Physicochemical approach to nanobubble solutions Kazunari Ohgaki a,Ã , Nguyen Quoc Khanh a , Yasuhiro Joden a , Atsushi Tsuji b , Takaharu Nakagawa b a Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan b Advanced Technologies Development Laboratory, Panasonic Electric Works Co., Ltd., Kadoma, Osaka 571-8686, Japan article info Article history: Received 2 June 2009 Received in revised form 30 September 2009 Accepted 3 October 2009 Available online 13 October 2009 Keywords: Gas solubility Solution Surface tension Nanostructure Stability Bubble abstract Small bubbles of nitrogen, methane, or argon with an average radius of 50 nm, as measured by scanning electron microscopy, were prepared under atmospheric conditions. The lifetime of the nanobubbles extended to more than two weeks. The total amount of gases in the nanobubble solutions reached 600 cm 3 per 1 dm 3 of water, and the liquid density was about 0.988 g/cm 3 . The internal pressure of the nanobubbles was estimated to be 6 MPa. The number of nanobubbles was 1.9  10 16 bubbles per 1 dm 3 of water. These findings show that almost no gas samples are dissolved homogeneously in the aqueous solution and that the vast majority is present in the form of nanobubbles, that is, nanobubbles should be thermodynamically unstable. Attenuated total reflectance infrared spectroscopy showed that the surfaces of the nanobubbles contain hard hydrogen bonds that may reduce the diffusivity of gases through the interfacial film. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Quite recently, extremely small gas-bubbles in water have been the focus of much attention in various research fields because of their astonishing effects; active effect for creatures like oysters; symbiosis of freshwater and saltwater fishes in the same aquarium; preservation of vagus nerve of rats; rapid reaction of gas hydrates. At this point, applied studies on nanobubbles have clearly preceded fundamental ones. These gas–liquid systems are sometimes referred to as ‘‘nanobubble solutions’’ although, strictly speaking, nanobubbles should be thermodynamically unstable in aqueous solution. Some fundamental aspects of nanobubble solutions, such as bubble size, gas content, and internal pressure, remain unclear. In particular, there is a problem regarding the kinetic stability of nanobubbles. Under atmospheric conditions, the surface tension of pure water against air is 72 mN/m. The internal pressure of a nanobubble is given by the Young–Laplace expres- sion as 2g/r , where g is the interfacial tension and r is the bubble radius; the internal pressure of a 50-nm nanobubble should therefore be about 3 MPa. The crucial questions are how nanobubbles can exist over significant periods of time and how it is possible to achieve a low gas permeability (and hence a high resistance to gas diffusion) at the surface of a nanobubble. We first established a method for the preparation of aqueous solutions of large numbers of nanobubbles. This method is capable of producing a high density of nanobubbles in an aqueous solution (1.9  10 16 bubbles and 580 m 2 per 1 dm 3 of water). The resulting nanobubble samples could then be used in subsequent experiments. Secondly, the sizes of nanobubbles were measured directly by means of scanning electron microscopy (SEM), and the total amounts of gas and the densities of the solutions were measured. The structures of the bulk solutions were analyzed by means of Raman spectroscopy; by this means, we investigated whether most of the gases remained within nanobubbles or were dissolved in the aqueous solution. As a result of all these experiments, we identified some important facts regarding the stability of nanobubbles, including their average radius, their internal pressure, and their so-called ‘‘supersaturated’’ concentration. Finally, we briefly examined the microstructure of the inter- facial film of nanobubbles by means of attenuated total reflec- tance infrared (ATR-IR) spectroscopic analyses of the hydrogen- bonding behavior of the interfacial film. It is important to identify the mechanism by which a high resistance to mass transfer of gases through the interfacial film arises, as this is an important factor relating to the stability of nanobubbles. 2. Experimental section 2.1. Preparation of nanobubbles The schematic diagram of experimental apparatus for pre- paration of nanobubble solutions is shown in Fig. 1 . The gas ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.10.003 Ã Corresponding author. Tel./fax: +81 6 6850 6290. E-mail address: ohgaki@cheng.es.osaka-u.ac.jp (K. Ohgaki). Chemical Engineering Science 65 (2010) 1296–1300