State of Oxygen Molecules in Aqueous Supersaturated Solutions Yuelong Li and Vitaly Buckin* School of Chemistry, College of Life Science, University College Dublin, Belfield Campus, Dublin 4, Ireland ABSTRACT: The state of oxygen in aqueous supersaturated solutions prepared by different methods was studied using high- resolution ultrasonic spectroscopy in combination with other techniques. This allowed for nondestructive evaluation of the properties of oxygen solute particles, composed of oxygen molecules and surrounding (coordinating) molecules of water, at equilibrium, supersaturated conditions, and different temperatures and concen- trations of O 2 . The results were compared with the behaviors of other types of solutes in water, including H 2 O 2 , which has similar molecular size and mass to O 2 but is characterized by a significantly different type of interaction with water molecules. Additionally, theoretical modeling was performed to assess the ultrasonic characteristics of dispersions of oxygen nanobubbles stabilized by a surface electrical charge. The obtained data indicate a clathrate-like organization of water in the coordination shells of single molecules of O 2 . We did not find any signs of formation of clusters of oxygen molecules in supersaturated solutions. No quantifiable presence of oxygen nanobubbles in the solutions was detected. The state of O 2 molecules was not affected by supersaturation within the analyzed concentration range of oxygen. The results also demonstrated the potential of the ultrasonic technique in precision real-time nondestructive monitoring of oxygen solubilization and outgassing processes. ■ INTRODUCTION Solubilization of oxygen gas in aqueous solutions plays an important role in the functioning of biological systems, as well as in various industrial processes. 1−5 This includes oxygen supersaturated solutions where the concentration of oxygen exceeds equilibrium level. Their applications range from facilitation of the surface oxidation of silicon 6,7 in semi- conductor industries to various treatments in medicine, including therapeutic oxygen for hypoxia, 8 oxygenation of arterial blood, 9 wound healing, 10 and stimulation of immune activity. 11−18 Oxygen supersaturated solutions can be pro- duced through physical agitation in combination with elevated partial pressure of oxygen or chemically. 8,19 In the second case, oxygen is formed directly from chemical reactions such as electrolysis of water 20 or decomposition of hydrogen peroxide, 2H 2 O 2 → 2H 2 O+O 2 . The chemical production is utilized in a variety of industrial processes, especially because its speed can be controlled by adding catalysts such as catalase or iron. 21 The development of oxygen supersaturated aqueous solutions for specific applications, including those with improved “stability”, e.g., the rate of release of oxygen with time, requires understanding of the state of dissolved oxygen and its dependence on oxygen concentration. Most previous data on the state of oxygen in aqueous solutions were obtained at equilibrium conditions. Specifically, theoretical calculations (see, for example, refs 22−24), spectroscopic 23,25 and other experimental studies, 26 have demonstrated that interaction between the dissolved molecules of oxygen and the surrounding molecules of water is extremely weak compared to water−water interactions. Solubilization of oxygen in water is characterized by negative entropy and enthalpy effects and large heat capacity increase. 27 The heat capacity of solubilization of oxygen gas in water at 25 °C is 200 J mol −1 K −1 corresponding to 24 R. 26,28 This value is close to those observed for gases with similar molecular size, such as argon or methane. 26,28 Based on the above, molecules of oxygen dispersed in water can be considered as hydrophobic solutes promoting the formation of clathrate-like “cages” of water molecules surrounding them 26,29−31 or other structures expelling of “fast” molecules of water. 30,32 The effect of supersaturation on the state of oxygen (O 2 ) is not well studied. Of particular interest are the properties of water surrounding O 2 , formation of O 2 clusters with O 2 concentration, and formation of thermodynamically stable or long-lived nanobubbles. 33−36 These subjects are important for understanding of the processes of bubble nucleation and of the nature of the supersaturation limit, 37 corresponding to the maximum concentration of gas dissolved in a liquid above which bubbles are formed spontaneously. Currently, classical theories have difficulties in quantitative description of this limit and also the dependence of this limit on the nature of the gas dissolved, 38−42 including oxygen. 19,43 The topic of oxygen/gas nanobubbles is also important for practical applications. As the density of gas at atmospheric pressure is significantly lower than that of water, large, micron-scale, gas bubbles are subjected to creaming accelerated by coalescence. The Received: February 1, 2019 Revised: March 11, 2019 Published: March 15, 2019 Article pubs.acs.org/JPCB Cite This: J. Phys. Chem. B 2019, 123, 4025-4043 © 2019 American Chemical Society 4025 DOI: 10.1021/acs.jpcb.9b01057 J. Phys. Chem. B 2019, 123, 4025−4043 Downloaded via MIAMI UNIV on October 21, 2019 at 17:27:02 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.