Ion-specific effects on the bubble coalescence in aqueous medium

The flotation froth must have low durability to be useful for the mineral separation process. For this reason, suitable frothers are used. Their role is to suppress the coalescence of the bubbles in the liquid phase of the reactor and to form a short-lasting froth cap above the level of the liquid. Indeed, one could state the question; what makes the bubble coalesce in pure water? They coalesce to decrease their surface energy, which is related to the hydrophobicity of the bubbles. In such a case, why do frothers with very small concentrations, practically not affecting the surface tension value, obstruct their coalescence? The answer, in our opinion, is the hydration repulsion between the bubbles, which are covered by frother with a hydrophilic head pointing outward the bubble. This makes the bubble more hydrophilic even when its surface tension values are practically the same as this one in the absence of a frother. Such a more hydrophilic bubble with a scarce frother adsorption layer has no sufficient Gibbs elasticity to underpin a stable froth cap above the level of the liquid. Some time ago an excellent group from Istanbul Technical University built up a unique setup, able to determine the fraction of bubble coalescence versus the frother concentration. We show in this work that the experimental curve determined by this setup can be converted to an experimental level of hydrophilicity of the bubbles versus the concentration of the frothers. Hundred percent of the primary bubbles coalesce in pure water, where they are the most hydrophobic. At 100% hydrophilicity, the bubbles do not coalesce but form a froth cap, which is short-lasting. Within this context, in this study, the effects of NaCl and CaCl2 on the coalescence of the bubbles being either frother-free or in the presence of sodium dodecyl sulfate (SDS) were investigated. Their foamability was studied as well. The fraction of the coverage of SDS, NaCl, and CaCl2 species on the surface of the bubble and the level of hydrophilicity versus their bubble percentage coalescence were determined. The experimental data were related to the adsorption energy of SDS, Na+, Cl-, and Ca2+ on the air/water interface and their Gibbs hydration energy. It was established a linear dependence between the value of CCC and its corresponding level of coverage of the added agent.