stability mechanism

Journal Pre-proofs Bulk nanobubbles: production and investigation of their formation/stability mechanism Elisavet D. Michailidi, George Bomis, Athanasios Varoutoglou, George Z. Kyzas, George Mitrikas, Athanasios Ch. Mitropoulos, Eleni K. Efthimiadou, Evangelos P. Favvas PII: DOI: Reference:

S0021-9797(19)31560-7 https://doi.org/10.1016/j.jcis.2019.12.093 YJCIS 25833

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

24 September 2019 4 December 2019 20 December 2019

Please cite this article as: E.D. Michailidi, G. Bomis, A. Varoutoglou, G.Z. Kyzas, G. Mitrikas, A. Ch. Mitropoulos, E.K. Efthimiadou, E.P. Favvas, Bulk nanobubbles: production and investigation of their formation/ stability mechanism, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis. 2019.12.093

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Bulk nanobubbles: production and investigation of their formation/stability mechanism Elisavet D. Michailidia,b, George Bomisc, Athanasios Varoutoglouc, George Z. Kyzasc, George Mitrikasa, Athanasios Ch. Mitropoulosc, Eleni K. Efthimiadoub*, Evangelos P. Favvasa** aInstitute

of Nanoscience and Nanotechnology, NCSR “Demokritos”, Terma Patriarchou Grigoriou and Neapoleos, Aghia Paraskevi, 153 41, Attica, Greece bInorganic Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografou 157 71, Greece cHephaestus Laboratory, Department of Chemistry, Hellenic International University, St. Lucas, 654 04, Kavala, Greece

Abstract Nanobubbles (ΝΒs) have attracted concentrated scientific attention due to their unique physicochemical properties and large number of potential applications. In this study, a novel nanobubble generator with low energy demand, operating continuously, is presented. Air and oxygen bulk nanobubbles (NBs@air and NBs@O2) with narrow size distribution and outstanding stability were prepared in water solution. The bulk NBs’ behavior was evaluated taking into consideration the hydrodynamic diameter and ζ-potential as a function of processing time, gas type, pH value and NaCl concentration. According to the results the optimum processing time was 30min, whereas the effect of water salinity was stronger in NBs@O2 than NBs@air. In order to investigate further the NBs properties, Electron Paramagnetic Resonance (EPR) spectroscopy was applied for quantitative analysis of free radicals following the spin trapping methodology. The mechanism of bulk NBs’ generation and their extremely long-time stability can be attributed mainly to the hydrogen bonding interactions. The formation of a diffusion layer, by absorption of OH- due to electrostatic interaction, contributing to negative surface charge, whereas the interaction of ions with the surface hydroxylic groups provide the equilibrium between the protonation and deprotonation of water and finally the formation of a stable interface layer. A remarkable highlight of this work is the long-time stability of generated bulk NBs which is up to three months. Keywords: Nanobubbles; oxygen and air NBs; bulk solution; stable system; free radicals; long stability; ions distribution model. Corresponding authors:

* Eleni

K. Efthimiadou (E-mail: [email protected], Tel. +30 2107274858) P. Favvas (E-mails: [email protected] & [email protected], Tel. +30 2106503658). **Evangelos

1. Introduction Nanobubbles can exist on surfaces [1-2] (surface or interfacial NBs) and as dispersed in a liquid phase [3, 4] (bulk NBs). It is observed that nanobubbles demonstrate an extremely long lifetime, ranging to several weeks [5] or even for months [6]. The term “nanobubbles” (NB) refers to nanoscopic gaseous cavities, which diameter is less than 1μm. An alternative and equivalent term which is also being used in the literature is this of ultrafine bubbles. The international standards organization is currently evaluating standards for ultrafine bubbles (ISO/TC281) 7]. Taking into consideration the classical thermodynamics, a paradox seems to appear in systems containing nanobubbles [8, 9] due to the fact that the longevity of nanobubbles is not in 2𝛾

