Stable Air Nanobubbles in Water: the Importance of Organic

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Stable Air Nanobubbles in Water: the Importance of Organic Contaminants Fredrik Eklund* and Jan Swenson

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Division of Biological Physics, Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden ABSTRACT: Nanobubbles, surprisingly stable submicrometer gas bubbles in water, appear to be common in water and biological fluids and are of great interest in technical applications ranging from ultrasound contrast agents to flotation in the mining industry. Nanobubbles on surfaces have been more researched than freely floating bulk nanobubbles, and the reason for their stability appears to be better explained. The stability of bulk nanobubbles is less well explained, several theories exist, and even their existence is sometimes questioned. In the present study, an attempt was made to generate nanobubbles through hydrodynamic cavitation as well as through vigorous shaking in test tubes, and it was found that none of these methods generated a detectable concentration of possible bulk nanobubbles if pure water was used, with or without a small addition of NaCl, the equipment was cleaned properly, and certain plastic materials were avoided. These results indicate that trace organic contaminants are necessary for nanobubble stabilization. Experiments were also made with the dissolution of a high concentration of inorganic salts, which generated bubbles by creating air supersaturation. Light scattering submicron particles were found in all solutions and appeared to be actual gas bubbles in at least one case. However, in many cases, these light scattering particles were unaffected by vacuum and pressure and appear, therefore, to be something else other than air bubbles. It is concluded that, in future research on nanobubble stability, it is very important to avoid contamination, as well as to ascertain that light scattering objects really are bubbles and not oil droplets or particles.

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INTRODUCTION Stable submicrometer gas bubbles in water is a growing research area. It is also an area of increasing industrial activity and interest, often in applications where larger microbubbles are used today. Possible applications include contrast agents in ultrasonic imaging, floatation in water treatment and mining industry, aeration and disinfection of water, cleaning, and many more.1−3 Many different types of nanobubble generators are already commercially available, and although the market is still in its infancy, it is safe to say that the potential societal and environmental value of nanobubble technology is great. Whereas the industrial interest is mainly focused on free floating bulk nanobubbles, much of the fundamental research has focused on bubbles attached to surfaces. In comparison with bulk nanobubbles, surface nanobubbles have been more researched and the reason for their stability appears to be well explained.4 In contrast, the reason for the stability of bulk nanobubbles is less clear,3 and even their existence is sometimes questioned. Generation of nanobubbles and microbubbles can occur by many different routes. To form bubbles, gas can by different mechanical means be finely dispersed into the liquid. Bubbles can also be nucleated in the solution by sonication or other means of generating cavitation or by electrolysis. In research, bubbles are often nucleated by changing the chemical composition of the solution to decrease the solubility of air © XXXX American Chemical Society

and create a great enough supersaturation for spontaneous bubble nucleation to occur. But whereas many different methods of generating bubbles have been demonstrated in research papers and in patents, it is still unclear why such small gas bubbles can be stable and what affects their stability. It should in this context be mentioned here that micro- and nanobubbles used as contrast agents are a special case, which will not be the focus of this paper. These usually contain very hydrophobic fluorinated compounds, which gives a much higher bubble stability compared to air.5 In other applications, as well as in nature, bubbles of air and its components is what is of interest and is also the focus of this paper. The reported lifetime of nanobubbles range from minutes and hours to days and weeks.3,6 However, Epstein and Plesset’s classical analysis7 based on diffusion theory, estimates the lifetime of an air nanobubble in water to be orders of magnitude shorter. The analysis takes into account two driving forces for diffusion, the degree of gas saturation of the liquid and the Laplace pressure, which is an effect of surface tension and curvature. For a bubble with a clean air−water interface and a diameter of 1 μm or less, the Laplace pressure will be very high and provide a strong driving force for dissolution. Received: May 24, 2018 Revised: July 31, 2018

