Nanobubbles of air in water tend to have a negative charge. Intuitively, one could think the water is not charged, air is not charged: why are nanobubbles charged? Looking closer, water has both OH- and H3O+ (even without any acid or base at pH7) and it seems that the negative OH- ions tend to go to the air-water interface. Why is that?
Ulf Nobbmann
One route of producing nanobubbles is to sonicate ultra-pure ('18 megohm') water. In this case the conductivity of the resulting liquid is increased by a factor of 7 or 8 orders of magnitude). The suggestion is that the nanobubbles ahere to the generated particulate matter from the disintegration of the sonication probe. Given that these contaminants are likely to be metallic in nature then the only bubbles that could adhere and be stabilized in this regime would need to possess a negative charge. Other 'free' bubbles would immediately collapse and dissolve.
As we know, the average size of the bubbles generated by sonication is in the range of 300 to 500 nm. The zeta potentials of bubbles in both pure water and alkyl polyglycoside (AG) solutions at all pH values are negative. As the chain length of AG increases, zeta potentials significantly decrease at high pH. Even with a very high concentration of H+ ions in solution the bubbles are charged negatively because the interface is covered with slightly acidic alcohol groups of AGs. At high pH, the less polar the surfactant, the more negative the charge, since nonpolar surfactant molecules induce the adsorption of OH− ions, rather than H+ ions that prefer hydration by water molecules.
First of all, thanks to both Alan F Rawle and Mani Sharma . I understand that contaminants and additives may support the stabilization and charge of nano bubbles. I am still intrigued by the possibility that neither appears to be a requirement for the negative charge. In a recent article Ferraro et al. claim evidence of nanobubbles that cannot contain organic or inorganic impurities. Furthermore, they show some pH dependence, supporting the view that primarily OH- ions are the reason of charge and stability. What draws the OH- to the interface and why can they stay there? Article A Henry's law method for generating bulk nanobubbles
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