Influence of dissolved gas on bubble-particle heterocoagulation

The heterocoagulation of single nitrogen bubbles with moderately hydrophobised quartz particles in a very dilute particle dispersion has been studied at pH 5.6. Bubble-particle capture efficiencies have been determined for aqueous dispersions in equilibrium with air (standard conditions) and where the dispersion has either been degassed or intentionally saturated at pressures between 1 and 4 atm with air, N-2, He, H-2, Ar, O-2 and CO2. For CO2 and air the capture efficiency is increased compared with standard conditions, but is decreased for the other gases. The results do not simply reflect solubility differences between the various gases, rather they indicate that CO2 plays a special role. A mechanism by which CO2 is generated in the intervening thin aqueous film between a hydrophobic particle and a nitrogen bubble, leading to an increased capture efficiency, is proposed. The disruption of water structure by HCO3- near the hydrophobic particles may also make a contribution. These results and the proposed mechanism have implications for the origin of the long range force of hydrophobic attraction.