Foaming in chemical surfactant free aqueous dispersions of anatase (titanium dioxide) particles.

Steady-state dynamic aqueous foams were generated from surfactant-free dispersion of aggregated anatase nanoparticles (in the micrometer size range). In order to tune the particle surfaces, to ensure a critical degree of hydrophobicity (so that they disperse in water and generate foam), the particles were subjected to low-temperature plasma treatment in the presence of a vapor-phase silane coupling agents. From ESCA it was shown that hydrophobization only occurred at a small number of surface sites. Foamability (foam generation) experiments were carried out under well-defined conditions at a range of gas flow rates using the Bikermann Foaming Column.1 The volume of the steady-state foams was determined under constant gas flow conditions, but on removing the gas flow, transient foams with short decay times (<5 s) were observed. The foamability of the steady-state foams was found to be dependent on (a) the time of plasma treatment of the particles (surface hydrophobicity), (b) the particle concentration in the suspension, and (c) the state of dispersion of the particles. High foamability was promoted in the neutral pH regions where the charged particles were highly dispersed. In the low and high pH regions where the particles were coagulated, the foamability was considerably reduced. This behavior was explained by the fact that the large coagula were less easily captured by the bubbles and more easily detached from the interface (during the turbulent foaming conditions) than individual dispersed particles.