Zeta potential of microbubbles in aqueous solutions: electrical properties of the gas-water interface

Microbubbles are very fine bubbles and appropriate for the investigation of the gas-water interface electrical charge, because of their long stagnation, due to slow buoyancy, in the electrophoresis cell observation area. This study investigated the zeta potential of microbubbles in aqueous solutions and revealed that the bubbles were negatively charged under a wide range of pH conditions. The potential was positive under strong acidic conditions, and the inorganic electrolytes decrease the potential by increasing the amount of counterions within the slipping plane. OH(-) and H(+) are crucial factors for the charging mechanism of the gas-water interface, while other anions and cations have secondary effects on the zeta potential, because counterions are attracted by the interface charge. The addition of a small amount of propanol and butanol provided significant information for considering the mechanism of the gas-water interface charge. Even though these alcohols did not have any electrical charge, they had a strong effect on the gas-water interface charge and dispersed the zeta potential of the microbubbles in the aqueous solution. These alcohols tended to adsorb to the interface and affect the hydrogen-bonding network at the interface, so that it was concluded that the gas-water interface electrical charge must be related to the difference of the construction of the hydrogen-bonding network between the bulk water and the gas-water interface.