Article pubs.acs.org/Langmuir
Generation and Stability of Bulk Nanobubbles Seung Hoon Oh and Jong-Min Kim* School of Mechanical Engineering, Chung-Ang University, Seoul 156-756, Korea ABSTRACT: Recently, extremely small bubbles, referred to as nanobubbles, have drawn increased attention due to their novel properties and great potential for various applications. In this study, a novel method for the generation of bulk nanobubbles (BNBs) was introduced, and stability of fabricated BNBs was investigated. BNBs were created from CO2 gas with a mixing method; the chemical identity and phase state of these bubbles can be determined via infrared spectroscopy. The presence of BNBs was observed with a nanoparticle tracking analysis (NTA). The ATR-FTIR spectra of BNBs indicate that the BNBs were filled with CO2 gas. Furthermore, the BNB concentration and its ζpotential were about 2.94 × 108 particles/mL and −20 mV, respectively (24 h after BNB generation with a mixing time of 120 min). This indicates the continued existence and stability of BNBs in water for an extended period of time.
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INTRODUCTION In recent years, extremely small bubbles, referred to as nanobubbles, have been explored for various applications due to their unique physicochemical properties. Nanobubbles fall into two categories: surface nanobubbles and bulk nanobubbles (BNBs). These small bubbles can be used in a wide variety of applications such as ultrasonic targeted imaging and treatment,1 agriculture and animal husbandry,2 fuels,3 water treatment,4 and cleaning.5,6 For example, Ebina et al.2 demonstrated that water containing oxygen and air nanobubbles may be an effective tool for various health-related applications. They reported that bubbles significantly promote the size and weight of plants (Brassica campestris), fish (sweetfish and rainbow trout), and mice (DBA1/J mice). Although BNBs have drawn great attention in various fields and have been studied for decades, they are still viewed as controversial. There are still many questions related to the presence and stability of BNBs after fabrication, size control during BNB fabrication, and the principles behind the potential applications of BNBs. In recent years, various techniques for BNB fabrication have been suggested; however, most of these are unable to initially produce uniformly sized BNBs. Thus, an additional step is needed to obtain uniform bubbles. In this step, target BNBs are separated from freshly fabricated bulk micro- and nanobubble solutions by physical filtration, floatation, and centrifugal force.7−10 These separation methods are effective for selecting nanosized bubbles; however, these methods may affect the bubble yield and stability, cause sample contamination, and lead to more material waste.11 Meanwhile, Kukizaki et al.12 directly fabricated uniformly sized BNBs using Shirasu-porous-glass (SPG) membranes as the gas−liquid dispersion medium. In this method, the SPG membranes have small pores, and gas is pressed through the SPG membrane into a continuously flowing liquid. Consequently, © XXXX American Chemical Society
monodispersed BNBs were stably produced from the membranes, and the particle size dispersal coefficient of the fabricated BNBs was calculated to be in the range 0.45−0.48. However, this system requires a mesoporous or macroporous material, a pressurized gas storage system, and a hydrodynamic system. In our study, a novel and simple method for BNB fabrication was introduced. BNBs were generated by using carbon dioxide (CO2) gas and distilled (DI) water. In addition, the effect of mixing time on the characteristics of the fabricated BNBs was experimentally investigated. Also, the presence and stability of fabricated BNBs were investigated using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and nanoparticle tracking analysis (NTA).
