Investigation of asphaltene aggregation with synthetic model compounds: an experimental and computational study

Aggregation of asphaltenes has attracted interest due to the impact on the crude-oil industry. Despite extensive studies on the molecular-structure of natural asphaltene, fundamental knowledge of their aggregation is incomplete. It is unclear how the driving forces of association are related to the molecular architecture and the solvent species, which ultimately affect the aggregation mechanism. In this dissertation, dynamic-light-scattering (DLS) experiments and molecular-dynamics (MD) simulations were performed to investigate the relation between asphaltene chemical-structure and solvent species. The model compounds studied isolate the driving forces of aggregation by varying the peripheral chain-length and functional-groups (triphenylene-cored models) in organic solvents. The results isolate the structure-function relationships. Increasing the chain length imposes restriction upon the nanoaggregate formation, while non-centrosymmetric models appear to be more prone to aggregation. Furthermore, polar components in asphaltene molecular-architecture are observed to increase aggregation potential, more than π-stacking. Hexabenzocoronene-cored models exhibit a structurally selective aggregation mechanisms, as the planar molecules are more liable to aggregate and precipitate than the non-planar models due to π-stacking hindrance. The motivation behind the development and testing of model polyaromatic compounds lies in the pursuit of isolating the source structural-dependence of the compounds interactions. This is done by assessing the solute-solute and solute-solvent associations by experimental and computational approaches, to underpin the structure-to-function relation dictated by aromatic and/or polar molecules in aromatic or aliphatic solvents. This dissertation provides insight for the aggregation of model compounds of varying molecular architectures, and sheds light on the intermolecular interactions affected by these variations and the solvent species

Revealing the photophysics of gold-nanobeacons via time-resolved fluorescence spectroscopy

We demonstrate that time-resolved fluorescence spectroscopy is a powerful tool to investigate the conformation states of hairpin DNA on the surface of gold nanoparticles (AuNPs) and energy transfer processes in Au-nanobeacons. Long-range fluorescence quenching of Cy5 by AuNPs has been found to be in good agreement with electrodynamics modelling. Moreover, time-correlated single-photon counting (TCSPC) is shown to be promising for real-time monitoring of the hybridization kinetics of Au-nanobeacons, with up to 60% increase in decay time component and 300% increase in component fluorescence fraction observed. Our results also indicate the importance of the stem and spacer designs for the performance of Au-nanobeacons

Combined Experimental and Computational Study of Polyaromatic Hydrocarbon Aggregation:Isolating the Effect of Attached Functional Groups

To establish, and isolate, the influence of different chemical functional groups on the aggregation of polyaromatic hydrocarbons, a series of triphenylene-based compounds were investigated using a combined experimental and computational approach. Containing alkoxy side chains of varying lengths or amide appendages, both with and without a terminating carboxylic acid, their aggregation structures, sizes, and kinetics in toluene were studied over several length scales, using a combination of dynamic light scattering and diffusion-ordered nuclear magnetic resonance spectroscopy, complemented with molecular dynamics simulations. There is a strong correlation between molecular architecture and aggregation mechanisms: the addition of polar functional groups and heteroatoms resulted in compounds that are more prone to aggregation and form large, micrometer-sized clusters, while the increased steric hindrance imposed by alkoxy side chains led to stable nanometer-sized aggregates. These conclusions underline the strong structure-function relationship of polyaromatic hydrocarbons, such as asphaltenes, examined here over multiple length scales in a single solvent. We also demonstrate the importance of using complementary techniques to study the aggregation process of polyaromatic hydrocarbons that could form aggregates of various sizes over different time scales

Combined Experimental and Computational Study of Polycyclic Aromatic Compound Aggregation: The Impact of Solvent Composition

The aggregation of polycyclic aromatic compound (PAC) molecules is sensitive to the solvent they are dissolved or suspended in. By using both dynamic light scattering and diffusion-ordered nuclear magnetic resonance spectroscopy, in combination with molecular dynamics simulations, the effect of chemical structure on the aggregation of PACs in both aromatic and alkane solvents were systematically investigated. A suite of triphenylene-based PACs offers a robust platform to understand the driving forces of aggregation mechanism across both nanometer and micrometer scales. Both the configuration, either parallel or otherwise, and the arrangement, whether compact or loose, of molecules in their aggregates are determined by a fine balance of different interactions such as those between the polar groups, π–π interactions between the aromatic cores, steric hindrance induced by the side chains, and the degree of solvation. These results suggest that molecular architecture is the major factor in determining how the model compounds aggregate. The shift from aromatic to aliphatic solvent only slightly increases the likelihood of aggregation for the model compounds studied while subtle differences in molecular architecture can have a significant impact on the aggregation characteristics

Clustering behaviour of polyaromatic compounds mimicking natural asphaltenes

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