Inversion of ‘dry water’ to aqueous foam on addition of surfactant

Abstract

Addition of charged surfactant to mixtures of air, water and hydrophobic silica nanoparticles under high shear induces transitional phase inversion from a water-in-air powder to an air-in-water foam. Optical and electron microscopy reveal the non-spherical shape of both drops and bubbles, respectively, in these materials due to their partial coverage by particles. Complementary experiments are described to elucidate the origin of phase inversion. From surface tension and contact angle measurements, the ratio of adsorption of surfactant at air–water and solid–water interfaces is determined. Particles become increasingly hydrophilic on adding surfactant since molecules adsorb tail down exposing charged head groups to the aqueous phase. The increased particle dispersibility into water and the generation of negative zeta potentials confirm this scenario. In addition, the ability of the same silica nanoparticles to act as antifoams of aqueous surfactant foams formed under low shear is investigated. The effectiveness of these particles as antifoams decreases both on increasing the surfactant concentration and the time particles and surfactant are in contact before foaming. Both trends are in agreement with the above-mentioned findings that when particles are rendered more hydrophilic via surfactant adsorption, they no longer break foam films between bubbles but remain dispersed in the aqueous phase.