Probing Deformable Oil-Water Interfaces by Atomic Force Microscopy and Cascade Partial Coalescence Measurements

  • Author / Creator
    Kuznicki, Natalie P.
  • Phase separation of stable oil-in-water and water-in-oil emulsions poses major challenges for a variety of industries, including the petroleum industry. The stability and destabilizing of petroleum emulsions highly depend on interfacial properties such as surface/interfacial charges, interfacial rheology, etc. Contrarily, for surfactant-free systems, determining the charge of liquid-liquid interface is very challenging due to inherent droplet deformations and large distribution of emulsified droplet sizes. Limited understanding is available on “clean” and “contaminated” droplets. Coated substrates, often used to study interfacially active materials, behave very differently from curved deformable interfaces. This thesis focuses on understanding the stability and characteristics of both charge-stabilized “clean” and surfactant-stabilized “contaminated” industrial systems. For “clean” oil-in-water systems, the occurrence of cascade partial coalescence was studied to link the observed partial oil droplet coalescence in surfactant-free electrolyte solutions to charged interfaces and determine the ζ-potential (linked to surface potential) of liquid-liquid interfaces. For a variety of fluids, it has been observed that a small droplet of organic liquid 1, slowly approaching through an immiscible liquid 2 a liquid 2- liquid 1 interface, underwent partial coalescence upon reaching the interface. During this process, only a part of the initial “mother” droplet passed through the interface and a smaller “daughter” droplet was left behind. We successfully linked the sizes of the last “mother” and “daughter” droplets to the surface potential bounds for a given set of liquids using the balance of van der Waals and electrostatic double layer forces, together with viscous resistance and apparent weight of the drop. Multiple emulsions are very difficult to remove from the water-crude oil system due to their special physical and interfacial properties, such as intermediate density and viscosity. Indigenous surface-active species, such as asphaltenes, form rigid skins at oil-water interfaces and alter surface properties of fine particles present in the system, causing further separation difficulties. In our study of “contaminated” systems using atomic force microscopy (AFM), we focus on interactions between a silica sphere and interfacial materials stabilizing water-in-diluted bitumen emulsions. The rapid dynamic aging of the interface results in formation of a rigid “skin” around water droplets, which changes the rheological properties of the interface, with the interface becoming non-Laplacian. To accurately describe the deformation of the oil-water interface, a viscoelasticity factor is successfully incorporated into the high force formula of the augmented Stokes-Reynolds-Young-Laplace (SRYL) equation to account for interfacial elasticity arising from the aging of the system. Incorporation of elasticity significantly improved predictions of droplet deformation in asphaltene-in-solvent solutions by SRYL equation for aged droplets. The AFM results show the droplet becoming less deformable and a rapid increase in adhesion force over time. After the addition of a demulsifier (ethyl cellulose, EC), droplets immediately become more deformable and unstable, with the interface becoming Laplacian again. Within minutes of EC addition, immersion of the probe into the droplet is observed. These changes at the interface, measured with AFM, provide insights on the effectiveness of demulsification by different chemicals, such as EC, at a fundamental level, showing the promise to ultimately reduce the cost of commercial operations in the petroleum industry. The major contributions to science of this thesis are developing a novel model describing the stability of “clean” surfactant-free systems using cascade partial coalescence measurements, and quantifying the forces present in “contaminated” water-in-oil emulsion systems by AFM colloidal probe force measurements. Introducing a novel viscoelasticity parameter in the high force SRYL equation to account for surface elasticity makes the SRYL model more accurate and versatile for predicting droplet deformation in non-Laplacian systems.

  • Subjects / Keywords
  • Graduation date
    Fall 2016
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • 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.