Bulk Nanobubbles from Acoustically Cavitated Aqueous Organic Solvent Mixtures

We investigate the existence and stability of bulk nanobubbles in various aqueous organic solvent mixtures. Bulk nanobubble suspensions generated via acoustic cavitation are characterized in terms of their bubble size distribution, bubble number density, and zeta potential. We show that bulk nanobubbles exist in pure water but do not exist in pure organic solvents, and they disappear at some organic solvent-water ratio. We monitor the nanobubble suspensions over a period of a few months and propose interpretations for the differences behind their long-term stability in pure water versus their long-term stability in aqueous organic solvent solutions. Bulk nanobubbles in pure water are stabilized by their substantial surface charge arising from the adsorption of hydroxyl ions produced by self-ionization of water. Pure organic solvents do not autoionize, and therefore, nanobubbles cannot exist in concentrated aqueous organic solvent solutions. Because of preferential adsorption of organic solvent molecules at the nanobubble interfaces, the surface charge of the nanobubbles decreases with the solvent content, but the strong hydrogen bonding near their interfaces ensures their stability. The mean bubble size increases monotonically with the solvent content, whereas the surface tension of the mixture is sharply reduced. This is in agreement with literature results on macro- and microbubbles in aqueous organic solutions, but it stands in stark contrast to the behavior of macro- and microbubbles in aqueous surfactant solutions.