accordance with the Young-Laplace Law, 𝛥𝛲 = 𝑅 , where 𝛥𝑃 = 𝑃𝑣𝑎𝑝 ― 𝑃𝑙𝑖𝑞 is the pressure which is defined as the difference of the pressure between the pressure inside (vapor phase) and outside (liquid phase) of the bubble, respectively, and γ is the surface tension. Thus, it would be expected that bubbles which their diameter is in the nanoscale dissolve immediately, within a few microseconds, in favour of larger ones according to the phenomenon of Ostwald ripening [10]. The extraordinary longevity, along with the fact that NB systems have special physicochemical properties [11, 12], have triggered the interest of the scientific community and at numerous studies is discussed the reasons and the phenomena behind this property. The growing significance of nanotechnology as well as the special properties of nanobubbles has drawn huge attention in many sectors due to their wide range of potential applications, including mining industry [13-17] medical [18-21] agriculture [22-24] wastewater treatment [25-28] and surface cleaning [29-33]. The research concerning the formation and the properties of nanobubbles is clearly defined between surface and bulk NBs. This work is focused on bulk nanobubbles. Conceivably, the first direct evidence of bulk NBs, with size less than 1 μm was reported by Johnson and Cooke in 1981 [34]. They claimed that bubbles generated by shear in seawater were observed to remain stable for extended periods (>22 h) due to the formation of films on the bubble-water interface, formed from naturally present surfactants. Only a few studies were published, concerning bulk NBs, until almost a decade later when Bunkin et al. [35, 36] noted the existence of stable microbubbles in dilute solutions of electrolytes. A significant number of

publications, after 2000, are focused on bubble generation and their properties. Kim et al. in 2000 reported the generation of bulk nanoparticles, attributed to NBs, by sonication [37]. Between 2001 and 2009, Kikuchi et al. [38-40] conducted experiments upon the generation of nanobubbles by electrolysis. In 2004, Oeffinger and Wheatley employed NBs, stabilized by the addition of surfactants, as ultrasound contrast agents [41]. NBs were produced by the sonication of a perfluorocarbon gas. Najafi et al. generated NBs in a closed cuvette by increasing the temperature, thus reducing the solubility of dissolved gases and precipitating NBs [42]. Ohgaki et al. achieved the formation of NBs by the injection of gas (N2, CH4, and Ar) into water solution. They concluded that the concentration of NBs was 1.9x1016 bubbles per dm3 and persisted for up to 2 weeks [11]. In this work bulk NBs with oxygen and air were generated in water by counterflow hydrodynamic cavitation and their properties have been investigated. The NBs’ hydrodynamic diameter, size, and ζ-potential were performed by Dynamic Light Scattering (DLS) as a function of processing time, gas type, pH value and NaCl concentration. Based on the experimental results about NBs’ stability under different conditions it is concluded that the optimum processing time was 30min, whereas the effect of pH and salinity was stronger in NBs@O2 than NBs@air. EPR spectroscopy was applied for quantitative analysis of free radicals based on spin trapping methodology. In order to investigate the exact mechanism of the NBs long-time stability, an analysis of water-gas interface interactions is proposed.

2. Devices, Materials & Methods The 3D sketch of both NBs’ generator and the entire apparatus are presented in Figure 1 [43-45]. Bulk NBs are produced through hydrodynamic cavitation under pressure of ~3 bar. The most innovative feature of the apparatus is the formation of a metastable fluid cylinder of length L and radius R, between 4.5< L< 2πR combined with the counter-flow of the mixed fluid and the surface roughness. The nanobubbles’ generator takes advantage of a Venturi tube, which in the most widely is used in the hydrodynamic cavitation devise. When the gas-liquid mixture passes through a Venturi tube, bubbles are formed due to the decrease and subsequent increase in the local pressure. In this generator, in addition to the hydrodynamic cavitation, due to the Venturi tube, rough/fractal surface characteristics are also exploited to affect the fluid flow, transmuting the system from a liquid/gas mixture to a colloidal phase.

Figure 1. Sketches of apparatus for NB’s production (left), cross-sectional representation of NB generator (right-up) and proposed counter flow diagram (right-down). Left: 1) NBs’ generator, 2) thermostable liquid vessel, 3) transparent tube for liquid observation, 4) 1st pump, 5) 2nd pump, 6) pressure transmitter, 7) gas input through the 1st pump, and 8) gas input through directly the generator.

