Effect of high salt concentrations on the stabilization of bubbles by silica particles

The stabilization of air bubbles by hydrophobically modified silica particles has been investigated in detail. The silica particles used had a nominal primary particle size of 20 nm and were made hydrophobic by treatment with dichlorodimethylsilane to yield particles with varying percent grafting of alkyl chains ("% SiOR"). Contact-angle (theta) measurements of pure water droplets on flats made from compressed samples of the particles showed a steep increase in theta above ca. 20% SiOR. Other measurements also showed a significant increase in theta when the salt concentration was raised to 1-3 mol dm(-3). Bubbles were formed in a sonicated dispersion of particles by suddenly lowering the pressure. Maximum stability was obtained with 33% SiOR particles and 2-3 mol dm(-3) NaCl. Under these conditions, theta was around 40 degrees. Above a threshold size of around 70 microm, bubbles were extremely stable to disproportionation and coalescence and bubble stability increased significantly with an increase in the NaCl concentration from 0.5 to 3 mol dm(-3). Furthermore, rheological measurements showed that at NaCl concentrations in this range weak particle gels were formed with a finite yield stress. The strength of these gels increased with an increasing NaCl concentration between 0.5 and 3 mol dm(-3) and with an increasing time of aging the dispersions, implicating this as part of the mechanism leading to an increased bubble stability in these systems. Dispersions in the absence of NaCl showed little or no foamability at all. Use of CaCl2 and Al(NO)3 at similar ionic strengths showed that equivalent stability could not be obtained with these salts. Atomic force microscopy (AFM) measurements of the adhesion between a pure (0% SiOR) silica sphere and flat showed a significant increase in the adhesion between 0.5 and 3 mol dm(-3) NaCl, even though in this concentration range no significant change in the electrostatic repulsion might be expected. It is concluded that the increased particle-particle adhesion, effective hydrophobicity, and bubble-stabilization properties of the particles at high NaCl concentrations are probably due to the collapse of protruding polysilicic acid chains on the surface of the silica.