Solvent and Concentration Effects Governing the Hierarchical Organization of Asphaltenes: A Small-angle X-ray Scattering Study

摘要

Asphaltenes are a group of planar molecules found in crude oil and are prone to aggregation which often causes blockage of pipes along the oil production stream resulting a huge economic downside. In an effort to understand the hierarchical organization of asphaltene in different solvents and concentrations, the solution structure of various asphaltene solutions were studied using small-angle x-ray scattering (SAXS). Solvents were chosen based on miscibility maps. Such maps are generated on the critical value of the solubility parameter difference between asphaltenes or asphaltene components, and a range of solvents. The critical factors that affect asphaltene and asphaltene component miscibility with a particular solvent depend on the Hildebrand solubility parameter (delta H) which accounts for Asphaltene type and its source, dipole moment of the solvent, self-association of the asphaltene; the degree of self-association of strongly polar and hydrogen-bonding solvents; free volume differences between asphaltene components and solvents etc. SAXS, (a technique used for the determination of the microscale or nanoscale structure of particle systems in terms of such parameters as averaged particle sizes, shapes, distribution, and surface-to-volume ratio etc.) was used to study modified asphaltenes with polar groups removed at concentrations of 1 to 50 mg/ml in toluene. Additionally, SAXS measurement of different asphaltene concentrations using heptane-extracted asphaltene in a strongly polar solvent THF (tetrahydrofuran) were carried out at concentrations ranging from 1 to 500 mg/ml to find develop a quantitative description of the fractal aggregate nature of asphaltenes as a function of concentration. Concentration-dependent asphaltene SAXS measurements in benzene ranging from 1 mg/ml to 100 mg/ml were carried out using both High-Flux and High-Resolution mode. A mass fractal model over a Q-range of 0.008 - 0.4 A-1 was used in part of the analysis. The varying cutoff length, primary radius parameters, and the growing mass fractal dimension, all suggest that at a certain chain length, asphaltene nanoaggregates (NA) begin to collapse onto themselves to form a larger and denser aggregate. The experimental data has been fit with several models including the Unified Power Law and the Ellipsoidal/Spherical Hayter Mean Spherical Approximation models to compare their output parameters and develop a consistent view of asphaltene hierarchical structure. Furthermore, results will be compared and integrated with statistical mechanics theories such as the DLVO (Derjaguin--Landau--Verwey--Overbeek) theory that models absorption and the aggregation of nanoparticles in aqueous systems and describes the stability of colloidal dispersions. Future work will also include the use of other solvents, as well as the impact of pressure and temperature on the nanostructure of these systems. Moreover, the relation between the molecular dynamics parameters and the self-aggregation formed in the solution will be investigated by using a variety of solvents (i.e Weakly polar or non-polar) to observe its effects on the miscibility and the structural network of aggregated Asphaltene.