Molecular and Surface Interaction Mechanisms of Asphaltenes in Organic and Aqueous Media

• Author / Creator

  Zhang, Ling

• Asphaltenes are the heaviest component in crude oil and bitumen. The molecular interactions and interfacial behaviours of asphaltenes are closely related to many challenging issues in oil production, which are believed to play an important role in stabilizing water-in-oil (W/O) and oil-in-water (O/W) emulsions, surface wettability change of clays, stabilizing solid particles, and fouling problems. In this work, the intermolecular and surface interactions of asphaltenes, extracted from different oil resources, in both organic and aqueous media have been experimentally investigated using several complementary techniques including surface forces apparatus (SFA), atomic force microscope (AFM), quartz crystal microbalance with dissipation monitoring (QCM-D), contact angle goniometer and in-house built computer-controlled 4-roll mill fluidic device. The interactions between asphaltenes were measured using SFA in heptol solvent with different ratio of n-heptane. The results showed that the interactions gradually changed from pure repulsion to weak adhesion as the ratio of toluene increased and theoretical fitting of the force curves also determined the force origin. Micropipette tests results showed asphaltenes at water/oil interfaces inhibited the coalescence of the emulsion drops and 4-roll mill tests revealed that the interfacial sliding between emulsion drops was required to destabilize the asphaltene layer at interface which could increase the probability of emulsion coalescence. The adsorption mechanisms of asphaltenes on silica surface were studied in toluene using QCM-D and SFA. It was found the adsorption of asphaltenes was dependent on the sources of the asphaltenes and showed various behaviours under different flow and static conditions. The asphaltenes precipitated from a crude oil was found to be able to continuously adsorb on silica surface under flow condition whereas the adsorption reached the equilibrium state under static condition. Combined with the elemental and X-ray photoelectron spectroscopy (XPS) analysis, Ca and O might play an important role in the aggregation of asphaltenes even at a low concentration in toluene resulting in continuous adsorption behaviour. The interaction mechanisms between asphaltene surfaces in aqueous solutions were investigated using SFA and AFM. The SFA results showed repulsion was measured during approach while adhesion was detected during separation. The measured long-range repulsion could be affected by the solution pH, salinity and Ca2+ addition whereas could not be described by the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. However, theoretical calculations based on the DLVO theory reasonably agreed with the measured force curves between AFM tip and asphaltene surfaces, indicating the DLVO interaction origin of asphaltenes in aqueous solutions at nanoscale. Pancake-like domains were observed on asphaltene films in aqueous solutions and were significantly affected by the solution conditions, which might be due to the interactions of asphaltene molecules (particularly the polar groups), water, and the supporting solid substrate. van der Waals interaction was calculated between water and mica surface across the asphaltene film and a water diffusing process was proposed for the paterning phenomena observed. The results in this work provide insights into the fundamental understanding of molecular and surface interaction mechanisms of asphaltenes in both organic solvents and aqueous solutions, with important implications to the stabilization/destabilization mechanisms of emulsion drops and fine solids in the presence of asphaltenes in oil production.

• Subjects / Keywords

  • Emulsions
  • Surface interactions
  • Molecular interactions
  • Asphaltenes

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3X63BB1J

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.