“Small-Angle Neutron Scattering Determination of Second Virial Coefficients for Asphaltenes in thePresence of Various Dispersants”

摘要

The use of small-angle neutron scattering (SANS) is indispensable for studying the aggregationbehavior of asphaltenes in crude oils and model solvents. Of particular interest are SANS studies thatprovide fundamental information on asphaltene aggregation mechanisms and the dependence on chemicalcomposition, solvency, and key additives that can mediate the effects of aggregation. Recent SANSexperiments have been performed on solutions of asphaltenes and resins in solvent mixtures containingheptane, toluene, and methanol as a function of solute concentration. The scattering curves obtained werefit to a mass fractal and monodisperse form factor models to obtain information concerning aggregate size,volume, molecular weight, and second virial coefficient values. Discrepancies between calculated andmeasured aggregate volume fractions were reconciled by assuming 30-70 % (v/v) entrainment of solvent inthe interstitial voids within aggregates. Subsequent evaluation of the second virial coefficient for the resinsolutions suggested that inter-aggregate interactions were repulsive in nature (A2 ~ 10-4 mol cm3/g2).Moreover, the magnitude of the virial coefficient increased as the solvent quality improved from 40:60heptane: toluene (v/v) to pure toluene, suggesting that the resin aggregates became increasingly dominatedby steric interactions. Virial coefficients for Hondo asphaltenes in toluene were also positive (A2 ~ 2x10-5mol cm3/g2), but less repulsive than the corresponding resins; however, the virial coefficient approachedzero with the addition of sufficient heptane flocculant (suggestive of non-interaction between the largeaggregates). Asphaltenes formed smaller, slightly less repulsive aggregates (MW ~ 190 kDa, A2 ~ 1.1x10-5mol cm3/g2) in mixtures of 90:10 toluene: methanol (v/v) compared to corresponding solutions in puretoluene (MW ~ 330 kDa), suggesting that the former solvent is more effective at dispersing nanoparticleaggregates via disruption of intra-aggregate polar and/or hydrogen bonding interactions. The addition ofdispersants, such as petroleum resins, to asphaltenes in toluene was effective at reducing molecular weightand increasing steric interactions between aggregates. Resin adsorption to polar and aromatic sites onasphaltenes likely results in the formation of a surface layer with aliphatic tail groups mainly presented tothe surrounding media. The adsorbed resin layer sterically hinders additional asphaltene adsorption. Theaddition of b-cholanic acid-- a model naphthenic acid previously shown in some cases to increase thestability of asphaltenic emulsions-- to solutions of asphaltenes in toluene did not significantly affectaggregate molecular weight or virial coefficient values. These results suggest that the careful measurementof thermodynamic quantities, such as second virial coefficients, can enable the design of more efficientdispersants and demulsifiers for petroleum mixtures.