Different Effects of Monovalent Anions and Cations on the Bubble Coalescence and Lifetime of Aqueous Films between Air Bubbles

Ions of inorganic salts are known to affect bubble coalescence via the surface charge density as described by the classical DLVO theory of colloid stability. Our experimental results obtained for monovalent salts of different concentrations show different effects of monovalent anions (F-, Cl-, Br- and I-) and cations (Li+, Na+, K+ and Cs+) on the lifetime of liquid films between two bubble surfaces. These results highlight the significant effects of ion radius and polarizability on the interaction of salt ions with water molecules in saline liquid films and at the air-water interface. They also show the important role of approach speed between the bubble surfaces. For each salt, there is a critical concentration beyond which the film lifetime rapidly increases and bubble coalescence is inhibited. These results are relevant to a number of chemical engineering processes taking place in saline water, including mineral separation by flotation using air bubbles in bore water.