@article{rakotondradany_fenniri_rahimi_gawrys_kilpatrick_gray_2006, title={Hexabenzocoronene model compounds for asphaltene fractions: Synthesis & characterization}, volume={20}, ISSN={["0887-0624"]}, DOI={10.1021/ef060130e}, abstractNote={Asphaltenes are the fraction of bitumen with the highest molecular weight, containing polyaromatic hydrocarbons rich in heteroatoms and polar groups that result in strong self-association under extraction and upgrading conditions. The synthesis of alkylated hexabenzocoronenes is here reported to provide new insight into the behavior of bitumen residue fractions. The self-association behavior of these polyaromatic model compounds is investigated over a range of temperatures using vapor pressure osmometry (VPO), nuclear magnetic resonance (NMR), optical and electron microscopy, X-ray and small-angle neutron scattering, and calorimetry (DSC/TGA). In addition to these experimental studies, computational studies are used to determine the contribution of alkyl−alkyl and π−π stacking interactions to this association behavior. Experimental and computational results are compared to asphaltene properties under extraction and upgrading conditions, as well as to the archipelago and pericondensed models proposed for a...}, number={6}, journal={ENERGY & FUELS}, author={Rakotondradany, Felaniaina and Fenniri, Hicham and Rahimi, Parviz and Gawrys, Keith L. and Kilpatrick, Peter K. and Gray, Murray R.}, year={2006}, month={Nov}, pages={2439–2447} }
 @article{gawrys_blankenship_kilpatrick_2006, title={On the distribution of chemical properties and aggregation of solubility fractions in asphaltenes}, volume={20}, ISSN={["1520-5029"]}, DOI={10.1021/ef0502002}, abstractNote={Asphaltenes from B6, Canadon Seco (CS), and Hondo (HO) crude oils were separated on a preparatory scale into 20−30 so-called “fine fractions” by sequential precipitation from mixtures of n-heptane and toluene. Chemical analyses were performed to measure the carbon, hydrogen, nitrogen, sulfur, oxygen, and trace metals contents of the fractions. Small-angle neutron scattering (SANS) was performed on fractions dissolved or dispersed in mixtures of toluene-d (or methylnaphthalene-d) and methanol-d to determine the average aggregate size. Statistical analyses were performed to calculate the weighted moments of each elemental parameter assuming that the data were “normally” distributed. High values of the standard deviation and skewness because of the early precipitation of metal oxides and inorganic salts suggested that a Gaussian distribution was inappropriate to describe the behavior of most metals. H/C, N/C, and S/C ratios in B6 and HO asphaltenes appeared to obey a Gaussian distribution, with mean values a...}, number={2}, journal={ENERGY & FUELS}, author={Gawrys, KL and Blankenship, GA and Kilpatrick, PK}, year={2006}, pages={705–714} }
 @article{gawrys_blankenship_kilpatrick_2006, title={Solvent entrainment in and flocculation of asphaltenic aggregates probed by small-angle neutron scattering}, volume={22}, ISSN={["0743-7463"]}, DOI={10.1021/la052509j}, abstractNote={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 angstroms, the average particle radius from 25 to 125 angstroms, and approximately 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.}, number={10}, journal={LANGMUIR}, author={Gawrys, KL and Blankenship, GA and Kilpatrick, PK}, year={2006}, month={May}, pages={4487–4497} }
 @article{gawrys_kilpatrick_2005, title={Asphaltenic aggregates are polydisperse oblate cylinders}, volume={288}, ISSN={["1095-7103"]}, DOI={10.1016/j.jcis.2005.03.036}, abstractNote={Small-angle neutron scattering (SANS) has proven to be very useful for deducing the sizes and morphologies of asphaltenic aggregates in solution. A wide variety of intra-particle structure factors have previously been applied to SANS scattering spectra, but the studies often provided limited information concerning the quality of the fits and the Q range over which the models were applied. Selection of an appropriate form factor that closely approximates the structure of asphaltenic aggregates is important for determining the properties of asphaltenic aggregates, such as the radius of gyration (RG), molar mass, and apparent fractal dimension. This study evaluates various mono- and polydisperse intra-particle structure factor models as applied to four asphaltene scattering spectra. Agreement of the model fit parameters (I0 and RG) with those obtained from Guinier analyses suggests that such a form factor model is physically reasonable. Reduced χ2 values for each non-linear least squares fit indicates how well a given model fits to the entire Q range studied for the scattering intensity distribution. In the polydispersity analyses, an analytical function is introduced to model the scattering behavior of oblate cylinders with a Schultz distribution of radii. Results indicate that the polydisperse radius oblate cylinder model best approximates the shape of asphaltenic aggregates.}, number={2}, journal={JOURNAL OF COLLOID AND INTERFACE SCIENCE}, author={Gawrys, KL and Kilpatrick, PK}, year={2005}, month={Aug}, pages={325–334} }
