Proving and interpreting the spontaneous formation of bulk nanobubbles in 2 aqueous organic solvent solutions: effects of solvent type and content

12 Abstract 13 We show that the mixing of organic solvents with pure water leads to the spontaneous 14 formation of suspended nano-entities which exhibit long-term stability on the scale of months. 15 A wide range of solvents representing different functional groups are studied: methanol, 16 ethanol, propanol, acetone, DMSO and formamide. We use various physical and chemical 17 analytical techniques to provide compounded evidence that the nano-entities observed in all 18 these aqueous solvent solutions must be gas-filled nanobubbles as they cannot be attributed to 19 solvent nanodroplets, impurities or contamination. The nanobubble suspensions are 20 characterized in terms of their bubble size distribution, bubble number density and zeta 21 potential. The bubble number density achieved is a function of the type of solvent. It increases 22 sharply with solvent content, reaching a maximum at an intermediate solvent concentration, 23 before falling off to zero. We show that, whilst bulk nanobubbles can exist in pure water, 24 cannot exist in pure organic solvents and they disappear at some organic solvent-water ratio 25 depending on the type of solvent. The gas solubility of the solvent relative to water as well 26 the molecular structure of the solvent are determining factors in the formation and stability of 27 bulk nanobubbles. These phenomena are discussed and interpreted in the light of the 28 experimental results


Introduction
Bulk nanobubbles (BNBs) are an emerging field which is attracting much attention from 37 researchers and industrial practioners alike. Their existence has been reported in a number of 38 recent experimental studies [1][2][3][4] and strong indirect evidence has been provided to show that 39 such nano-entities must be gas bubbles. 5,6 Nonetheless, considerable speculation and 40 controversy still exist about the existence and nature of BNBs, their origin and their 41 extraordinary longevity which contradicts predictions of the diffusive gas transport based on 42 the Epstein and Plesset theory. 7 Those who dispute the existence of BNBs tend to attribute 43 them to supramolecular structures, solvent or oil nanodroplets or simply impurities and 44 contamination, based on either questionable experimentation or sheer speculation. 8-15 A 45 similar debate which lasted for many years took place after surface nanobubbles emerged and 46 became the focus of attention about two decades ago. 16 The present situation is exacerbated 47 by the lack of a full rational explanation of the mechanism behind the long-term stability of 48 BNBs. 6,[17][18][19] A number of speculative interpretations have been postulated but a complete 49 physical model has yet to emerge. 50 51 Despite such scepticism, however, many applications have been suggested for BNBs, including 52 control of the nucleation mechanism in cavitation/boiling, 20 facilitating oxygen supply to 53 marine/aquatic life (plant and fish), 21 detoxification of water, 3,22 enhanced remediation of 54 organic contaminants, 23 drag reduction, 24 prevention of surface fouling, 25,26 enhanced 55 ultrasound imaging of small cell lung cancer, [27][28][29] oxygenation of hypoxic conditions for 56 cardiac resuscitation, 30 enhanced seed germination, 31,32 and improved efficiency of IC 57 engines. 33,34 This nonexhaustive list highlights the wide and versatile interest in BNBs. BNBs 58 offer significant advantages over microbubbles due to their persistence, negligible buoyancy 59 and huge relative surface area. However, it is still not understood how they can be produced 60 in an efficient, consistent and controlled fashion, especially in large volumes, and how they 61 can be efficiently exploited in all the pertinent technologies.   The BNB suspensions are visualized and the nanobubbles measured by a nanoparticle tracking 90 analysis technique, and their surface charge is measured in terms of their zeta potential. We 91 study the influence of the type of solvent and its mole fraction on the existence of BNBs and 92 their bubble number density. We also monitor the long-term stability of the BNB suspensions 6.7 at a temperature of 20 ˚C, was used in all experiments. All solvents and reagents used were 102 of the highest purity grade available on the market. All glassware was cleaned by immersion 103 for 30 min in a 10% aqueous solution of potassium hydroxide (KOH, Sigma Aldrich, UK) 104 placed inside an ultrasonic bath, followed by rinsing with ultrapure water, drying in a 105 microwave oven and flushing with a stream of high-purity dry nitrogen gas. Analytical grade 106 (99.9% pure) methanol, ethanol, 2-propanol, acetone, dimethyl sulfoxide (DMSO) and 107 formamide were purchased from Fisher Scientific (UK) and were handled in glassware in order 108 to avoid contamination from plastic products. Prior to experimentation, purified water and all 109 stock solutions were initially examined for any nanoscale entities using the Nanosight   115 BNBs were formed by mixing an organic solvent at different concentrations within the range 116 0.01−0.9 mole fraction with pure water at room temperature, in 100 mL glass beakers, as 117 illustrated in Figure 1. The order of mixing (i.e. adding solvent to water or adding water to 118 solvent) had no effect on the bubble number density or nanobubble size distribution, within 119 experimental error. We selected a range of solvents: methanol, ethanol, propanol, acetone, 120 DMSO and formamide; their physical properties are summarized in Table 1 125 The concentration, mean size, and size distribution of BNB suspensions was measured using 126 nanoparticle tracking analysis (NTA) afforded by a NanoSight instrument (NS300, Malvern, 127 UK), as described in our previous work. 6 Further details are summarised in the Supporting 128 Information. 136 We shall start by assuming that the entities formed in the aqueous solvent solutions are BNBs 137 and make some general observations about their characteristics and behaviour. We will then 138 present multiple pieces of evidence to show that the observed entities are indeed BNBs and not 139 any of the common nano-scale impurities sometimes associated with BNBs. Subsequently, we   In conclusion, the various physical and chemical analytical techniques used above have 240 produced multiple evidence which, taken together, provides conclusive proof that the nano-241 entities spontaneously formed by water-solvent mixing cannot be attributed to solvent 242 nanodroplets or any type of common impurities or contamination as speculated in some 243 literature reports [8][9][10][11][12][13][14][15] and, therefore, must be indeed gas-filled BNBs.

