Confirmation of the Heterocoagulation Theory of Flotation

To model the flotation process, we have used the microscopic method developed by Scheludko et al., 11 to study the stability of an aqueous thin film containing tetradecyltrimethylammonium bromide (C 14 TAB) between an air bubble and a silica substrate. The experiments were performed at a range of C 14 TAB concentrations and pH values. Spontaneous rupture of the thin aqueous film was interpreted in terms of the earlier proposed heterocoagulation mechanism and resulted from the preferential adsorption of relatively low surfactant concentrations at the vapor/solution interface causing a net positive charge while the solution/silica interface remained negatively charged. This attractive electrostatic interaction was sufficient to overcome the van der Waals repulsion. At higher amine concentrations, the negative charge at the solution/silica interface was reversed. Finally, on approaching the critical micelle concentration (cmc), both interfaces were sufficiently positively charged to cause the restabilization of the film by electrostatic repulsion. In addition, in dilute solution, during the three-phase-contact (TPC) expansion or dewetting step following film rupture, it was suggested that the movement of TPC across the silica substrate leads to transfer of amine from the vapor/solution interface to the vapor/silica. This process resembles a Langmuir−Blodgett deposition process and emphasizes the importance of the solution/vapor interface in the dewetting process.