Multi-step formation of asphaltene colloids in dilute solutions

Measurements of refractive index (RI) time series were used to study molecular aggregation in toluene solutions of n-C7 asphaltenes from four geographically diverse native crude oils. Asphaltene concentrations varied from 0.15 to 4500 mg/L. The results confirmed that measured parameters of aggregation stages are kinetically governed. Time-independent data were registered only in samples equilibrated for at least one week and revealed a multi-stage manner of aggregation before formation of colloidal particles at the "critical nanoaggregate concentration" (CNAC). Data analysis indicates that irrespective of the origin of asphaltene samples, all aggregation stages, including CNAC, are observed at the same concentrations of a "low-soluble" Al fraction of asphaltenes, although a relative content of this fraction decreases with increasing density of a parent crude oil. In particular, the onset of aggregation commences at a "critical dimerization concentration" (CDC) -0.48 mg/L of A1 asphaltenes. In the suggested model, the observed changes of mean RI below and above CDC are quantitatively explained by different volumes of toluene solvation shells around monomers and dimers of Al asphaltenes. (C) 2015 Elsevier B.V. All rights reserved.