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DOI: 10.1021/acs.langmuir.8b01724 Langmuir XXXX, XXX, XXX−XXX

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long time and quickly and easily be remeasured. For these reasons, DLS was selected for this study. The present work aimed at examining the influence of water chemistry on generation and stability of bulk nanobubbles. More specifically, it addressed the question if stable nanobubbles can exist in pure water or aqueous solutions of inorganic salts with no added organic substances. Three methods for nanobubble generation were used; cavitation in a liquid flow through a narrow constriction, vigorous shaking in a test tube, and dissolution of inorganic salts in water. Great care was taken to avoid contamination. With the first two bubble generation methods, the effect of avoiding contamination versus not avoiding contamination was demonstrated to affect whether any light scattering bubbles or particles were detected and several contamination sources were found. Salt dissolution experiments were made to examine if there is any ion-specific effect on bubble stability, and it was also investigated whether the light scattering objects found in such solutions are in fact nanobubbles or something else.

This will cause such a bubble to dissolve in far less than a second, not only in an undersaturated solution but also in a moderately oversaturated solution. For very oversaturated solutions, the bubble will instead grow, acquire buoyancy and rise to the surface. Although this is a slower process than dissolution, it can also limit the lifetime of small bubbles. Either way, the lifetime of a submicron bubble is expected to be short. An explanation for this discrepancy between experiment and theory is needed, and several theories have been suggested. The existence of very small air bubbles, stabilized by a skin of organic substances, was suggested already in the 1950s,8 long before nanobubbles could be detected with light scattering methods. Johnson and Cooke9 later generated bubbles in filtered seawater and imaged them with optical microscopy. Images revealed that as the bubbles decreased in size and stabilized at a size of a few or less than one micron, their shape became irregular and nonspherical. This was interpreted as being due to a compressed layer of adsorbed organic substances, which gave the bubbles a very low surface tension. In addition to the extensive literature on surfactant stabilized microbubbles, nanobubble generation in artificial surfactant solutions have also been demonstrated in several cases.10−13 Whereas surfactants are generally recognized to stabilize bubbles, it is not generally recognized that all nanobubbles are stabilized by surfactants. Recently, it was instead suggested that not surfactants, but entirely hydrophobic substances adsorbed to nanobubbles can work to stabilize them.14 Whereas it is a common opinion that nanobubbles are stabilized by organic substances, it has also been claimed that inorganic ions alone can stabilize nanobubbles, even at concentrations as low as 10−6 M.6 Based on ATR-IR15 and Raman measurements16 on nanobubbles in distilled or deionized water, it has also been suggested that a specific water structure at the air−water interface prevents gas diffusion. Although questioned on theoretical grounds, these ideas appear to be confirmed experimentally by many papers that report the generation of nanobubbles in solutions of NaCl and other salts without the addition of any surfactants, as well as in distilled or deionized water. Obviously, a better understanding of why nanobubbles can be stable and how water chemistry affects their stability would be highly beneficial, as it would make it possible to ensure reproducible production methods and optimization of their stability. The most common method for detecting submicron bubbles has since the 1990s been Dynamic Light Scattering (DLS). In recent years Nanoparticle Tracking Analysis (NTA) has instead become more and more popular. The NTA method is based on light scattering in a microscope, where individual particles are tracked with a video camera. NTA can detect lower concentrations of particles/bubbles and better resolve a heterogeneous particle size distribution than DLS. DLS can, on the other hand, detect a wider range of particle sizes. In the NTA setup, the sample is pumped through a capillary and into a narrow sample chamber. As the same capillary and chamber is used for all measurements, there is a certain risk of cross contamination. The sample chamber should not be cleaned with liquids that can etch glass, something that further increases the risk of cross-contamination. It is also conceivable that the flow and pressure differences when loading the sample chamber could affect gas bubbles. For DLS measurement, the sample is placed in a clean test tube, which can be stored for

