Bulk Nanobubbles from Acoustically Cavitated Aqueous Organic Solvent Mixtures

Abstract

We investigate the existence and stability of bulk nanobubbles in various aqueous organic solvent mixtures. Bulk nanobubble suspensions generated via acoustic cavitation are characterised in terms of their bubble size distribution, bubble number density and their 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 stabilised by their substantial surface charge arising from the adsorption of hydroxyl ions produced by self-ionisation of water. Pure organic solvents do not auto-ionise and, therefore, nanobubbles cannot exist in concentrated aqueous organic solvent solutions. Due to preferential adsorption of organic solvent molecules at the nanobubble interfaces, the surface charge of the nanobubbles decreases with solvent content, but the strong hydrogen bonding near their interfaces ensures their stability. The mean bubble size increases monotonically with solvent content whilst 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 behaviour of macro and microbubbles in aqueous surfactant solutions.

Proving and interpreting the spontaneous formation of bulk nanobubbles in aqueous organic solvent solutions: effects of solvent type and content

“…16 The present situation is exacerbated by the lack of a full rational explanation of the mechanism behind the long-term stability of BNBs. 6,[17][18][19] A number of speculative interpretations have been postulated but a complete physical model has yet to emerge.…”

“…Various techniques have been suggested for BNB production, including hydrodynamic and acoustic cavitation, 6,18,[35][36][37][38] fluidic oscillation, 39 and nano-membrane filtration. 40 These methods suffer from a number of drawbacks as they tend to be energy intensive; they are prone to contamination arising from detachment of nanoparticles from surfaces; they lack control of bubble size, uniformity and concentration; and have low resistance to corrosive chemicals, which restricts the use of reactive gases and solutions.…”

Bulk Nanobubbles or Not Nanobubbles: That is the Question

“…Consequently, two possible case scenarios could be envisaged: (i) it may be possible that the nanobubbles do actually vanish as in the case of pure water, but the presence of ethanol at low temperatures inevitably leads to the generation of a new more concentrated suspension of nanobubbles; or (ii) we argued above that ethanol molecules adsorb on the surface of nanobubbles via hydrogen bonding, which is reflected in their low zeta potential. 3 The adsorbed ethanol molecules should, thus, provide a thick protective shell that shields the nanobubbles and prevents them from collapsing on thawing, but the low temperatures lead to the creation of a nanobubble surplus.…”

“…The presence of strong hydrogen bonds near the interface compensates for the lower surface charge and may be the prime factor in stabilizing bulk nanobubbles in a water− ethanol mixture. 3 Cryo-Scanning Electron Microscopy. Cryo-SEM micrographs of pure water and a nanobubble suspension produced in pure water captured at different magnifications are presented in Figure 5.…”

“…The most interesting theory perhaps is the ion-stabilized model proposed by Bunkin et al 21 It conjectures that the presence of negative electrostatic pressure because of adsorption of OH − ions in the form of an electric double layer at the nanobubble interface, akin to that observed around solid nanoparticles, balances the internal Laplace pressure and, therefore, no net diffusion of gas occurs. 3 Bulk nanobubbles have already attracted a lot of attention and various industrial and medical applications have been suggested. 17,22−30 However, despite such wide interest, this is still an emerging field and speculation remains rife about the existence of bulk nanobubbles and their stability as definite proof that the nano-entities observed are actually gas bubbles is still missing.…”