The hydrodynamic cavitation is well described by Bernoulli’s equation: 1

𝑃 + 2𝜌𝑈2 = 𝐶 (𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡)  𝑈2 +

2𝑃 𝜌

=

2𝐶 𝜌

where, P is the pressure, U is the water flow velocity at a point, and ρ is the liquid density. Therefore, the pressure becomes negative if the water flow velocity >

2𝐶 𝜌

, and this is a

threshold for starting the cavitation phenomenon [46]. For taking place hydrodynamic cavitation changes in flow direction and convergence must be occurred; the stream must be free of turbulences or/and splits. The cavitation number was introduced in order to measure the flow resistance versus cavitation. This magnitude, Kc, is defined as: Kc =

Ploc ― Pvap 1

2 2ρ1Uloc

, where Ploc is the ambient pressure,

Uloc is the velocity of the stream and Pvap is the pressure of vapor. Cavitation takes place when Kc is either equal to the initial cavitation number (Ki) or less than it, which is obtained from laboratory results (generally Ki<3)[46]. Deionized water (dH2O) was used for the preparation of all studied NBs solutions. The gas which is used to generate the nanobubbles was highly pure oxygen and filtered atmospheric air. Sodium hydroxide (NaOH, CAS No 1310-73-2, from Merk) and hydrochloric acid (HCl, 37%, CAS No 7647-01-0, from Merk) were used to adjust the pH. Sodium chloride (NaCl, CAS 764714-5, from PanReac) was used in order to evaluate the effect of salinity on the size distribution and stability of bulk nanobubbles. 5,5-dimethyl-1-pyrroline N-oxide (DMPO, CAS No 3317-61-

1, from Sigma-Aldrich) and 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, CAS No 14691-89-5, from Sigma-Aldrich) were used as free-radical spin-trapping agent and stable reference radical, respectively, for EPR measurements. A Malvern Zetasizer Nano ZS instrument was used for measuring the NB size distribution and ζ-potential. All samples were placed in Malvern DTS-1070 cuvette and measured at 25 oC. Each measurement was obtained as an average of three measurements and an average of 12 continues runs. In the case of ζ-potential each presented value is the average number of three measurements, and each measurement became as the average value of 100 runs. Electron paramagnetic resonance (EPR) measurements were performed at 25 oC using a Bruker ESP 380E spectrometer with a dielectric resonator (Bruker ER4118X-MD5). Spectra were obtained using 9.6 GHz of microwave frequency, 20.9 mW of microwave power, 100 kHz of modulation frequency, 0.3 mT of modulation amplitude, a field set of 343.5 mT with a scan range of 20 mT, and a total 10.5 min scan time over 30 accumulations. 5,5-dimethyl-1-pyrrolineN-oxide (DMPO) acting as a free-radical spin-trapping agent and was used without further purification. After preparation, samples of equal volumes were immediately placed in identical capillary tubes and the spectra were recorded exactly after 5 min from the preparation time. Radical concentrations were estimated based on a calibration curve obtained with measurements on five different concentrations of the stable TEMPO radical ranging from 10 μM to 10 mM. Spectra simulations were performed with the EasySpin package [47].

3. Result & Discussion 3.1. Nanobubbles’ evaluation According to the results the first evidence that nanobubbles have been produced is the observation of the Tyndall effect. Tyndall effect is a well-known phenomenon observed in colloidal solutions and used as a first proof to support that nanobubbles are existed into the aqueous solutions. This observation it is also reported as nanobubbles evidence in numerous relevant works [42, 48]. According to this phenomenon the NBs’ solution scatters the light of the laser beam (green laser wavelength: 495–570nm) indicating the existence of nanobubbles with diameter between 400 and 900 nm (Figure 2).

Figure 2. Tyndall effect observation: the scattering of a green laser beam through pure (untreated) water, water enhanced with air and with oxygen NBs. On the right is the phenomenon of laser beam scattering in the case of air-NBs in a dark room environment.