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EXPERIMENTAL SECTION

Materials and Chemicals. All glassware was cleaned by soaking in an alkaline cleaning solution (Hellmanex 2%), followed by ethanol 99.7% and nitric acid 1%, and generous rinsing with Milli-Q water after each step. It was then dried in a vacuum oven at 50−60 °C. Stainless steel was contacted with Hellmanex solution fortified with 0.3% KOH for at least 1h, followed by Ethanol 99.7% and Milli-Q rinsing. It was visually examined that Milli-Q water would form an uninterrupted film on the surface, indicating absence of hydrophobic contaminants. Plastic materials were mostly avoided, as they make out a contamination risk. Pieces of PTFE film were cleaned with Hellmanex and Ethanol in a beaker placed on a shaker. Syringe filters were flushed with at least 10 mL of Milli-Q water and the initial 2−3 mL of filtrate was discarded each time. Inorganic salts were used without further purification. The Milli-Q water used was filtered through a 0.22 μm filter (Millipak Express 20). The Milli-Q water was stored in a clean glass bottle for at least 24 h in advance to ensure equilibration with air. Pressurized air was used without any additional fine particle filter at the point of use. Generation of Bubbles by Hydrodynamic Cavitation. The equipment used was similar, but not identical, to that described in ref 17 and is described in Figure 1. A plastic pressure vessel was equipped

Figure 1. Equipment for hydrodynamic cavitation: (1) Pressure regulator for pressurized air; (2) and (3) Shutoff valves; (4) Pressure vessel; (5) Sample beaker; (6) Needle valve; (7) Outlet beaker. with an inlet for pressurized air and an outlet for the liquid which included a needle valve. All piping and valves were of stainless steel (Swagelok). Inside the pressure vessel, a clean glass beaker was placed so that the outlet pipe extended almost to the bottom of the beaker. The liquid could be oversaturated with air by keeping it under pressure for an extended period of time. To speed up the equilibration of pressurized air with water, liquid movement was created by placing the entire vessel on a shaking table. In the reported experiments, the water was kept under 3 bar for >1 h at 70−90 rpm The needle valve was set at 2.5 turns opening which gave a flow rate of about 0.2 L/s. B

DOI: 10.1021/acs.langmuir.8b01724 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Generation of Bubbles by Shaking in Glass Tubes. Nanobubbles were generated by vigorous vertical shaking of test tubes by hand for 30 s. Mainly 8 × 100 mm plain flat top glass tubes were used, which also served as cuvettes in the DLS measurements. In this case, silicon stoppers, plastic stoppers of unknown material, PTFE film, and flat glass (pieces of microscope slides) were used as stopper. In the latter two cases the stopper needed to be firmly pressed toward the tube to avoid leakage. All of the above stopper materials were cleaned with Hellmanex, ethanol, and Milli-Q water and dried under vacuum before use. In the case of glass, it was also soaked in 69% Nitric acid. Generation of Bubbles by Addition of Inorganic Salts. Around 20% by weight of several inorganic salts were added to 50 mL of Milli-Q water in a 100 mL glass bottle which was manually gently swirled, just enough to completely dissolve the salt. Dynamic Light Scattering (DLS). The instrument used was an ALV-5000 with eight detectors placed at angles from 29° to 149°. The photon count rate at 131° was used as a measure of the concentration of nanobubbles. A high angle was selected since larger particles/ bubbles that could interfere, primarily scatter at lower angles (forward direction) and considerably less on high angles (backscattering). The light scattering of a particle increases exponentially with the particle radius, therefore it takes very few large particles to give the same or stronger signals than many small. For this reason, DLS size measurements tend to indicate slightly larger particle sizes than what is actually the case. The scattering intensity from a particle is furthermore proportional to the difference in the refractive index between the particle and the liquid. Different concentrations of 0.1 μm polystyrene latex (PSL) particles were measured for comparison; 0.01 mg/L was found to give a scattering intensity of about 2.5 kcps (kilocounts per second) at 131°. Pure MQ water had a scattering intensity of 1.1 kcps, the PSL solutions thus had an excess scattering of 1.4 kcps compared to pure water. Said PSL concentration corresponds to 2 × 107 particles per cm3. Polystyrene has RI 1.59 and thus a difference of 0.26 compared to water. The difference between water (RI 1.33) and air (1.0) is 0.33, which gives comparably strong scattering. Therefore, the scattering intensity of a PSL particle can be expected to be rather similar to that of an air bubble of the same size. Vacuum and Pressure. Since DLS cannot differentiate between gas bubbles, solid particles, and liquid droplets, it need to be complemented with other analytical methods. Otherwise it cannot be ascertained that light scattering objects detected with DLS are actually gas bubbles. Many such methods have been reported, both direct methods such as electron microscopy,15 resonant mass measurement,18,19 and phase microscopy6 and indirect methods such as exposing the sample to vacuum18,20−22 or pressure.9,23−25 Exposing a sample to vacuum will decrease the concentration of dissolved gas in it, creating undersaturation once the pressure is returned to normal, which will force gas bubbles to dissolve. As the diffusion of gas molecules is slow in a static water sample, long time and small liquid depth is needed. On the other hand, exposing the sample to elevated pressure will first compress the bubbles−which may cause them to collapse,26 then force them to dissolve due to undersaturation of air while under pressure. The number of bubbles have been shown to decrease under pressure.9,24,26 The effect is comparatively fast. Samples in 8 mm test tubes, with a liquid depth of about 1.5 cm, were exposed to 0.1 bar pressure for 24 h or 5 bar for 10 min.