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EXPERIMENTAL SECTION
Materials. In this study, all materials were purchased from commercial sources. Distilled (DI) water (No. 119, HPLC grade, Duksan Pure Chemicals Co., Korea) and carbon dioxide (CO2) gas (purity: 99.999%, Shinyoung Gas Co., Korea), which can be identified using infrared spectroscopy, were used to fabricate BNBs in water. Also, a 4 mL vial (SL.Vi1151, SciLab Korea Co., Ltd., Korea) and its screw cap (SL.Vi1164, SciLab Korea Co., Ltd., Korea) were used in these experiments. All materials were used as-received, with the exception of the vial and the screw cap; before fabrication of the BNBs in water, the vial and its cap were cleaned using DI water to eliminate nanosized impurities from the vial, cap, and septa. Generation of Bulk Nanobubbles in Water. To generate the BNBs in water, a gas−liquid mixing method with a linear actuator was used (Figure 1). The actuator consists of an electrical motor, crank, connecting rod, and vial holder. The crank converts the rotational motion generated by the motor into linear motion, which is Received: February 14, 2017 Revised: March 24, 2017
A
DOI: 10.1021/acs.langmuir.7b00510 Langmuir XXXX, XXX, XXX−XXX
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presence of nanosized impurities in the DI water. However, no impurities were found in the DI water (data not shown; only a black background was obtained via the NTA Instruments). Unlike the case for pure DI water, laser-illuminated nanoparticles are clearly detected after BNB generation, which is performed under a gas−liquid ratio of 1:1 and a mixing time of 120 min. As shown in Figure 2a, detected BNBs are
Figure 1. A diagram of the BNB generator. transmitted via the connecting rod to the vial holder. Consequentially, the fluids (gas and liquid) in the vial are mixed together by the up-anddown movement of the actuator. In order to mix the CO2 gas and DI water in the vial, the gas−liquid ratio, motor velocity, and moving distance are set to 1:1, 90 rpm, and 20 cm, respectively. Also, to investigate the effect of mixing time on the generation of BNBs, gas− liquid mixing was performed for 30, 60, and 120 min at a given speed and moving distance. In this study, because BNBs are fabricated by gas−liquid mixing, fabrication time and mixing time mean the same thing. Fourier Transform Infrared Spectroscopy. To identify BNBs in solution by ATR-FTIR spectroscopy, the gas inside the BNBs must be IR-active. There are many IR-active gases, including CO2, CO, and CH4. Among these, CO2 is particularly suited for IR techniques because gaseous CO2 and dissolved CO2 have very different IR spectra. In this study, an ATR-FTIR spectroscopy instrument (Thermo Nicolet 6700, Scinco Co., Ltd., Korea) equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector was used to investigate the nature of the gas that filled the BNBs in solution. Infrared spectra were recorded with a resolution of 0.5 cm−1, and a total of 200 scans were measured in the range of 650−4000 cm−1 at ambient conditions. Particle Analysis. The concentration, size distribution, mean, and mode of the generated BNBs in solution were measured via nanoparticle tracking analysis (NTA), which is a nanoparticle visualization technique that provides size and concentration measurements (NanoSight LM10-HSBFT14 with a 405 nm blue laser, Quantum Design Korea, Korea). NTA is based on a laser-illuminated microscopic technique, and the Brownian motion of BNBs in solution is detected and recorded by a scientific CMOS camera. Subsequently, a particle tracking image analysis program (i.e., the NTA software) determines the particle size distribution by tracking the visualized particles simultaneously but separately. The detection range for the particle sizes is between 10 and 1000 nm, and the size of nanoparticles is determined using the Stokes−Einstein equation:
Dt =
KBT 4πηr
Figure 2. Particle analysis results obtained just after BNB generation (gas−liquid ratio of 1:1, mixing time of 120 min): (a) laser-illuminated BNBs in DI water and (b) size distribution of BNBs.
