Probing the Surface Interaction Mechanisms of Air Bubbles and Oil Droplets with Hydrophobic Polymers, Asphaltenes and Bitumen
Creators: Yang, Diling
Publisher: name: University of Alberta Library
Year: 2023

<p>Interfacial interactions among solid particles, gas bubbles and liquid drops play a critical role in numerous interfacial phenomena and biophysical and industrial processes, such as drug and gene delivery in biological systems, froth flotation and oil/water separation in mining and oil extraction industry. Polymers, asphaltenes, bitumen, gas bubbles, and liquid droplets are commonly involved in numerous engineering processes. It is of both fundamental and practical importance to characterize the interfacial interaction mechanisms of these species underlying the relevant industrial processes. In this thesis, the surface forces among bubbles, hydrophobic polymers, asphaltenes and bitumen in complex aqueous media have been directly quantified to reveal their interaction mechanisms.<br>
Polystyrene (PS) of different molecular weights have been used as model hydrophobic polymers, and their interactions with air bubbles and oil droplets in aqueous solutions have been investigated using the bubble probe atomic force microscope (AFM) technique. The measured forces were analyzed by a theoretical model based on Reynolds lubrication theory and augmented Young−Laplace equation by including the influence of disjoining pressure. Despite the surfaces of PS of different molecular weights showing similar hydrophobicity (i.e., water contact angle in the air), differences were detected in the strength of their hydrophobic (HB) interactions with air bubbles and oil droplets. It is found that molecular chain mobility plays an important role in surface hydrophobic interactions and surface hydrophobicity. It is also found that the hydrophobic interactions between PS and air bubbles or oil drops can be significantly suppressed due to the presence of surface-active species such as surfactants in the aqueous solutions. <br>
The surface forces of air bubbles, asphaltenes and asphaltenes-toluene droplets in various aqueous solutions have been quantified using an integrated thin film drainage apparatus and bubble probe AFM. The effects of asphaltenes concentration, pH, salinity, presence of Ca2+ and surfactants have been investigated. In solutions of high salinity (i.e., 100 mM NaCl), the HB interaction plays a critical role in the attachment of air bubbles and asphaltenes surfaces or oil droplets, by overcoming the repulsive van der Waals forces (VDW) and electrical double layer (EDL) interactions. Increasing the asphaltenes concentration in oil droplets enhances their HB attraction with bubbles due to strengthened asphaltenes adsorption and their aggregation at the interface of aqueous and oil media. Increasing aqueous solution pH weakens the HB interaction as the asphaltenes-toluene surfaces become more negatively charged and less hydrophobic under higher pH. Under low salinity conditions (i.e., 1 mM NaCl), the strong repulsive EDL and VDW interactions inhibit the bubble-oil droplet contact. Introducing Ca2+ ions and surfactants leads to strong steric repulsion, preventing the contact of bubbles and asphaltene-oil droplets. <br>
The interaction forces between air bubbles and various bitumen surfaces in complex aqueous media have also been systematically measured. AFM imaging results show that the bitumen surfaces become rougher in aqueous solutions of high NaCl and CaCl2 concentrations and a strongly alkaline environment. Surface force measurements demonstrate the impact of ionic strength, solution pH and presence of surfactants on the bubble–bitumen interaction and attachment. In solutions of low salinity, the EDL repulsion dominates the bubble-bitumen interaction and prevents the bubbles from attaching to the bitumen surfaces, and such effects are further enhanced with increasing solution pH. Under high salinity conditions, the EDL interactions are significantly suppressed, and the HB interaction overcomes the VDW repulsion, leading to the bubble-bitumen attachment. Increasing the solution pH weakens the bubble-bitumen HB attraction interaction. It is also found that the apparent HB interaction can be strengthened with the addition of calcium ions, contributing to the bubble-bitumen attachment. Interestingly, the addition of a small amount of surfactants to the aqueous solutions suppresses the bubble-bitumen attachment, due to the weakened hydrophobic interaction and interfacial steric interaction, even under high salinity conditions. <br>
This thesis work has advanced the fundamental understanding of surface interaction mechanisms among bubbles, hydrophobic polymers, asphaltenes, bitumen, and oil droplets in complex aqueous solutions at the nanoscale. The results offer useful insights on how to effectively modulate such interfacial interactions in oil production, water treatment and other interfacial processes.</p>