On the Size Distribution of Self-Associated Asphaltenes H. W. Yarranton,* ,† D. P. Ortiz, † D. M. Barrera, † E. N. Baydak, † L. Barre ́ , ‡ D. Frot, ‡ J. Eyssautier, ‡ H. Zeng, § Z. Xu, § G. Dechaine, § M. Becerra, § J. M. Shaw, § A. M. McKenna, ∥ M. M. Mapolelo, ∥ C. Bohne, ⊥ Z. Yang, ⊥ and J. Oake ⊥ † Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr. NW., Calgary, Alberta, Canada T2N 1N4 ‡ IFP Energies Nouvelles, 1 & 4, Avenue de Bois-Pre ́ au, 92852 Rueil-Malmaison, Cedex, France § Department of Chemical and Materials Engineering, University of Alberta, 114 Street 89 Avenue NW, Edmonton, Alberta, Canada, P6G 2M7 ∥ Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, 1800 E. Paul Dirac Dr., Tallahassee, FL, 32310-3706, United States ⊥ Department of Chemistry, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia, Canada, V8P 5C2 * S Supporting Information ABSTRACT: A variety of experimental techniques were applied to a single source asphaltene sample at the same experimental conditions in order to reveal the possible size distributions of asphaltene monomers and aggregates. The asphaltene sample was divided into solubility cuts by selective precipitation in solutions of heptane and toluene. Asphaltene self-association was assessed through a combination of density, vapor pressure osmometry (VPO), elemental analysis, Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry, and time-resolved fluorescence emission spectra measurements performed on each cut. The physical dimensions of the asphaltenes were assessed using SAXS, DLS, membrane diffusion, Rayleigh scattering, and nanofiltration measurements. Molecular and nanoaggregate dimensions were also investigated through a combination of interfacial tension, interfacial adsorption, and surface force measurements. All of the measurements indicated that approximately 90 wt % of the asphaltenes self-associated. Ultrahigh resolution spectrometry suggests that the nonassociated asphaltenes are smaller and more aromatic than bulk asphaltenes indicating that the associating species are larger and less aromatic. On the basis of VPO, the average monomer molecular weight was approximately 850 g/mol, while the molecular weight of the nanoaggregates spanned a range of at least 30000 g/mol with an average on the order of 10000 to 20000 g/mol. SAXS and DLS gave molecular weights 10 times larger. The physical dimensions of the nanoaggregates were less than 20 nm based on nanofiltration and with average diameters of 5 to 9 nm based on diffusion and Rayleigh scattering. SAXS and DLS gave average diameters of 14 nm and indicated that the nanoaggregates had loose structures. Film studies were consistent with the lower molecular weights and dimensions and also demonstrated that asphaltene monolayers swell by a factor of 4 in the presence of a solvent. The most consistent interpretation of the data is that asphaltenes form a highly polydisperse distribution of loosely structured (porous or low fractal dimension) nanoaggregates. However, the discrepancy between VPO and SAXS molecular weights remains unresolved. 1. INTRODUCTION After almost a century of research, the structure and molar mass distributions of asphaltene molecules and self-associated aggregates continue to be debated. The subject is challenging because asphaltenes are a mixture of hundreds of thousands of different chemical species and a fraction of the asphaltene molecules self-associate. This brief introduction reflects the span of current debates on pertinent topics but does not comprise an exhaustive review. Considerable progress has been made in establishing the average molecular weight of asphaltene monomers. Vapor pressure osmometry measurements extrapolated to zero concentration suggest average monomer molecular weights on the order of 1000 g/mol. 1,2 Significantly lower average molecular weights are inconsistent with the wide product distributions observed after hydrotreating or thermal cracking. 3 However, the size and structural distributions of the monomers are unknown. Few data are available for the size distribution of asphaltene monomers because they tend to self-associate at very low concentrations. High-resolution mass spectrometry provides qualitative results for heavy oil fractions and indicates a wide distribution of molecular weights ranging from a few hundred to approximately 1500 g/mol. 4 Structural distributions have been inferred from a variety of measurements 5−7 but with conflicting interpretations. The traditional view of asphaltene molecular structure is a “continent”, a highly condensed aromatic center containing some heteroatoms and some n- alkyl side chains. 8 A more recently proposed alternative structure is an “archipelago”, smaller condensed aromatic groups with some heteroatoms connected by alkyl bridges. 5 Received: April 22, 2013 Revised: July 21, 2013 Published: August 5, 2013 Article pubs.acs.org/EF © 2013 American Chemical Society 5083 dx.doi.org/10.1021/ef400729w | Energy Fuels 2013, 27, 5083−5106