Aggregation States of Asphaltenes: Evidence of Two Chemical Behaviors by 1H Diffusion-Ordered Spectroscopy Nuclear Magnetic Resonance

Abstract

The mean molecular weight and particle size of asphaltene aggregates extracted from Buzurgan feedstock have been evaluated by measuring the diffusivity by means of 1H diffusion-ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR). This technique, recently applied in the petroleum industry, appeared as a key tool to investigate the behavior of asphaltenes diluted in toluene over a wide range of concentrations (from 0.01 to 15 wt %). The results show that the diffusivity is highly dependent upon solute concentration. Indeed, interactions occurring in dilute systems differ from interactions occurring in the semi-dilute regime. In the dilute regime (below 0.25 wt %), physical characterization of the nanoaggregates detected by 1H DOSY NMR could be achieved. An average molecular weight of roughly 6900 g mol−1 (taken at the highest diffusion peak and given as a polystyrene equivalent) with a range of 1500−85000 g mol−1 was obtained, while a mean radius of 15.6 Å was determined from the solute diffusivity at infinite dilution [D∞asp ∼ (2.4 ± 0.1) × 10−10 m2 s−1 for Buzurgan asphaltenes]. Average masses (Mn and Mw) were also calculated from DOSY NMR data and compared to results obtained from size exclusion chromatography (SEC) analyses. We also used DOSY NMR techniques to investigate the molecular dynamics of asphaltenes. A concentration of 0.25 wt % was found to represent the onset of the aggregation process. We believe DOSY NMR allowed us to observe the beginning of aggregation in phase transition. The asphaltene system is very polydisperse. At low concentrations, it is a polydisperse diluted system, but increasing the solute concentration induces a distinction of different aggregates, presenting various sizes in a macroscopic homogeneous phase. When the distinction occurs, there is a zone that is richer in high molecular weight aggregates of asphaltenes, and as a result, a second zone is impoverished in high molecular weight aggregates. In a microscopic point of view, there is a difference in nano or macroaggregate concentration, whereas in a macroscopic point of view, an average density is observed. For the first time, a clear separation between two families of aggregates of asphaltenes is presented in the diffusion dimension for concentrations higher than 3 wt %. 1H DOSY spectra and diffusion profiles confirm these results. The key point of this study resides in the detection and presentation of two classes of aggregates of asphaltenes achieved for concentrated solutions, without any assumption concerning the composition of the mixture.