Asphaltene Aggregation and Solubility

Abstract

An\nattenuated association model describing the aggregation of asphaltenes\nin solution is extended to derive an equation for the weight-average\ndegree of association and account for phase behavior. The weight-average\nmolecular weight is calculated to be higher than number average, as\nit must be for a polydisperse material, but not by enough to explain\nthe very large differences in these quantities reported in the literature.\nBinodals and spinodals are calculated using expressions derived previously,\nbut modified to account for free volume (thermal expansion) differences.\nThe phase behavior of asphaltene solutions is examined in more detail,\nparticularly in the dilute solution regime. It is shown that the formation\nof nanoaggregates significantly affects the critical value of the\nχ interaction parameter. The phase diagram is highly asymmetric\nand the phase boundary approaches the pure solvent composition limit.\nThis has a number of implications in terms of asphaltene solution\ncharacterization and the nature of asphaltene solutions. The results\nindicate that there are toluene insoluble asphaltene components, but\nthese could exist as microphase-separated clusters stabilized against\nfurther aggregation by steric and kinetic factors. This would explain\nthe large difference between observed number and weight-average molecular\nweights. In addition, because of the shape of the binodal curve at\nlow concentrations, experimental data that have previously been interpreted\nin terms of a critical cluster or micelle concentration are shown\nto be consistent with a microphase separation.