represented as white dots on a black background. The larger BNBs shown in the figure are brighter and bigger than the smaller ones. The size distribution of BNBs in water is shown in Figure 2b. The distribution shows a single narrow peak, indicating that most of the nanoparticles are 62 nm in diameter. Also, the BNBs generated in the water have a concentration of (3.47 ± 0.39) × 108 particles/mL and a mean diameter of 88.50 ± 9.59 nm. As mentioned earlier, the gas and water in the vial are simply mixed together during the BNB fabrication process; there is no source that can make any nanosized particles (e.g., a porous gas−liquid dispersion system). Therefore, regarding these results, it is reasonable to assume that BNBs were successfully generated in DI water by the BNB generator, which mixes CO2 gas and DI water together. ATR-FTIR Measurement of BNBs in Water. In IR spectroscopy, some of the IR radiation is absorbed by the sample, while some of it passes through the sample. Because the frequency of the absorbed IR radiation depends on the molecular structure, there are no two molecules with the same IR spectrum. Similarly, because the IR spectrum depends on the phase state of CO2 molecules, IR spectroscopy can be used to directly identify the phase state of the CO2 molecules. It is known that the IR spectrum of gaseous CO2 consists of two branches with fine lines at about 2300−2380 cm−1, while that of dissolved CO2 exhibits a single peak at about 2340 cm−1.13 The ATR-FTIR spectrum of the BNBs in water clearly shows that the shape and position of the band matched the gaseous CO2 spectrum. The recorded IR spectrum shows two branches
(1)
where KB, T, and η are Boltzmann’s constant, the temperature, and the liquid viscosity, respectively, and Dt is the diffusion coefficient, which is an experimentally measured value based on the Brownian motion of the particle (as determined by the NTA method). ζ-Potential Measurement. To investigate the stability of BNBs in water, ζ-potential measurements were performed with a ζ-potential analyzer (ZetaPALS, Brookhaven Instruments, USA) at room temperature. All measurements were made at an electric field strength of 27.45−27.62 V/cm. Also, the refractive index and dielectric constant used to determine the ζ-potential were 1.331 and 78.54, respectively.
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RESULTS AND DISCUSSION Generation of Bulk Nanobubbles (BNBs) in Water. Water containing BNBs was fabricated by a BNB generator, which mixed CO2 gas and DI water together. Before beginning BNB fabrication, particle analysis was performed to examine the B
DOI: 10.1021/acs.langmuir.7b00510 Langmuir XXXX, XXX, XXX−XXX
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Langmuir with fine lines at about 2300−2380 cm−1 (see black solid line in Figure 3), and shape changes in spectrum (i) and (iii) are
concentration of BNBs is proportional to the mixing time, as shown in Figure 4a. With an increase in mixing time, the concentration of the fabricated BNBs increased. The measured concentrations at fabrication times of 30, 60, and 120 min were (0.97 ± 0.04) × 108, (2.44 ± 0.88) × 108, and (3.47 ± 0.39) × 108 particles/mL, respectively. Unlike the tendency shown with the concentration measurements, the mean diameter of BNBs decreased with increasing mixing time. As shown in Figure 4b, the mean diameter decreased from 120.67 ± 7.51 to 88.50 ± 9.59 nm. Additionally, the size distribution showed that smaller BNBs were steadily generated during the BNB fabrication process, as shown in Figure 5. At a mixing time of 30 min, the most
Figure 3. ATR-FTIR spectra of (i) BNBs in water, (ii) CO2-saturated water, and (iii) high-resolution (0.125 cm−1) spectrum of BNBs. CO2saturated water was prepared via CO2 gas dissolution at pressurized conditions.
attributed to the high resolution.14 In addition, in a different experiment, we also obtained the IR spectrum of dissolved CO2 molecules in DI water. Before measuring the IR spectrum, CO2-saturated water was prepared; a saturated condition for CO2 gas dissolution is created at 1.0−1.2 bar. Unlike the BNBs in water, the obtained IR spectrum from the CO2-saturated water showed a single peak at 2343 cm−1 (see black dashed line in Figure 3). Similarly, Zhang et al.15 performed ATR-FTIR spectroscopy to reveal the gas content of the surface nanobubbles; the recorded ATR-FTIR spectrum is the same as that of CO2 gas. As a result, the obtained ATR-FTIR spectrum of the BNBs reveals that the generated BNBs in water were filled with gaseous CO2 molecules. Effect of Mixing Time on the Generation of BNBs. To investigate the effect of mixing time on BNB generation in water, BNBs were fabricated in water under various conditions, and particle analysis using the NTA method was performed on these samples. The mixing times used to fabricate BNBs in water were 30, 60, and 120 min. Particle analysis results of the BNBs just after bubble fabrication are shown in Figure 4. The
Figure 5. Effect of mixing time on the size distribution of BNBs in water.
