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

Abstract

We show that the mixing of organic solvents with pure water leads to the spontaneous formation of suspended nano-entities which exhibit long-term stability on the scale of months. A wide range of solvents representing different functional groups are studied: methanol, ethanol, propanol, acetone, DMSO and formamide. We use various physical and chemical analytical techniques to provide compounded evidence that the nano-entities observed in all these aqueous solvent solutions must be gas-filled nanobubbles as they cannot be attributed to solvent nanodroplets, impurities or contamination. The nanobubble suspensions are characterized in terms of their bubble size distribution, bubble number density and zeta potential. The bubble number density achieved is a function of the type of solvent. It increases sharply with solvent content, reaching a maximum at an intermediate solvent concentration, before falling off to zero. We show that, whilst bulk nanobubbles can exist in pure water, they cannot exist in pure organic solvents and they disappear at some organic solvent-water ratio depending on the type of solvent. The gas solubility of the solvent relative to water as well as the molecular structure of the solvent are determining factors in the formation and stability of bulk nanobubbles. These phenomena are discussed and interpreted in the light of the experimental results obtained.

Response to “Comment on Bulk Nanobubbles or Not Nanobubbles: That is the Question”

“…However: (i) they do not provide any concrete evidence for the presence of such hydrophobic impurities in our system; (ii) they do not specify their chemical composition; and (iii) they do not specify at what water−ethanol ratios such hydrophobic impurities are supposed to be detectable. Furthermore, Rak & Sedlak fail to address why in our work: 1,4,29 (iv) the number per mL of nanoentities (we interpret as BNBs) observed varies strongly with the water−ethanol ratio, such that it reaches a maximum at an ethanol volume fraction of about 20% and it falls off to zero at a fraction of about 60%; and (v) why these nanoentities do not disappear when ethanol is evaporated from the suspension in a rotary evaporator. Incidentally, independent relevant work has more recently reported that actual BNB suspensions in water can be concentrated in a rotary evaporator with no loss of BNBs.…”

Generating Bulk Nanobubbles in Alcohol Systems

“…22 It was found that NBs could be produced through alcohol−water exchange, which was well characterized by the nanoparticle tracking analysis (NTA), Fourier transform infrared spectroscopy (FTIR), gas chromatography−mass spectrometry (GC−MS), and inductively coupled plasmamass spectrometry (ICP-MS). 27 It was also reported that a procedure containing pressurization−depressurization steps would controllably generate NBs in different solutions, which was confirmed by the X-ray fluorescence intensity of the element of the gas in solutions and by NTA. 30 Although these independent studies provide strong evidence for the existence of NBs, they focus on aqueous solutions as liquid carriers.…”

“…They pointed out that there were no ions on the surface of organic molecules, and NBs could not be formed in such a pure organic system . Recent studies carried out by Barigou et al also indicated that NBs were formed during water–organic solvent mixing, but no NBs were detected in pure organic systems . However, it seems that the electrostatic repulsion model does not work for bulk NBs in pure water (without surfactant) as the calculation showed that the repulsive pressure associated with electrostatic force caused by surface charge was one order of magnitude smaller than the Laplace pressure inside the bubble; thus, the electrostatic repulsive pressure could not balance with the Laplace pressure at all. , In 2015, Craig et al observed the formation of surface NBs on highly oriented pyrolitic graphite (HOPG) covered by some pure organic solvents and believed the existence of a three-dimensional hydrogen bond network that promoted the formation of the surface NBs .…”

“…Various methods for producing NBs have recently been developed, and evidences of the existence of NBs have been provided in a number of studies. − ,− For instance, NBs were generated via chemical reactions and by ultrasonic irradiation . It was found that NBs could be produced through alcohol–water exchange, which was well characterized by the nanoparticle tracking analysis (NTA), Fourier transform infrared spectroscopy (FTIR), gas chromatography–mass spectrometry (GC–MS), and inductively coupled plasma-mass spectrometry (ICP-MS) . It was also reported that a procedure containing pressurization–depressurization steps would controllably generate NBs in different solutions, which was confirmed by the X-ray fluorescence intensity of the element of the gas in solutions and by NTA .…”

Ultrasonic cavitation in CO2-expanded N, N-dimethylformamide (DMF)

“…of liquids [27] . Due to these changes, when ultrasound is applied to a CXL system, the physics and chemistry of non-linearly oscillating acoustic cavitation bubbles are strongly influenced by the dissolved CO 2 [28] , and gas oversaturation is a crucial parameter in determining the population of bubbles [29] . During the rarefaction of ultrasonication, the local pressure is lower than the saturation pressure of the CO 2 -expanded DMF, resulting in a decrease in the gas solubility of the corresponding local liquid.…”