251
The corollary of the results discussed above (Figure 3) is that BNBs cannot form in pure 252 solvents. These results seem to suggest, therefore, that a pure organic solvent acts as a gas sink 253 removing any excess gas from the solution and, consequently, it does not form nanobubbles.

254
Hence, since BNBs exist in pure water, as reported by a number of scientific reports including 255 our own, 5,6,17,48-50 then they should disappear at some organic solvent-water ratio, which is 256 confirmed by the present results. where, X m , X e , X w , X s , n w and n s are, respectively, gas mole fraction obtained when the saturated 273 solvent and water of a given mole fraction are mixed, gas mole fraction in that mixture, 51

Why does the number density of BNBs vary with the type of solvent?
297 Gas oversaturation is a useful parameter in determining the population of bubbles formed in 298 the solution. 55 As discussed above, the solubility of atmospheric gases (O 2 , N 2 ) in water has 299 been observed to be lower than in aqueous solutions of methanol, ethanol, propanol and acetone and, therefore, gas oversaturation is expected to be higher in the presence of such solvents, thus 301 resulting in the formation of BNBs. However, the same gases will have a different solubility 302 depending on the organic solvent present (Figure 8), which explains the differences in bubble 303 number density achieved in different aqueous solvent solutions.

305
To illustrate these differences, the values of the maximum number density of BNBs formed in 306 the aqueous solutions of these volatile solvents are compared in Figure 9. The low number of BNBs in methanol can be explained by its low gas solubility relative to 317 water, and this argument can also be used to explain the extremely low number of BNBs in the 318 case of formamide ( Figure 3). Formamide has the same oxygen solubility as pure water (~ 1.3 319 mM; 56 data for N 2 are not available) which means that the gas oversaturation is extremely low 320 and, hence, the lack of gas release to form BNBs in the mixture.