Clustering behaviour of polyaromatic compounds mimicking natural asphaltenes

Abstract

Clustering behaviour of hexa-tert-butylhexa-peri-hexabenzocoronene (HTBHBC) and its derivatives has been used as a model system to mimic that of natural asphaltenes. We used light scattering and 1H-NMR spectroscopy, complemented by molecular dynamics simulation, to examine HTBHBC and its derivatives in toluene and toluene/heptane mixture, over a range of concentrations. The dispersibility of HTBHBC in toluene was found to be strongly dependent on its concentration. At concentrations below 5 mg/mL, HTBHBC appears to be fully dispersed and clustering equilibrium was reached within minutes as shown by scattering intensity measurements. At greater concentrations, the scattering intensity was approximately similar for all concentrations initially, but then decreased very slowly towards an apparent clustering equilibrium within two weeks. The mean hydrodynamic diameter of clusters, measured by dynamic light scattering, was initially around 1 μm for all concentrations greater than 5 mg/mL and then gradually reduced to around 0.4 μm at clustering equilibrium. At concentrations of 10 mg/mL and above, solid deposits were observed in toluene solutions when equilibrium was reached. 1H-NMR spectroscopy showed that the precipitate was high purity HTBHBC possessing a planar structure, while the liquid phase contained a mixture of planar HTBHBC and its non-planar derivatives, forming colloidal clusters. The results show that the clustering process of asphaltene mimics in toluene can be extremely slow and great care should be taken when preparing equilibrated solutions. We also showed that observations of the solid-liquid equilibrium and clustering behaviour can be strongly dependent on the molecular structure of the polyaromatic compounds found in natural asphaltenes, and the structural and compositional evolution of colloidal clusters following an initial dispersion of asphaltene mimics in solvents.