Figure 2. Dynamic light scattering measurements, after 1 passage through a needle valve. Normalized correlation function vs correlation time in milliseconds. Red triangles, upper panel: Milli-Q water before cleaning of equipment. Blue circles, upper panel: Milli-Q water after cleaning. Blue circles, lower panel: 10 mM NaCl, after cleaning.

not well wetted by Milli-Q water before they were cleaned. A single passage of water, pressurized at 3 bar for 1h at 70 rpm, generated a small but measurable amount of submicron particles/bubbles. The average count rate at 131° was about 2 kcps versus 1.0−1.1 for pure Milli-Q. The signal is weak as the concentration is close to the detection limit and the plateau at higher correlation times indicate presence of large particles, which may skew the results. Nevertheless, the result is a clear indication of submicron objects of 300−350 nm diameter. Setting the needle valve at different constriction diameters did not significantly alter the result. Subsequently, the entire steel pipe assembly was filled with 2% Hellmanex fortified with 0.3% KOH for more than 1 h, followed by 99.7% ethanol and generous Milli-Q rinsing. After cleaning, the same procedure was repeated with Milli-Q water as well as 10 mM NaCl and now no detectable amount of particles/bubbles was found. The correlation function was only noise, similar to fresh Milli-Q water. The same result was achieved after the water was passed through the equipment 5 times in rapid succession. When instead a glass beaker was used that had hydrophobic patches that could not be removed by alkaline cleaning, a detectable concentration of particles/bubbles appeared again, but only after 3−5 passages through the equipment. When this beaker was again exchanged for a cleaner one, no particles/bubbles were detected. Commercial nanobubble generators often recirculate the water for an extended period of time, subjecting it to shearing forces to generate cavitation or dispersion of gas.

