Investigation of Aggregation Behavior and Molecular Weight Changes of Asphaltene Model Compound

Abstract

We investigate the aggregation of Violanthrone-78 (VO-78), a model asphaltene compound, in mixtures of toluene and n-heptane (heptol) using spectroscopic and dynamic and static light scattering techniques. The light scattering experiments were conducted using pure toluene and n-heptane, as well as 25/75, 50/50, and 75/25 heptol solvents. Furthermore, the concentration of VO-78 varied from 10-8 M to 10-3 M in these suspensions. The dynamic light scattering experiments were used to study the aggregate sizes from the intensity autocorrelation function at different n-heptane volume fractions. The results show the onset of aggregation of about 50 % n-heptane volume fractions. The aggregate sizes in toluene, within the range of concentrations examined, were below the instrument's detection limit (< 5 nm) but increased with increasing dosages of n-heptane. On the other hand, the aggregate size in pure heptane was observed to be greater than the maximum limit of the instrument (> 10 μm). Furthermore, the static light scattering experiments provided an anomalous behavior. Increasing concentrations of VO-78 in the solvent resulted in lowering the parameter Kc/Rq, where c is the molar concentration.