Ionic physisorption on bubbles induced by pulsed ultra-sound.

Ion flotation processes involve the use of bubbles in order to separate ionic species from a mixed solution. Due to bubble interfaces we may assume null curvature at the molecular scale, where selective ion adsorption might be more easily investigated than with liquid-liquid extraction. In contrast to a classical flotation set-up, where bubbles are introduced via a glass frit, we use here a controlled sono-device generating cavitation bubbles which are initially absolutely clean. Moreover we have a faster process with a smaller device. The liquid phase resulting from the coalescence of the overflowing foam is enriched in some ions versus the initial brine. We show here that this effect follows the Hofmeister series and can be attributed to a weak adsorption of hydrated ions at the surfactant-water interface. The selectivity of alkali metals physisorbed at interfaces is analysed through the concentrations of competing ions remaining in solution by inductively coupled plasma optical emission spectrometry. Cationic selectivity, which is independent of the method for obtaining a foam, is discussed via the Gibbs free energy difference for bulk to hydrated surfactant monolayer. Relative values of effective adsorption energies are determined versus sodium ions taken as reference and correspond to 1-3% of the total hydration free energy.