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RESULTS AND DISCUSSION Hydrodynamic Cavitation. Figure 2 shows the correlation function from DLS measurements of Milli-Q water passed through the equipment before it was cleaned and of Milli-Q and 10 mM NaCl passed through after the equipment was cleaned. The equipment was first tested with pure Milli-Q water directly after assembly. The steel pipes had been cleaned before assembly but not the needle valve and shutoff valves. The steel surfaces appeared to have a thin oily film and were C

DOI: 10.1021/acs.langmuir.8b01724 Langmuir XXXX, XXX, XXX−XXX

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above results indicate that the materials used can strongly affect the results. Dissolution of Inorganic Salts. Submicron particles were detected by DLS in freshly prepared solutions of all inorganic salts tested. The excess scattering intensities compared to salt solutions filtered at 0.02 μm at 131° are listed in Figure 4. After

The extended treatment times not only increase the possibility of bubbles to form, but also the possibility for impurities from the equipment to accumulate in the water. Shaking in Test Tube. Milli-Q water was shaken vigorously in test tubes with different stoppers covering the opening. As can be seen in Figure 3, the results were

Figure 4. Excess light scattering at 131° (kcps) in salt solutions. From left to right for each solution: (1) Freshly prepared solution; (2) After 1 day at ambient conditions; (3) After 1 day at 0.1 bar air pressure; (4) After 1 day and 10 min at 5 bar pressure.

passing the solutions through a 0.02 μm syringe filter, there was no correlation function and thus no detectable particle concentration in any of the solutions. The standard deviation in scattering intensity between fresh unfiltered solutions was below 0.5 kcps for three of the salts, but almost 3 kcps for the most scattering NaCl and about 4 kcps for CaCl2. Figure 4 represents a single set of experiments. As can be seen, the scattering intensity varied considerably between different salts and also between NaCl from different canisters. The fact that widely varying particle concentrations were found in different samples of the same salt (NaCl) is a strong indication that the light scattering signal has no correlation with the main constituents of the salt used. This view was further reinforced when CaCl2 from a different, new canister, of high purity was tested and gave a considerably lower scattering signal which was unaffected by pressure treatment (data not shown). It can also be mentioned that some previous authors have reported different results; for example, Sedlak,30 who interpreted the light scattering particles as solute clusters, reported relatively higher particle concentration in MgSO4 than in NaCl, whereas Yurchenko et al.31 reported the opposite, although for filtered samples. To determine if the light scattering particles were air bubbles or particles/droplets, test tubes were exposed to vacuum (0.1 bar) for 24 h. This treatment caused the light scattering to decrease substantially in the CaCl2 solution and also to some extent in MgSO4 and in one of the three NaCl solutions. Solutions were also exposed to 5 bar pressure for 10 min, which caused a smaller decrease in excess scattering in CaCl2 and no decrease in the other solutions. It was also observed that the scattering of the CaCl2 solution decays more rapidly with time than the other solutions. Although it has been suggested32 that droplets of moderately volatile hydrocarbons could respond similarly to vacuum, the results are a strong indication that the light scattering particles in the CaCl2 solution to a large extent are air bubbles. The results might not however prove with complete certainty that the particles unaffected by vacuum and pressure are not bubbles. Experimentally, Zhou et al.18 found that light scattering

Figure 3. Dynamic light scattering measurements of Milli-Q water shaken with different stopper materials. Upper panel: Normalized correlation function vs correlation time in milliseconds: red squares, plastic stoppers (unknown material); violet triangles, silicon stoppers; blue circles, PTFE film. Lower panel: Excess light scattering in counts/second at 131°.

considerably different. Silicon stoppers gave substantial scattering and an obvious submicron particle correlation. These stoppers also gave substantial light scattering in the water after only gently turning the tube upside down. Apparently, the material released particles or droplets of some sort. Other plastic stoppers of an unknown material also gave substantial scattering after shaking, but not after only contacting the water with the stopper. In this later case, pressure and vacuum treatment of the sample did not change the scattering intensity, the scattering objects were therefore probably not air bubbles in this case either. Finally, Milli-Q water shaken with a cleaned glass plate or PTFE film as a stopper did not scatter significantly more than fresh Milli-Q. If the PTFE film had not been previously cleaned, shaking did generate a little scattering, presumably due to particle contamination. It should also be mentioned that experiments were also made with test tubes with a conical ground glass stopper from two different suppliers. In this case, the ground glass joints appeared to release particles, also after thorough cleaning procedures. Generation of micro- and nanobubbles by vertical shaking has been reported in several papers.27−29 The D

