Particle affinity for oil-water interfaces and selective adsorption
Essenmacher, Scott D.
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Many applications require the selective removal of particulate contaminants from aqueous solutions. Standard froth flotation is a very efficient process that is commonly used in these applications to indiscriminately remove very hydrophobic micron-sized particulate contaminants. It is desirable to have a method that can take advantage of the efficiency of such a process and be selective with regards to what particles are removed from the solution. One such method would be to coat the air bubbles in froth flotation with a thin layer of oil. This allows for the possibility of selecting oils that have strong attractive intermolecular interactions with a targeted particle and weak attractive interactions with other particles in the solution, which could cause only the targeted particle to strongly adsorb to the oil-water interface and rise to the top of the suspension with the bubble, while the other particles remain in the suspension. Demonstrating this concept of using oil-coated air bubbles to selectively remove particulate contaminants from process effluents, called “affinity flotation,” would be novel. This study focuses on the first step required to prove affinity flotation, which is examining potential oil and particle combinations that could be used to demonstrate the idea of affinity flotation. The main goals of this study are to (1) find three oils that can selectively remove only one particle from an aqueous suspension via one of the following attractive intermolecular interactions: hydrophobic, π-π, and acid-base; and (2) quantify the strength of the affinity the particles have for each of the possible oil-water interfaces. Potential oil and particle combinations were chosen based off the propensity of their molecular structures to have one of the aforementioned intermolecular interactions between them. These potential combinations were then screened using simple foaming ability and foam stability tests. Confocal microscopy was used to verify that capillary foams could be made for the chosen oil and particle combinations. The strength of the affinity a particle has for an oil-water interface was determined by using data collected through measuring the oil-water interfacial tension, the three phase contact angle of a particle at the interface, and the particle size. Results suggest that all of the oils were selective to one type of particle (i.e. only one particle had a high affinity for the oil-water interface, while the other particles had a low affinity); however, only two of the oils (DINCH and heptane) were selective towards the particles that were originally chosen because they have primarily one type of attractive intermolecular interaction with the oil (PVC via acid-base with DINCH and HMDS modified silica via hydrophobic with heptane). The other oil, toluene, was selective for PVC, but not for PS. The attractive intermolecular interactions between toluene and PS were too strong, resulting in the undesirable result of PS having a low affinity for the toluene-water interface. The calculated strength of particle adsorption to oil-water interfaces for PVC and PS particles were high for combinations that produced stable capillary foams in the foaming ability and stability tests, and low for combinations that were either semi-stable or unstable. This demonstrates why these quick and simple tests can be used to predict when particles have a high affinity for the oil-water interface of an oil-coated air bubble, and confirms that DINCH maybe selective enough to be used as an oil to demonstrate affinity flotation by removing PVC particles from binary suspensions. In short, this study provided some progress towards developing a model system to use to demonstrate affinity flotation by finding two oil and particle combinations (DINCH-PVC and heptane-HMDS modified silica) that could potentially be used in such a system.