3.2. Free Radical Determination Free radicals could be generated by the collapse of ultrasound-induced cavitation bubbles when they are forcefully compressed by dynamic stimuli [49]. Electron paramagnetic resonance, EPR, spin trapping using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used for quantitative analysis of free radicals, resulting from the collapse of NBs without a dynamic stimulus. When reactive radical species are formed, they are trapped by the reagent. The resultant stable paramagnetic radical adducts can be observed on the EPR spectrum. The DMPO concentrations were 5, 25, 50 and 100 mM in NBs@air, NBs@O2 as well as in dH2O. 4-acetamido-2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO), which is a stable radical, was used to create a calibration curve in order to quantify the concentration of free radicals. The range of concentrations was 0.01, 0.05, 0.1, 0.5 and 1mM. In order to evaluate the possibility of radical formation in nanobubble-containing samples, we firstly compared the EPR spectra of three different samples in the concentration of 100 mM in DMPO, namely, (i) air-treated nanobubbles, (ii) oxygen-treated nanobubbles, and (iii) pure water, as a control system. Figure 3A shows that in the case of nanobubbles there is weak but detectable EPR signal with a reach hyperfine structure. Based on the calibration curve, a total spin concentration of about 1.5 μM for (i) can be inferred. The intensity of the EPR signal shows saturation for DMPO concentrations above 50 mM, as can be seen in Figure 3B. The appearance of EPR signal in samples (i) and (ii) is in line with previous studies that associated it to the formation of hydroxyl radicals (•OH) as a result of bubble collapse either as an inherent effect [49, 50] or promoted in the presence of dynamic stimuli [49-51].

The temperature increases during this process and free radicals (such as •OH) are created by decomposing water vapor and non-condensable gases (including air and oxygen) inside the bubbles [52]. However, this phenomenon could be observed also in the case of a system which takes advantage of the cavitation phenomenon for nanobubbles production [53]. Here must be noted that hydrodynamic cavitation has been attempted as an alternative to acoustic cavitation method [54, 55].

Figure 3. (A) EPR spectra of three samples containing 100 mM DMPO and (i) air nanobubbles, (ii) oxygen nanububbles, (iii) pure water. (B) Dependence of the total EPR signal intensity on DMPO concentration.

The EPR signals of Figure 3 imply the presence of two DMPO radical adducts namely DMPO/•OH and DMPO/•CH3. A quantitative analysis is provided by the simulation shown in Figure 4 where it is clear that the spectrum consists of two species; a weak four-line spectrum with intensity ratio 1:2:2:1, and a six-line spectrum of stronger intensity. The simulated hyperfine coupling parameters aH = 15.1 G, aN = 15.0 G for the former and aH = 23.4 G, aN = 15.6 G for the latter are typical for the DMPO/•OH and DMPO/•CH3 adducts, respectively.

Figure 4. Analysis of the EPR spectrum of sample (i). Upper traces: experimental (black) and simulated (gray) spectrum. Lower traces: isolated EPR spectra of the two components used for simulation (for simulation parameters see the text).

The analysis indicates that the signal is dominated by a methyl trapped radical, DMPO/CH3 with estimated ratio 9:2 (CH3:OH) compared to DMPO/•OH. This implies the existence of organic species in solution that are attacked by the hydroxyl radicals to create methyl radicals, which, in turn, are trapped by DMPO to form DMPO/•CH3. The •OH radicals which are released by the NBs collapse cause a direct oxidation of the spin trapping agent DMPO forming an imposter spin-adduct •DMPO-OH [56]. According to literature if the contaminant is some organic molecules like DMSO it is observed •CH

3

due to the existence of •OH radicals (see the reactions). This reaction can be attributed to

the formation of •CH3 radicals. NBs → NBs + •OH DMPO + •OH → •DMPO-OH (CH3)2SO + •OH → CH3(OH)SO +•CH3 However, whether the contamination source is related to the preparation of nanobubble water or degradation of spin trap is not clear yet. As a next step, in the near future, more stable spin traps like BMPO (5-Tert-Butoxycarbonyl-5-Methyl-1-Pyrroline N-oxide) will be used to confirm and investigate further this point.

3.3. Size distribution and ζ-potential Size distribution Dynamic Light Scattering (DLS) measurements were performed in order to determine the hydrodynamic diameter and the colloidal behaviour of the produced NBs. The NBs behavior was evaluated taking into consideration the hydrodynamic diameter and ζ-potential as a function of processing time, gas type, pH value and NaCl concentration. Processing time is an important parameter to be considered, as the size distribution seems to be time-depended. The size of NBs is linearly depended on the production time (Figure 5) and decreases as the processing time increased. For NBs@air, the mean size is about 850 nm for 10 min processing time, decreased to 600nm for 20 min and finally to 430nm for 30min, whereas NBs@O2 tend to be a little smaller; about 650nm for 10min processing time, 520nm for 20min and 350nm for 30min. In both cases, no further decrease in nanobubbles’ size was observed after processing time of 30 minutes. Α critical time is required until the system becomes in equilibrium status and the maximum fragmentation

in

the

gas

phase

occurs. This time depends on the geometrical characteristics of the generator parts, such as the size of the orifices, the surface roughness, the effective

surface

area,

the

counterflow length, etc. and the applied feed fluid pressure. In this, Figure 5 NBs’ size vs production processing time for NBs@air (▲) and NBs@O2 (■).

