina State UniVersity, Raleigh, North Carolina 27965-7905 ReceiVed September 14, 2005. In Final Form: January 30, 2006 While small-angle neutron scattering (SANS) has proven to be very useful for deducing the sizes and masses of asphaltenic aggregates in solution, care must be taken to account for solvation effects within the aggregates so as to not err in the characterization of these important systems. SANS measurements were performed on solutions of asphaltenes dispersed in deuterated solvents in which a broad spectrum of solute and solvent chemical compositions was represented. Fits to the scattering intensity curves were performed using the Guinier approximation, the Ornstein- Zernike (or Zimm) model, a mass-fractal model, and a polydisperse cylinder model. The mass-fractal model provided apparent fractal dimensions (2.2-3) for the aggregates that generally decreased with increasing aggregate size, indicating increased surface roughness for larger aggregates. The polydisperse cylinder model provided typical values of the particle thicknesses from 5 to 32 Å, the average particle radius from 25 to 125 Å, and ∼30% radius polydispersity. Subsequent calculation of average aggregate molar masses suggested a range of solvent entrainment from 30 to 50% (v/v) within the aggregates that were consistent with previous viscosity measurements. Additional calculations were performed to estimate the proportion of microparticle to nanoparticle aggregates in the solutions. The results indicate that the inclusion of solvation effects is essential for the accurate determination of aggregate molecular weights and fractal dimensions. Introduction Petroleum asphaltenes are well-known for their tendency to associate in solution and adsorb at interfaces, implicating them in petroleum production problems such as organic deposition 1,2 and water-in-crude oil emulsion formation. 3-11 Fundamental research has focused on establishing a link between asphaltene chemical composition, molecular structure, and colloidal proper- ties. These efforts are complicated by the fact that asphaltenes, the portion of crude oil insoluble in n-heptane (or n-pentane), 12 are comprised of a polydisperse mixture of chemically hetero- geneous species 13 that can vary significantly from one crude oil to another. In general, the asphaltene molecular structure is characterized by the presence of fused ring aromatic moieties, small aliphatic side chains, and polar heteroatom-containing functional groups. 14-18 The chemical composition of asphaltenes is also polydisperse, with typical atomic H/C ratios varying between 1.0 and 1.3 and N, S, and O contents of a few weight percent. 19-22 Fourier transform infrared (FTIR) and X-ray absorption near-edge (XANE) spectroscopy reveal several polar functional groups, such as carboxylic acids, carbonyls, phenols, pyrroles, and pyridines, that are capable of participating in proton donor-acceptor interactions. 23-25 The aggregation mechanism for asphaltenes is primarily governed by van der Waals dispersion interactions, electrostatic interactions between molecular charges, hydrogen bonding of polar moieties, and orientation-dependent repulsive steric in- teractions, with lesser contributions stemming from intermolecular charge transfer and weak inductive interactions. 26 A recent * Corresponding author. 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