Abstract
Micro- and nanobubbles (MNBs) are potentially useful for industrial applications such as the purification of wastewater and the promotion of physiological activities of living organisms. To develop such applications, we should understand their properties and behavior, such as their lifetime and their number density in solution. In the present study, we observed oxygen MNBs distributed in an electrolyte (NaCl) solution using a transmission electron microscope to analyze samples made with the freeze-fracture replica method. We found that MNBs in a 100 mM NaCl solution remain for at least 1 week, but at higher concentrations decay more quickly. To better understand their lifetimes, we compared measurements of the solution's dissolved oxygen concentration and the ζ-potential of the MNBs. Our detailed observations of transmission electron microscopy (TEM) images allows us to conclude that low concentrations of NaCl stabilize MNBs due to the ion shielding effect. However, higher concentrations accelerate their disappearance by reducing the repulsive force between MNBs.
Highlights
Small gas bubbles have great potential in industrial applications such as the purification of wastewater, water-quality improvement, sterilization, decolorization, and the promotion of physiological activities of living organisms [1]
We argue that a small amount of NaCl in solution helps maintain the micro- and nanobubbles (MNBs) in the solution via the ionic shielding effect
We used a transmission electron microscopy (TEM)–freeze-fracture replica method to observe the distribution of micro- and nanobubbles of oxygen (O2 –MNBs) in solutions of NaCl concentration below 3.5 wt % at 293 K
Summary
Small gas bubbles have great potential in industrial applications such as the purification of wastewater, water-quality improvement, sterilization, decolorization, and the promotion of physiological activities of living organisms [1]. These bubbles include micrometer-scale bubbles, or microbubbles (MBs), as well as nanometer-scale bubbles, or nanobubbles (NBs). Understanding the effect of electrolytes on the MNB lifetime is important for the fundamental knowledge of the origin of the surface charge of MNBs, and for the industrial applications of MNBs in various conditions, such as the promotion of physiological activities of living organisms in the culture medium or in sea water. To better understand the mechanism through which electrolytes affect the O2 -MNB distribution, we measured pH and the amount of dissolved O2 (DO) of the NaCl–MNB solutions as well as the ζ-potential of the O2 -MNB
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