Effect of pH and salt concentration on the phase inversion of particle-stabilized foams.

The effect of pH and salt concentration on the phase inversion of silica particle-stabilized foams is presented. Inversion from a water-in-air powder to an air-in-water foam can be achieved by increasing the pH of the aqueous phase. By contrast, an increase in the salt concentration causes a nonfoaming aqueous dispersion to foam. The results are rationalized in terms of changes in the hydrophobicity of the solid surfaces, probed by measurement of the contact angles of water drops on hydrophobized glass slides in air.