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soluble surfactant is expected to be expelled into the bulk of the liquid as the surfactant layer is compressed. It need to be considered that poorly water-soluble impurities may play a role also in these cases. Laboratory glassware may have adsorbed surfactants from previous experiments or from detergents, and will also rapidly adsorb organic compounds directly from the air. Metal surfaces often have a film of organic substances from the manufacturing process, for example, from cutting liquids, which may contain oil as well as surfactants. Chemicals may contain organic substances as well, which could originate from the manufacturing process, or could migrate into the chemical from its’ container40 or be absorbed from surrounding air. Finally, water is also a possible source of contamination. Even very pure water, such as Milli-Q, may contain trace amounts of organic substances. The output of such purification equipment is dependent on the quality of the feedwater and the specific types of filters selected. To achieve a very low concentration of organic contamination is not easy. In the present study, an effort was made in this respect, but admittedly there is room for further improvements. Nanobubbles have been reported in pure water27,15,41 and aqueous solutions of inorganic salts.6,42,12,10,39,43 In most of these papers, however, cleaning of glassware and other equipment was reported to be made only by rinsing with pure water or not reported at all. In the cases where cleaning procedures were not reported, it can of course not be concluded with certainty that good cleaning procedures were not in place, but neither can the opposite. In some cases, deionized or distilled water was used directly without further purification. Inorganic salts were used “without further purification”. It is therefore quite possible that various impurities were present that influenced the experiments.

particles which resisted vacuum treatment still appeared to be bubbles, according to Resonant Mass Measurements (RMM). RMM determined the particle density to be lower than that of water, which would indicate that they were indeed bubbles. Ideally, additional methods should be employed to differentiate between bubbles and particles/droplets with certainty. Discussion. In the case of salt solutions, particles which are not bubbles may very well consist of insoluble substances present in the salts. The concentration of insoluble matter in the salt need not be very high for it to be detected by DLS. In one paper that confirms this,33 particles detectable by DLS were found in a saturated ammonium chloride solution. The particles were collected, chemically analyzed, and found to be present in the ammonium chloride salt at less than 1 mg/kg, which satisfied the chemical specification of the product, but it was nevertheless enough to be detectable by DLS. Thus, even laboratory-grade chemicals with a tight specification can very well contain a detectable concentration of insoluble particles. Another type of light scattering impurity, which could be mistaken for bubbles, is small droplets of poorly water-soluble substances like oil and fat. Such droplets can be stabilized by surfactants or consist of substances which are surface active themselves, although poorly water-soluble. One example from the nanobubble field is the paper by Häbich and co-workers,21 where light scattering particles in mixtures of ethanol and water was in their case shown to consist of water insoluble substances, presumably solubilized by ethanol molecules. It was shown that addition of decane to purified ethanol, subsequently mixed with water, would yield the same result. In another study, it was shown that light scattering particles in mixtures of tertiary butyl alcohol and water consisted of insoluble substances originating from the alcohol.34 Berkelaar and co-workers35 studied surface nanobubbles with atomic force microscopy (AFM) and found that apparent nanobubbles were in fact droplets of polydimethylsiloxane (PDMS), originating from plastic syringes. In our study, it is possible that the light scattering particles detected when performing cavitation experiments before cleaning the equipment (Figure 2) were dispersed oil droplets, originating from the pipe fittings. Surface active compounds are well-known to stabilize air bubbles in water. Air microbubbles with an adsorbed layer of poorly water-soluble surfactants have been observed to shrink and become stabilized at a size of 1−2 μm.36,37,9 As the air diffuses out into the liquid due to Laplace pressure, the surfactant molecules reach a very high packing density, which causes the surface tension to approach zero.37 When this state, where the adsorbed surfactant layers are very compressed, has been reached, further diffusion into or out of the bubble will be slow and such bubbles considerably more stable.38 As the Laplace overpressure equals the surface tension divided by the particle radius, this state should be attainable at different bubble sizes.22 Furthermore, o/w (oil-in-water) emulsions can be generated with droplet sizes ranging from below 100 nm to many microns. If gas diffusion is slow enough, it is logical that an air-in-water emulsion will follow the same laws as an o/w emulsion and also be attainable at a range of different sizes. The studies made on 1−2 μm large bubbles can therefore very well be of significance for smaller nanobubbles. Whereas the above concerns poorly water-soluble surfactans, stable nanobubbles have also been reported in solutions of well watersoluble surfactants in several papers.39,10,11 This cannot be explained with the same mechanism as above, since a water-