 @article{akbarzadeh_bressler_wang_gawrys_gray_kilpatrick_yarranton_2005, title={Association behavior of pyrene compounds as models for asphaltenes}, volume={19}, ISSN={["1520-5029"]}, DOI={10.1021/ef0496698}, abstractNote={Asphaltene association in solution has been studied extensively using methods such as vaporpressure osmometry and neutron scattering. These methods give relative data on association as a function of solvent strength, temperature, and concentration, but interpretation of the results is hampered by the polyfunctional nature of asphaltenes and the distribution of molecular weight. In this work, we present data on association of representative model structures for asphaltenes, as measured using vapor-pressure osmometry in o-dichlorobenzene at 75-130 °C, and using smallangle neutron scattering in toluene. A series of compounds were synthesized based on the fourring aromatic compound pyrene. The synthetic compounds were designed to give interactions between aromatic rings, alkyl chains, and selected functional groups through aromatic interactions, hydrogen bonding, and polar interactions. Even in this strong solvent, polar interactions between oxygen functional groups gave average molecular weights of up to twice the true value, indicating dimer formation. An alkyl pyrene compound, dipyrenyl decane, gave much less significant association. Pyrene itself exhibited little or no self-association.}, number={4}, journal={ENERGY & FUELS}, author={Akbarzadeh, K and Bressler, DC and Wang, JN and Gawrys, KL and Gray, MR and Kilpatrick, PK and Yarranton, HW}, year={2005}, pages={1268–1271} }
 @article{gawrys_kilpatrick_2004, title={Asphaltene aggregation: Techniques for analysis}, volume={32}, ISSN={["1525-6030"]}, DOI={10.1081/CI-120030536}, abstractNote={Abstract The study of asphaltene colloidal properties is motivated by their propensity to aggregate, flocculate, precipitate, adsorb onto interfaces and, hence, pose considerable challenges for the petroleum industry. Asphaltenes are defined as the toluene (or benzene) soluble, yet n‐heptane (or n‐pentane) insoluble, portion of crude oil. The “solubility class” definition of asphaltenes generates a broad distribution of molecular structures that vary greatly among crude sources. Asphaltenes are generally characterized by fused ring aromaticity, small aliphatic side chains, and polar heteroatom‐containing functional groups (e.g., carboxylic acids, carbonyl, phenol, pyrroles, and pyridines) capable of donating or accepting protons inter‐ and intra‐molecularly. Molar H/C ratios between 1.0–1.2 and N, S, and O content of a few weight percent suggest that the asphaltene backbone mostly contains fused aromatic carbon interspersed with occasional polar functional groups. The most plausible mechanisms of asphaltene aggregation involve dispersion interactions between aromatic rings, polar and hydrogen bonding interactions between heteroatoms, and other charge transfer interactions. Understanding asphaltene chemistry and the fundamental mechanisms of colloid formation has been the driving force behind much petroleum research of the last half‐century.}, number={3}, journal={INSTRUMENTATION SCIENCE & TECHNOLOGY}, author={Gawrys, KL and Kilpatrick, PK}, year={2004}, pages={247–253} }
 @article{spiecker_gawrys_kilpatrick_2003, title={Aggregation and solubility behavior of asphaltenes and their subfractions}, volume={267}, ISSN={["0021-9797"]}, DOI={10.1016/S0021-9797(03)00641-6}, abstractNote={Asphaltenes from four different crude oils (Arab Heavy, B6, Canadon Seco, and Hondo) 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 subfractions indicated strong cooperative asphaltene interactions of a particular subfraction that is polar and hydrogen bonding. This subfraction had lower H/C ratios and modestly higher N, V, Ni, and Fe contents than the less polar and more soluble subfraction of asphaltenes. VPO and SANS studies indicated that the less soluble subfractions formed aggregates that were considerably larger than the more soluble subfractions. In general, asphaltene aggregate size increased with decreasing solvent aromaticity up to the solubility limit, beyond which the aggregate size decreased with heptane addition. The presence of a low wavevector Q feature in the scattering curves at 25 degrees C indicated that the individual aggregates were flocculating; however, the intensity of the feature was diminished upon heating of the samples to 80 degrees C. The solubility mechanism for Canadon Seco asphaltenes, the largest aggregate formers, appears to be dominated by aromatic pi-bonding interactions due to their low H/C ratio and low nitrogen content. B6 and Hondo asphaltenes formed similar-sized aggregates in heptol and the solubility mechanism is most likely driven by polar interactions due to their relatively high H/C ratios and high nitrogen contents. Arab Heavy, the least polar asphaltene, had a H/C ratio similar to Canadon Seco but formed the smallest aggregates in heptol. The enhancement in polar and pi-bonding interactions for the less soluble subfraction indicated by elemental analysis is reflected by the aggregate size from SANS. The less soluble asphaltenes contribute the majority of species responsible for aggregation and likely cause many petroleum production problems such as pipeline deposition and water-in-oil emulsion stabilization.}, number={1}, journal={JOURNAL OF COLLOID AND INTERFACE SCIENCE}, author={Spiecker, PM and Gawrys, KL and Kilpatrick, PK}, year={2003}, month={Nov}, pages={178–193} }