abundant nanobubble has a diameter of 95.0 nm (i.e., the mode diameter is 95.0 nm); with an increase in mixing time, the mode diameter decreases from 95.0 to 62.0 nm. However, the mixing time has almost no effect on the dispersity of BNBs. The dispersity of the fabricated BNBs was determined using the particle size dispersal coefficient, δ:12 90
d − 10d 50 d
δ= 90
50
(2) 10
where d, d, and d are the diameters corresponding to 90, 50, and 10 vol % on the relative cumulative bubble size distribution curve, respectively. This means that as the particle size dispersal coefficient becomes smaller, the size variation between BNBs decreases. According to eq 2, the particle size dispersal coefficients of the BNBs fabricated at mixing times of 30, 60, and 120 min are 0.97 ± 0.08, 1.19 ± 0.10, and 1.02 ± 0.04, respectively. Based on these results, mixing the gas and liquid together is very effective for fabricating uniformly sized BNBs. Also, this means that the number of generated BNBs has a positive correlation with the mixing time; however, the mean and mode diameters of BNBs show a negative correlation. In addition, the mixing time has virtually no effect on the dispersity of BNBs. Stability of BNBs. As was shown in the particle analysis results of BNBs, smaller BNBs were steadily generated during BNB fabrication and the BNB concentration increased with mixing time. It is possible that once the BNBs are formed in solution, they do not easily disappear during the fabrication process. According to a study performed by Ljunggren et al.,16 the lifetime of a bubble can be described as Figure 4. Effect of mixing time on (a) concentration and (b) mean diameter of BNBs in water.
t= C
Kd0 2 12RTD
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where Pi, Pa, σ, and d represent the internal pressure of the bubble, the ambient pressure, the surface tension of the liquid, and the bubble diameter, respectively. In other words, the internal pressure Pi of a bubble with diameter d exceeds the ambient pressure Pa by 4σ/d. From eq 4, the internal pressure of the generated BNBs would be approximately 3.4 MPa (using d = 88.50 nm and σ = 0.073 N/m). However, the average internal pressure is found to be about 2.25 MPa from the Soave−Redlich−Kwong equation of state,21,22 which is significantly less than that expected from the Young−Laplace equation. Therefore, it is expected that the surface tension of BNBs was much smaller than that obtained at macroscopic scale. For the surface tension of nanobubbles, it has been shown that the surface tension of nanobubbles was lower than those of the gas−liquid interface at the macroscopic scale.23−27 Tolman23 showed theoretically that the surface tension could be expected to decrease with decrease in droplet size, and it decreased significantly at very small sizes. In addition, Attard24 and Zhao et al.25 showed experimentally that the surface tension of nanobubbles was less than that of saturated water and varies with the curvature of the gas−liquid interface. However, the internal pressure is still greater than atmospheric pressure and such a high pressure inside of the bubble would not allow BNBs to exist in a stable form; they would be expected to collapse in a very short time. One of the main factors supporting the stability of BNBs in solution is the negative ζ-potential, which generates repulsive forces between neighboring BNBs.18,28 The ζ-potentials of water containing BNBs were measured over a period of 24 h (Figure 7). The average measured ζ-potential value for the
where K, d0, R, T, and D are the Henry law constant, bubble diameter at t = 0, gas constant, temperature, and diffusion constant, respectively. Thus, the expected lifetime of generated BNBs would be approximately 0.41 μs (using K = 2.98 × 103 J/ mol, d0 = 88.50 nm, R = 8.314 J/(K mol), T = 298.15 K, and D = 1.92 × 10−9 m2/s). Therefore, if the lifespan of BNBs is about 0.41 μs, the BNB concentration will not increase and/or the increment of the concentration will not be proportional to the mixing time. The particle analysis results 24 h after BNB generation, with a mixing time of 120 min, are shown in Figure 6. This indicates
Figure 6. Particle analysis results obtained 24 h after the end of the BNB fabrication process.