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CONCLUSION Much remain to be learned about air nanobubbles in water and what stabilizes them. In the present study, three different methods were used in an attempt to generate stable nanobubbles: hydrodynamic cavitation in a needle valve, vertical shaking in test tubes, and dissolution of inorganic salts in water. Dynamic light scattering (DLS) was used to detect the presence of submicron particles or bubbles. When light scattering decreased substantially following exposure to mild vacuum or pressure, the light scattering objects were likely nanobubbles and not solid particles or droplets. Impurities were found to affect the results of all three bubble generation methods. Hydrodynamic cavitation and shaking were not found to generate any detectable concentration of nanobubbles, when contamination was minimized. Light scattering particles were found in all inorganic salt solutions tested, but appeared to be actual air bubbles only in some cases. The particle concentration varied widely between several different NaCl samples and appear to be related to impurities rather than the salt itself. The present results indicate that the source of nanobubble stability is organic impurities in low concentrations, presumably surface-active substances. A literature survey shows that in previous reports on nanobubbles in pure water or inorganic salt solutions, thorough contamination control was in most cases not reported. This means that there is virtually no experimental evidence that supports previous claims that inorganic ions or a shell with a special water structure alone can stabilize nanobubbles. In future research on nanobubble stability, great care should be E

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(16) Zhang, X.; Liu, X.; Zhong, Y.; Zhou, Z.; Huang, Y.; Sun, C. Q. Nanobubble Skin Supersolidity. Langmuir 2016, 32 (43), 11321− 11327. (17) Azevedo, A.; Etchepare, R.; Calgaroto, S.; Rubio, J. Aqueous dispersions of nanobubbles: Generation, properties and features. Miner. Eng. 2016, 94, 29−37. (18) Zhou, C.; Cleland, D.; Snell, J.; Qi, W.; Randolph, T. W.; Carpenter, J. F. Formation of Stable Nanobubbles on Reconstituting Lyophilized Formulations Containing Trehalose. J. Pharm. Sci. 2016, 105 (7), 2249−53. (19) Kobayashi, H.; Maeda, S.; Kashiwa, M.; Fujita, T. In Measurement and Identification of Ultrafine Bubbles by Resonant Mass Measurement Method, International Conference on Optical Particle Characterization (OPC 2014), SPIE, 2014; p 5. (20) Qiu, J.; Zou, Z.; Wang, S.; Wang, X.; Wang, L.; Dong, Y.; Zhao, H.; Zhang, L.; Hu, J. Formation and Stability of Bulk Nanobubbles Generated by Ethanol-Water Exchange. ChemPhysChem 2017, 18, 1345. (21) Häbich, A.; Ducker, W.; Dunstan, D. E.; Zhang, X. Do stable nanobubbles exist in mixtures of organic solvents and water. J. Phys. Chem. B 2010, 114, 6962−6967. (22) Yount, D.; Gillary, E.; Hoffman, D. A microscopic investigation of bubble formation nuclei. J. Acoust. Soc. Am. 1984, 76 (5), 1511− 1521. (23) Aya, N.; Iki, N.; Shimura, T.; Shirai, T.; Tuziuti, T.; Yasui, K.; Kanematsu, W. Measurement of the change in the number of ultrafine bubbles through pressurization. Proc. SPIE 2014, 9232, 92320T. (24) Tuziuti, T.; Yasui, K.; Kanematsu, W. Influence of increase in static pressure on bulk nanobubbles. Ultrason. Sonochem. 