Stability of aqueous films on hydrophobic methylated silica

Abstract

Experiments in which a small gas bubble was slowly advanced towards a polished silica plate under dilute aqueous solutions have shown that “thick” equilibrium films are formed not only on clean, hydrophilic, silica, but also on methylated, hydrophobic, silica. The latter films are metastable, collapsing when disturbed and forming a contact angle which may be as large as 90°. Surprisingly, the thicknesses of the metastable films are apparently independent of the contact angle. The variation of film thickness with electrolyte concentration indicates that over the measured range (60 to 220 nm) the stability is due to an electrical double layer associated with the clean or the methylated silica/water interface. The instability at smaller film thicknesses has been attributed basically to reduced hydrogen bonding at the solid/water interface. The findings have a bearing on the theory of froth flotation—in particular, on the kinetics of particle/bubble contact and the “salt flotation” of weakly hydrophobic materials such as coal.