that the lifespan of generated BNBs is longer than 24 h. As previously mentioned, the concentration of BNBs was initially (3.47 ± 0.39) × 108 particles/mL at a mixing time of 120 min. However, the BNB concentration decreased from (3.47 ± 0.39) × 108 to (2.94 ± 0.16) × 108 particles/mL over 24 h; only 15.27% of the BNBs disappeared, unlike the theoretical prediction. This means that the fabricated BNBs disappear slowly and can be maintained in water for a long time. On the other hand, the mean diameter of BNB increased from 88.50 ± 9.59 to 110.00 ± 4.58 nm over 24 h. It is known that there are two main particle coarsening pathways: Ostwald ripening and Smoluchowski ripening. Assuming that the BNBs were well dispersed in water, the distance between neighboring BNBs would have been about 14.23 μm at just after fabrication or about 322 times as large as the mean radius of BNBs. Thus, collision between BNBs would be infrequent in the BNB water used in this study,17 and therefore Ostwald ripening likely dominated the growth of BNBs. Similarly, other researchers have also demonstrated that BNBs can remain in liquid for a long time.18,19 Ushikubo et al.18 fabricated BNBs using oxygen gas and showed that the presence of BNBs could be detected via dynamic light scattering (DLS) for 6 days. Also, Liu et al.19 reported that BNBs could be generated with nitrogen gas and remain in water for 7 days. They also showed that the stability of BNBs was not disrupted by degassing under a pressure of 0.02 MPa. It is generally known that bubbles can disappear via the following paths: (1) bubbles rise to the surface due to buoyant forces, (2) bubbles dissolve away, and (3) the collision and coalescence of bubbles. However, it has been reported that bubbles that are smaller than about 5 μm in diameter have very small buoyancy forces and do not rise.20 In addition, due to their thermodynamic instability, the stability of BNBs has been debated for a long time. According to the Young−Laplace equation (eq 4), the internal pressure of a bubble dramatically increases as its diameter decreases: 4σ Pi = Pa + (4) d
Figure 7. Measured ζ-potential over a period of 24 h.
water containing BNBs was about −20 mV. During the measurement time, the ζ-potential of the BNBs in water was maintained almost same value. At a high absolute ζ-potential, particles tend to repel each other and avoid agglomeration in a colloidal dispersion. In the case of a BNB solution, such a high ζ-potential could cause repulsive forces between neighboring BNBs; these forces would contribute to the stabilization of BNBs. Ushikubo et al.18 suggested the strong possibility of long-term existence of nanobubbles in water; they concluded that the stability of nanobubbles is supported by the electrically charged liquid−gas interface, which creates repulsive forces that prevent the coalescence of bubbles. Also, Cho et al.28 showed that ζ-potential became less than −20 mV for pH greater than 6 because of the adsorption of OH− ions. Similarly, Tyrrell and Attard29 reported that bubble size increases as the pH is decreased and vice versa. Also, authors showed that discrete nature of the bubbles is enhanced as a result of the increase in surface charge and corresponding repulsion between neighboring sites at high pH. These mean that surface potential and D
DOI: 10.1021/acs.langmuir.7b00510 Langmuir XXXX, XXX, XXX−XXX
