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Jhonny A. Rodrigues

Surfactant and Colloid Group, Department of Chemistry
University of Hull, Hull, United Kingdom

Addition of a charged surfactant (sodium dodecyl sulphate) to mixtures of air, water and hydrophobic silica nanoparticles under high shear induces the transitional phase inversion from a water-in-air powder to an air-in-water foam. Electron microscopy (cryo-SEM) reveals the non-spherical shape of both water drops and air bubbles as shown below respectively, in which the particle coverage varied at the air-water interface.

Complementary experiments are described to elucidate the origin of this 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 dispersibility of surfactant-coated particles 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.