system this time is measured equal to 30 min. After this time no noteworthy

changes are occurred, and that's why the diagram’s data are limited between the time of 10 and 30 minutes. Thus, it is expected that the critical time to reach the equilibrium, will change by changing the water feed pressure, gas flow and temperature. ζ-potential One of the main factors supporting the stability of bulk NBs in solution, is the negative ζpotential, which generates repulsive forces between neighboring bulk NBs [3, 57]. In both cases, of air and O2 “dissolved” gases, the ζ-potential has a negative value. The negative zeta potential

value could be explained by the excess of hydroxyl ions (OH-) relative to hydrogen ions (H+) at the gas–water interface in pure water [3, 57-59]. According to the literature, the zeta potential is generally negative and the values varied depending, mainly, on the kind of gas introduced [10]. The charging mechanism of nanobubbles is also attributed to the preferential adsorption of OHin electrolytic solution [42]. The ζ-potential value depends on the production time (Figure 6) and a decrease, as an absolute value, as the processing time is increased. Between the two systems, the NBs@O2 provides higher ζ-potential values than the NBs@air for the

three

different

generation

processing time. This behavior is also reported from other researchers and can be attributed to the fact that the produced

colloidal

system

of

NBs@air provides a smaller pH compared to that of NBs@O2, a critical factor that determinates the ζ-potential magnitude [59]. Also, Figure 6 ζ-potential-processing time for NBs@air and NBs@O2.

after the processing time of 30 minutes the ζ-potential is stabilized

following the behavior of the processing time to the nanobubbles size (see the discussion at the “size distribution” section). The concentration of ions at and near the interface is an important factor of the nanobubbles charge. Numerous previous studies have recognized that the gas-water interface charge is caused by the adsorption of OH- onto the interface [58]. Figure 7 shows the effect of NaCl concentration on the size and ζ-potential of the air and O2 nanobubbles, respectively. In both cases of air and oxygen NBs, the surface charge of the gaswater is strongly affected by the pH of the solution. Whilst in the case of air nanobubbles an initial retreat of the absolute value of the ζ-potential is observed at the salinity of 1 mM of NaCl, at a concentration of 10 mM NaCl the ζ-potential value was increased by 50%. Based on the assumption that the hydration energy in the case of the Cl- anion (-317 kJ/mol) tended to remain longer at the gas-water interface than the Na+ cation (-406 kJ/mol) can explain

the fact that the ζ-potential increases by increasing the NaCl concentration [60]. The effect of the salinity on the NBs@air is recorded in the direction of changing the NBs’ size, linearly, from about 300nm at pure water up to ~800nm at a salinity of 10mM NaCl. On the other hand, in the case of NBs@O2, by adding NaCl the size of the NBs decreased from 450nm to about 250 and 300nm at 1 and 10mM NaCl concentration respectively.

NBs@air: NBs’ size at different NaCl concentrations (left), and ζ-potential at different NaCl concentrations (right).

NBs@O2: NBs’ size at different NaCl concentrations (left), and ζ-potential at different NaCl concentrations (right).

Figure 7 The effect of salinity in nanobubbles size and ζ-potential behavior.

The results provide evidence that the observed NB’s size can be attributed to the electric double layer formation as well to salinity variations which affect also the zeta ζ-potential at the air-water and solid-water interfaces [58]. The phenomenon of the observed changes at both NBs size and ζ-potential, finally could be attributed to the formation of a diffused layer which is created due to the interactions of the dissolved Na+ and Cl- ions with the external negatively charged nanobubbles interface. Due to these interactions the hydrodynamic diameter of the measured NBs is bigger than the original one, according to DLS measurements. The system of water and gas can be assumed as a thermodynamically equilibrium system, at certain fixed conditions (temperature and pressure).