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(3) Oh, S. H.; Yoon, S. H.; Song, H.; Han, J. G.; Kim, J.-M. Effect of hydrogen nanobubble addition on combustion characteristics of gasoline engine. Int. J. Hydrogen Energy 2013, 38 (34), 14849−14853. (4) Agarwal, A.; Ng, W. J.; Liu, Y. Principle and applications of microbubble and nanobubble technology for water treatment. Chemosphere 2011, 84 (9), 1175−1180. (5) Zhu, J.; An, H.; Alheshibri, M.; Liu, L.; Terpstra, P. M.; Liu, G.; Craig, V. S. Cleaning with Bulk Nanobubbles. Langmuir 2016, 32 (43), 11203−11211. (6) Zhang, M.; Seddon, J. R. Nanobubble−Nanoparticle Interactions in Bulk Solutions. Langmuir 2016, 32 (43), 11280−11286. (7) Krupka, T. M.; Solorio, L.; Wilson, R. E.; Wu, H.; Azar, N.; Exner, A. A. Formulation and characterization of echogenic lipid− pluronic nanobubbles. Mol. Pharmaceutics 2009, 7 (1), 49−59. (8) Zong, Y.; Wan, M.; Wang, S.; Zhang, G. Optimal design and experimental investigation of surfactant encapsulated microbubbles. Ultrasonics 2006, 44, e119−e122. (9) Feshitan, J. A.; Chen, C. C.; Kwan, J. J.; Borden, M. A. Microbubble size isolation by differential centrifugation. J. Colloid Interface Sci. 2009, 329 (2), 316−324. (10) Oeffinger, B. E.; Wheatley, M. A. Development and characterization of a nano-scale contrast agent. Ultrasonics 2004, 42 (1), 343− 347. (11) Hwang, T. L.; Lin, Y. K.; Chi, C. H.; Huang, T. H.; Fang, J. Y. Development and evaluation of perfluorocarbon nanobubbles for apomorphine delivery. J. Pharm. Sci. 2009, 98 (10), 3735−3747. (12) Kukizaki, M.; Goto, M. Size control of nanobubbles generated from Shirasu-porous-glass (SPG) membranes. J. Membr. Sci. 2006, 281 (1), 386−396. (13) Lohse, D.; Zhang, X. Surface nanobubbles and nanodroplets. Rev. Mod. Phys. 2015, 87 (3), 981. (14) Zhang, X. H.; Quinn, A.; Ducker, W. A. Nanobubbles at the interface between water and a hydrophobic solid. Langmuir 2008, 24 (9), 4756−4764. (15) Zhang, X. H.; Khan, A.; Ducker, W. A. A nanoscale gas state. Phys. Rev. Lett. 2007, 98 (13), 136101. (16) Ljunggren, S.; Eriksson, J. C. The lifetime of a colloid-sized gas bubble in water and the cause of the hydrophobic attraction. Colloids Surf., A 1997, 129, 151−155. (17) Weijs, J. H.; Seddon, J. R.; Lohse, D. Diffusive shielding stabilizes bulk nanobubble clusters. ChemPhysChem 2012, 13 (8), 2197−2204. (18) Ushikubo, F. Y.; Furukawa, T.; Nakagawa, R.; Enari, M.; Makino, Y.; Kawagoe, Y.; Shiina, T.; Oshita, S. Evidence of the existence and the stability of nano-bubbles in water. Colloids Surf., A 2010, 361 (1), 31−37. (19) Liu, S.; Kawagoe, Y.; Makino, Y.; Oshita, S. Effects of nanobubbles on the physicochemical properties of water: The basis for peculiar properties of water containing nanobubbles. Chem. Eng. Sci. 2013, 93, 250−256. (20) Zimmerman, W. B.; Tesař, V.; Bandulasena, H. H. Towards energy efficient nanobubble generation with fluidic oscillation. Curr. Opin. Colloid Interface Sci. 2011, 16 (4), 350−356. (21) Soave, G. Equilibrium constants from a modified RedlichKwong equation of state. Chem. Eng. Sci. 1972, 27 (6), 1197−1203. (22) Ohgaki, K.; Khanh, N. Q.; Joden, Y.; Tsuji, A.; Nakagawa, T. Physicochemical approach to nanobubble solutions. Chem. Eng. Sci. 2010, 65 (3), 1296−1300. (23) Tolman, R. C. The effect of droplet size on surface tension. J. Chem. Phys. 1949, 17 (3), 333−337. (24) Attard, P. Direct Measurement of the Surface Tension of Nanobubbles. arXiv preprint arXiv:1505.02217, 2015. (25) Zhao, B.; Wang, X.; Wang, S.; Tai, R.; Zhang, L.; Hu, J. In situ measurement of contact angles and surface tensions of interfacial nanobubbles in ethanol aqueous solutions. Soft Matter 2016, 12 (14), 3303−3309. (26) Moody, M. P.; Attard, P. Curvature-dependent surface tension of a growing droplet. Phys. Rev. Lett. 2003, 91 (5), 056104.