2017, 38, 347−350. (25) Hervey, E. N.; Barnes, D. K.; McElroy, W. D.; Whiteley, A. H.; Pease, D. C. Removal of gas nuclei from liquids and surfaces. J. Am. Chem. Soc. 1945, 67 (1), 156−157. (26) Yount, D. E.; Strauss, R. H. Bubble formation in gelatin: A model for decompression sickness. J. Appl. Phys. 1976, 47 (11), 5081−5089. (27) Oh, S. H.; Kim, J. M. Generation and Stability of Bulk Nanobubbles. Langmuir 2017, 33 (15), 3818−3823. (28) Kwan, J. J.; Borden, M. A. Microbubble dissolution in a multigas environment. Langmuir 2010, 26 (9), 6542−6548. (29) Kowacz, M.; Lopes, J. N. C.; Esperança, J. M. S. S.; Rebelo, L. P. N. Hollow calcite rhombohedra at ionic liquid-stabilized bubbles. CrystEngComm 2012, 14 (18), 5723. (30) Sedlak, M.; Rak, D. Large-scale inhomogeneities in solutions of low molar mass compounds and mixtures of liquids: supramolecular structures or nanobubbles? J. Phys. Chem. B 2013, 117 (8), 2495− 504. (31) Yurchenko, S. O.; Shkirin, A. V.; Ninham, B. W.; Sychev, A. A.; Babenko, V. A.; Penkov, N. V.; Kryuchkov, N. P.; Bunkin, N. F. IonSpecific and Thermal Effects in the Stabilization of the Gas Nanobubble Phase in Bulk Aqueous Electrolyte Solutions. Langmuir 2016, 32 (43), 11245−11255. (32) Hain, N.; Wesner, D.; Druzhinin, S. I.; Schonherr, H. Surface Nanobubbles Studied by Time-Resolved Fluorescence Microscopy Methods Combined with AFM: The Impact of Surface Treatment on Nanobubble Nucleation. Langmuir 2016, 32 (43), 11155−11163. (33) Ward, M. R.; Mackenzie, A. M.; Alexander, A. J. Role of Impurity Nanoparticles in Laser-Induced Nucleation of Ammonium Chloride. Cryst. Growth Des. 2016, 16 (12), 6790−6796. (34) Sedlak, M.; Rak, D. On the origin of mesoscale structures in aqueous solutions of tertiary butyl alcohol: the mystery resolved. J. Phys. Chem. B 2014, 118 (10), 2726−37. (35) Berkelaar, R. P.; Dietrich, E.; Kip, G. A. M.; Kooij, E. S.; Zandvliet, H. J. W.; Lohse, D. Exposing nanobubble-like objects to a degassed environment. Soft Matter 2014, 10, 4947. (36) Kovalenko, A.; Polavarapu, P.; Pourroy, G.; Waton, G.; Krafft, M. P. pH-controlled microbubble shell formation and stabilization. Langmuir 2014, 30 (22), 6339−47.

taken to avoid contaminants from water, chemicals, and other materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +46-709793918. ORCID

Fredrik Eklund: 0000-0003-0582-3404 Jan Swenson: 0000-0001-5640-4766 Funding

The research leading to these results received funding from EKWA AB. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Fredrik Höök for valuable discussions.

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REFERENCES

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DOI: 10.1021/acs.langmuir.8b01724 Langmuir XXXX, XXX, XXX−XXX