Based on the above-mentioned protocol, the dissolved ions formed an electric double layer (EDL) from the adsorbed ions at the interface inducing an external inward radial force due to Coulomb interactions [60]. At low concentrations of NaCl, the ζ-potential of bubbles is significantly negative indicating the preferable adsorption of negative ions. It is demonstrated that H+ and OH- have a significant effect on the gas-water interface electrical charge. Additionally, taking into consideration the hydrogen bonding interactions, the charging mechanism can be explained. The hydrogenbonding network at the gas/water interface is believed to be different from that in bulk water, because intermolecular interactions on the interface are not compensated. Another crucial parameter, which strongly affects the physicochemical characteristics of the colloidal system NBs-water, is the pH of the solution. The average size and ζ-potential of both NBs@air and NBs@O2 with different pH, 5, 7 and 9, are shown in Figure 8.

NBs@air: NBs’ size at different pH (left), and ζ-potential at different pH (right).

NBs@O2: NBs’ size at different pH (left), and ζ-potential at different pH (right).

Figure 8 The effect of pH in nanobubbles size and ζ-potential behaviour.

In the case of NBs@air the smallest size, is at about ~200 nm at pH 7 contrary to pH 9, for which the size increases to 800 nm. NBs@O2 shows a different behaviour, where the average size

decreases as the pH increases from 5 to 9. at 800, 600 and 200 nm at pH of 5, 7 and 9 respectively. It is worth mentioning that in both cases, as illustrated in Figures 6 and 7, the NBs have negative zeta potential under the three studied pH conditions. The negative value increases as a function of pH alterations, highlighting that in alkaline conditions (pH 9) the maximum values have been achieved (-15 for NBs@air and -20mV NBs@O2). The high colloidal stability can be attributed to the high ζ-potential value supporting their stability in which it should be much more stable in alkaline conditions. These results are in good agreement with those obtained by other preparation methods in the recent literature [61]. Similar results are reported also by Calgaroto et al., a team which studied the effect of pH (between pH 2 and pH 8.5) in the presence of 10-2 mol/L NaCl and in the presence of alkyl methyl ether monoamine or sodium dodecyl sulphate surfactant (10-4 mol/L). At both these systems the increase of pH modifies the ζ-potential values in the direction of lower values. In specific, a threshold point of the pH 4.5 is where the ζ-potential value became negative. In this pH range the size of the NBs also decreases, in the case of 10-2 mol/L NaCl. This is similar with what we observed in the case of oxygen NBs (Fig. 8). This observed strong effect of pH on the ζ-potential of the NBs’ solution, indicated that both H+ and OH- play a very important role in the gas-water interface charge by adsorption of these ions at the interface. The fact that the interface is positively charged under slightly basic conditions (at pH 9) shows that there is an excess of H+ over OH- at the interface.

A. NBs@air: NBs’ size at dilute solutions of water-NB and EtOH (left), and ζ-potential at dilute solutions of water-NB and EtOH.

B. NBs@O2: NBs’ size at dilute solutions of water-NB and EtOH (left), and ζ-potential at dilute solutions of water-NB and EtOH.

Figure 9 Relationship of NBs’ ζ-potential and the concentration of ethanol mixed with NBs-enhanced water.

According to the literature, it is well known that the addition of alcohols in distilled water is used to investigate the alterations of zeta potential and NBs’ size based on the affection of hydrogenboundary network at the interface [62]. In order to investigate this phenomenon different amounts of ethanol (20, 40, 60 and 80 %) were added in NBs’ solution and after their homogenization the ζ-potential and size were measured. Based on the results, (Fig. 9A), the size of NBs@air increases after the addition of 20% ethanol from 400 nm to 600 nm, where no further affection is observed. Additionally, the ζ-potential value decreases from -10 to -1mV after the serial additions, (Fig. 9A). This behavior can be attributed to the adsorption of ethanol on the gas-water interface causing significant changes in hydrogen bonding network causing heterogenicity in the interface network due to the formation of ethanol-water interactions. These interactions, either thermodynamically or kinetically, lead to releasing of water molecules in bulk phase affecting the NBs’ surface charge and size [62]. Figure 9 presents the results of NBs@O2 size and ζ-potential after the addition of ethanol, which each of them is affected due to heterogenicity of formed interface. The size reduced, from ≈600 to 300 nm when ethanol is added, indicating the replacement of water molecules with ethanol on NBs’ interfaces causing a dynamic phenomenon. This phenomenon can be explained by the existence of higher oxygen radical concentration and/or the changes on hydrogen bonding network. The ζ-potential decreases, as absolute value, from -15 to -2 mV.

