Using Foams for the Oxidative Dissolution of Copper Particles: Transport of Reactants Is Controlled by the Foam Structure

We study the oxidation of micrometric copper particles in rotating aqueous foams by oxygen O₂ present in the gas bubbles and in the presence of protons H⁺ in the continuous phase. Our goal is to understand the link between the reaction kinetics and the transfer of the three reactants present in three different phases in the foam by monitoring the foam structure using simple experimental tools. Measuring the bubble size evolution and the foam liquid fraction enables us to obtain an insight into the transfer of O₂ by diffusion in the thin liquid films, while measuring the foam liquid fraction and using a pH-sensitive dye enable us to observe the distribution of protons within the foams. Finally, the trapping of particles in the dry foams can be observed directly with a camera. We vary the rotating speed, which influences the amount of liquid effectively incorporated inside the foams, which in turn controls the quantity of particles and protons within the foams. We prove that the foam structure controls the transfer of reactants and, hence, the reaction kinetics. Wetter foams ensure a steady transport of particles and protons, resulting in a steady increase in the amount of copper dissolved over time. In the drier foams, the O₂ transfer is faster, so that the kinetics is faster than in wet foams at short times. Yet the transport of the particles and the continuous phase is hampered, leading to slower kinetics at longer times. These results will guide the design and handling of reactive foams used as chemical reactors.