Coulomb repulsion play a role in the stabilization/evolution of the BNBs. In addition, it is known that BNBs stabilize themselves through ionic shielding and diffusive shielding.22,30−32 For example, Ohgaki et al.22 showed that the interface of BNBs consists of hard hydrogen bonds that can markedly reduce the diffusivity of gas from the BNBs. In summary, ζ-potential measurements showed that the BNBs in water were negatively charged. The measured ζpotentials were strong (about −20 mV) and were maintained throughout the ζ-potential measurement. As a result, this high ζ-potential value may be related to the stability of BNBs in water. The repulsive forces generated by the electrically charged surfaces of BNBs can help avoid the coalescence of neighboring nanobubbles.
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CONCLUSIONS In this study, BNBs were fabricated in water by mixing a gas and liquid together, and the effect of the mixing time on the characteristics of the generated BNBs was investigated. BNBs were successfully generated by mixing CO2 gas and water under various mixing times (30, 60, and 120 min). The concentration of the BNBs generated in water tends to increase with increasing mixing time (from (0.97 ± 0.04) × 108 particles/mL for 30 min to (3.47 ± 0.39) × 108 particles/mL for 120 min), but the mean diameter of BNBs decreased slightly from 120.67 ± 7.51 to 88.50 ± 9.59 nm with increasing mixing time. Moreover, ATR-FTIR spectroscopy showed that the fabricated BNBs consist of gaseous CO2 molecules. The concentration of BNBs was stably maintained for 24 h after BNB generation, indicating the presence and stability of BNBs (a few hundred nanometers in diameter) for a long time. The ζ-potential measurements showed that the surfaces of BNBs were negatively charged and experienced no significant change over 24 h. These results support that BNBs are successfully generated in water by a simple gas−liquid mixing method and generated nanobubbles remain in water for a long time.
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AUTHOR INFORMATION
Corresponding Author
*Tel +82 2 820 5728; Fax +82 2 824 5728; e-mail 0326kjm@ cau.ac.kr (J.-M.K.). ORCID
Jong-Min Kim: 0000-0001-9206-0382 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017007923) and the Technological Innovation R&D Program (S2409594) funded by the Small and Medium Business Administration (SMBA, Korea).
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REFERENCES
(1) Yang, H.; Cai, W.; Xu, L.; Lv, X.; Qiao, Y.; Li, P.; Wu, H.; Yang, Y.; Zhang, L.; Duan, Y. Nanobubble−Affibody: Novel ultrasound contrast agents for targeted molecular ultrasound imaging of tumor. Biomaterials 2015, 37, 279−288. (2) Ebina, K.; Shi, K.; Hirao, M.; Hashimoto, J.; Kawato, Y.; Kaneshiro, S.; Morimoto, T.; Koizumi, K.; Yoshikawa, H. Oxygen and air nanobubble water solution promote the growth of plants, fishes, and mice. PLoS One 2013, 8 (6), e65339. E
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Article
Langmuir (27) Fang, C.-K.; Ko, H.-C.; Yang, C.-W.; Lu, Y.-H.; Hwang, S. Nucleation processes of nanobubbles at a solid/water interface. Sci. Rep. 2016, 6, 24651. (28) Cho, S.-H.; Kim, J.-Y.; Chun, J.-H.; Kim, J.-D. Ultrasonic formation of nanobubbles and their zeta-potentials in aqueous electrolyte and surfactant solutions. Colloids Surf., A 2005, 269 (1), 28−34. (29) Tyrrell, J.; Attard, P. Atomic force microscope images of nanobubbles on a hydrophobic surface and corresponding force− separation data. Langmuir 2002, 18 (1), 160−167. (30) Seddon, J. R.; Lohse, D.; Ducker, W. A.; Craig, V. S. A deliberation on nanobubbles at surfaces and in bulk. ChemPhysChem 2012, 13 (8), 2179−2187. (31) Bunkin, N.; Shkirin, A.; Ignatiev, P.; Chaikov, L.; Burkhanov, I.; Starosvetskij, A. Nanobubble clusters of dissolved gas in aqueous solutions of electrolyte. I. Experimental proof. J. Chem. Phys. 2012, 137 (5), 054706. (32) 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.
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