Figure 10. Proposed interactions that took place through different pH conditions.

Figure 10 represents a schematic illustration of ions distribution at and near the gas-water interface in an aqueous solution in different pH values. It is known that the surface charge is strongly affected by pH variations that cause changes in equilibrium between protonation and deprotonation of water. Based on this equilibrium between protonation and deprotonation of water molecules it is proposed that in the NBs surface are formed inta- and inter molecular interactions affecting the size and the ζ-potential value. According to this concept can be formed different types of inter and itra-molecular interactions like repulsive due to the same charge, electrostatic interactions due to opposite charge and finally hydrogen bonding interactions when the charge in neutral. All these situations can be in equilibrium if the pH values range between different values confirming that our system is potential. According to the generation procedure NBs are produced in deionized water in pH 5.4. It is concluded that in pH=5.4 higher concentration of H3O+ (C= 10-5.4) exist in the solution and the first layer around the NBs consisted by H3O+. These cations attract the OH- ions through electrostatic interactions creating an additional layer contributing to decrease of negative charge. When the pH becomes basic, pH 9, H3O+ concentration (C= 10-9<<10-5.4) is lower than at 5.4 increasing the concentration of OH- ions in the NBs’ surface increasing the negative charge of them which is in good agreement with the experimental results (Fig. 8). Due to that, the OH- ions which are on the NBs’ surface repulsed each other decreasing the hydrodynamic radius. When NaCl was added in a range of concentrations (0 mM, 1 mM and 10 mM NaCl) the Na+ interacts with the negative charged surface of the NBs affecting their hydrodynamic diameter by developing a new layer. On the Na-layer was absorbed also Cl- due to electrostatic interactions increasing in this way the ζ-potential (Fig. 11 right).

Size and ζ-potential influenced by NaCl addition

Figure 11. Proposed distributions of ions at and near the gas-water interface in an aqueous solution of NaCl and different pH.

Nanobubbles Long Stability in Time The high negative ζ-potential values can be attributed to the electrically charged surface of NBs. This induced repulsion forces which prevent the nanobubble coalescence and this may increase their longevity [60]; being stable and without large size fluctuations for three months. To the extent of the authors’ knowledge this is the first time that stable NBs are reported for long time. The evolution of the NBs’ diameter as a function of time is a critical issue studied also by other researchers. In specific, Sjogreen et al. reported that oxygen nanobubbles of a size of about 500 nm were recorded extremely larger, equal to 1900 and 2300 nm after 24 and 192 hours respectively [63]. The longer-lived nanobubbles were reported by Azevedo et al. for a period of time of 14 days for NBs@air [64]. As it is shown in Table 1, the size of the nanobubbles is changed, mainly increased, after the end of production process ( t=0), by a factor of about 30-100%, whereas, in the case of NB@air system, the average size varies between 430 and 640nm.

Nanobubbles’ Size (nm) NBs@O2 NBs@air 0 350 430 1 395 440 2 100 & 420* 460 Days * 3 140 & 420 460 4 100 & 400* 440 5 100 & 460* 440 1 420 460 * 2 120 &460 460 Weeks 3 120 & 560* 460 4 180 & 560* 500 * 2 180 & 640 590 Months 3 200 & 660* 640 Table 1. Nanobubbles stability, diameter size and ζ-potential, as a function of time after the end of production process (*bimodal peak values). Time

