School of Chemistry

The examination of several different experimental systems presented in this thesis work are linked through their singular aim of investigating the interactions of deformable interfaces, in this case air bubbles, in aqueous polyelectrolyte solutions. Direct force measurements were used to probe depletion and structural force interactions between colliding air bubbles at equilibrium timeframes. In addition, air bubbles were generated without the presence of interface stabilising molecules within a block-and-break microfluidic device to probe the interactions from generation to equilibrium. Direct force measurements were conducted between interacting bubble pairs in aqueous solutions of polyvinylpyrrolidone (PVP), a neutrally charged polymer, and sodium poly(styrene sulfonate) (NaPSS), a negatively charged polyelectrolyte. The selection of these molecules allowed a comparison between neutral and charged polymers and their influence on the measured force interactions. It was found that for measurements conducted in the presence of PVP, neither a structural force nor a depletion interaction was able to be measured. This was largely due to an increase in solution viscosity with increasing concentration of PVP, which results in an increase in the hydrodynamic fluid flow that subsequently overwhelms any potential presence of a depletion interaction. Also, the polydispersity of the molar mass of the PVP would appear to be responsible for the non-observance of structural forces in this system. This conclusion is based on the results of force measurements conducted in aqueous solutions containing monodisperse NaPSS. Uncharged polymers have also been shown to have a lower osmotic pressure when compared to a charged polyelectrolyte, which decreases the magnitude of the depletion interaction. The measurements conducted in the presence of NaPSS with the deformable interface of bubbles were shown to be more sensitive to the presence of the polyelectrolytes when compared to similar measurements using rigid interfaces. The study involved the use of both polydisperse and monodisperse molar mass distributions and the experimental factors that were examined were NaPSS concentration, bubble collision velocity and polyelectrolyte molar mass. Structural forces were measured with the use of a monodisperse sample but only a depletion interaction when the polydisperse molar mass distribution was present. This demonstrates that polydispersity in molar mass results in the structural forces to be smoothed. The dispersity of the various molar mass distributions was manipulated to further investigate the role they play on the observed interactions. The polydisperse samples were dialysed, which removed the low molar mass molecules and the monodisperse samples were blended to create a bidisperse mixture. When the dispersity was decreased through dialysis, structural forces were observed and the bidisperse mixture only allowed the presence of a depletion interaction to be measured. These measurements further highlight the role that molar mass dispersity plays on the observed interactions and shows how these interactions can be manipulated. These measurements were then compared with an analytical model based on polyelectrolyte scaling theory (depletion interaction) or an empirical model (structural forces) in order to explain the effects of concentration and bubble deformation on the interaction between bubbles. The modelling highlighted that these interactions can be accounted for by polyelectrolyte scaling theory taking into account the structural properties of the polymer in solution in the dilute and semi-dilute polymer concentration regimes. It was also shown in all measurements that depletion and structural forces were overwhelmed by hydrodynamic fluid flow at increase bubble collision velocities. Bubbles were generated for the first time within a block-and-break microfluidic device. The original design allowed water droplets in oil to be generated and design changes were required to ensure air bubbles could be formed. Block-and-break devices offer the advantage, that the generated bubble sizes are flowrate independent instead of other design types where the bubble size varies with flowrate. Air bubbles were able to be generated in solutions of pure water, NaPSS and sodium dodecyl sulfate (SDS) and their size was shown to be flowrate independent. The microfluidic device designs were modified to allow staged amphiphilic addition of SDS and NaPSS after the bubble was generated in pure water and this has the potential to allow complexity to be increased stepwise throughout an experiment. The combination of both direct force measurements and microfluidic studies allowed bubbles in the presence of aqueous solutions of NaPSS to be studied from generation through to their equilibrium form.

The dissolution of quantum sized CdS and MnO2 particles in water was conducted using 20 kHz ultrasound. CdS particles were found to dissolve chemically via an oxidation process while MnO2 particles dissolved via a reductive process. It was found that the dissolution of the colloids could be controlled via the addition of surface active chemicals to solution and by varying the saturation gas type. In the presence of Na2S or propan-2-ol and argon gas, the dissolution of CdS was inhibited, whereas the addition of alcohols (methanol, ethanol, propan-2-ol, butan-1-ol and pentan-1-ol) to the MnO2 system led to an increase in the amount of dissolution for a given time of sonication. This increase in dissolution was found to be dependent on the ability of the surface active radical scavenger to accumulate around the bubble interface during the cavitation process. Eventually, at higher alcohol concentration there was a plateau or a limiting value reached for the efficiency of colloid dissolution which was common for each alcohol. (For complete abstract open document)

School of Chemistry - Theses

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