Here the data are referred to NBs produced at 10min of generator working time. Exception is observed for NB@O2 NBs, where after 2 days the size distribution is best described by a bimodal curve. As the oxygen has relatively high-water solubility (2.293x105 mol gas/mol H2O) this phenomenon could be attributed to the fact of a new equilibrium stage of the dissolved oxygen phase and the bulk water phase. As already mentioned, both NB@air and NBs@O2 are extremely stable even after a period of 3 months and even more. This does not depend on properties such as solubility and density of the nanobubbles’ gases. Note that the solubilities of N2 and O2 are 1.183 and 2.293x105 mol gas/mol H2O respectively, whereas their corresponding densities are 1.1449 and 1.3080 g/L [49]. Ushikubo et al. [60] suggested that the stability of nanobubbles is mainly due to the magnification of electrostatic repulsive forces caused by overlapping electrical double layers of the neighbouring bubbles. Ohgaki et al. [11] suggested that the stability of a nanobubble is strongly related to hydrogen bonding at water-gas interface. More recently, Wang, Liu, and Dong reported [65] that the surface of a nanobubble is kinetically stable and the water–gas interface is gas impermeable. Tolman et al. [66] predict a decrease of the surface tension for large curvature on small scales. In agreement with the hypothesis of a lower internal pressure of nanobubbles based on molecular simulation data, Nagayama et al. concluded that there are too few vapor atoms inside nanobubbles [67], so the interior gas pressure would not be high enough to support the force balance of a nanobubble.

Another theory from Lohse and Weijs [68] claims that the stability of nanobubbles originates from the slow rate of dissolution of gas into a surrounding liquid already saturated with it. A pertinent article by Ducker [69] suggests that a film of water insoluble contaminant at the vapor– liquid interface decreases the surface tension and increases contact angle of nanobubbles. Seddon et al. [70] provided a model for this remarkable nanobubbles stability to bulk dissolution for surface nanobubbles. Their argument is that the gas in a nanobubble is of Knudsen type. This leads to the generation of a bulk liquid flow, which effectively forces the diffusive gas to remain local. Several theories have been proposed as explanation for the long-term stability of bulk nanobubbles. One of the most dominant theories suggests that the stability of the nanobubbles is caused by the fact that their surface is charged. Each nanobubble is surrounded by a double layer [71, 72]. The developed double layer plays a critical role in the formation and stability of nanobubbles in aqueous solutions by providing a fairly high repulsive force, which prevents inter-bubble aggregation and coalescence of the stable bubbles. Ke et al. suggest that, in alkaline conditions, the charged nanobubbles develop negative electrostatic pressure on the interface which balances the Laplace pressure inside the NBs. Thus, the gas cannot diffuse during the equilibrium, contributing to their stability [73]. Recently, Nirmalkar et al., explain the stability of bulk NBs based on the DLVO theory. They support that DLVO provides a good exposition for the colloidal stability of NB suspensions and not only for isolated NBs [74]. Various models for NB stability are reviewed by and by Calgaroto et al., Oh et al., and Yasui et al., [13, 79, 80]. A review article, where numerous research groups including, Fox et al., [75], Kobayashi et al., [76], Azmin et al., [77], Weijs et al., [78], discuss and suggest different model for the explanation of the NBs’ stability, has been recenty published by Yasui et al. [80].

Conclusions In this study, bulk NBs@air and NBs@O2 were produced by using a generator which takes advantages of counterflow hydrodynamic cavitation. The size of produced NB was between 190 and 680 nm. The optimum, the time for producing smaller NBs, processing time was found at 30min. The effect of water salinity was stronger in the case of NB@air than NB@O2. Furthermore, the optimum NB@air size was measured at about 240nm, at pH=7, whereas the ζpotential was measured equal to -11.1, -13.2 and -14.8 mV at pH 5, 7 and 9 respectively. According to the results the studied NBs are extremely stable in terms of size for a period of

three months. Electron paramagnetic resonance revealed the presence of •OH radicals in low concentration for all NBs samples. A hydrogen interaction at various pH as well as the structure of NBs at different salinities and pH was proposed. Further studies are needed in order to investigate in depth the mechanism of NB formation. In specific, molecular dynamic calculations is a promising methodology for investigate the molecule-molecule interactions into the bulk phase and interface regions. Conclusions concerning the mechanism of the creation of the diffuse layer could be supported by the combination of the experimental data and the molecular dynamic calculations. Furthermore, attention will be given to the measurement/calculation of the NBs population. For this purpose, advanced experimental techniques such as neutron or/and x-ray small angle scattering will be taken place.

Acknowledgements This work is supported by the program of Industrial Scholarships of Stavros Niarchos Foundation. The authors EDM, GM, EKE and EPF would also like to acknowledge the support of the project MIS 5002567, implemented under the “Action for the Strategic Development on the Research and Technological Sector”, funded by the Operational Program "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014–2020) and co-financed by Greece and the European Union (European Regional Development Fund).

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