Resonance and relaxation effects governing the sorption of O2 and CO2 in deionized water under dynamic conditions at 21 ± 1 °C

Abstract

The behavior of water samples (deionized, deaerated and washed with He) was investigated in dynamics by physicochemical and electrochemical methods. The susceptibility of water samples to sorption of O2 and CO2 from air depends on their pretreatment. Sorption of O2 and CO2 accompanied by the desorption of dissolved helium is a multivariable nonlinear process, presumably, based on resonance and relaxation phenomena within the surface layer. Sorption of O2 and CO2 in water is considered as an interaction between gas molecules and ensembles of H2O molecules, which is assumed to be based on resonance effects. Sorption of gases leads to a, decrease in the surface tension at the water/gas interface. The effect is most pronounced in case of He-washed samples. The decrease in the surface energy upon CO2 absorption and the increase in the relaxation properties of the water surface are probably factors that facilitate sorption of gases, including O2. A fraction of sorbed O2 are contained in nanobubbles formed via water surface relaxation. A change in the viscosity of water samples during the sorption of gases may serve as a relaxation factor favorably affecting the formation and stability of nanobubbles in the surface layer and in bulk water. Irradiation of water with a laser light at λ = 633 nm can generate nano-cavities in the laser beam. In case of very low dissolved O2 concentrations, nanobubbles have no noticeable effect on the oxygen reduction reaction. Water samples washed with helium demonstrate a higher physical and chemical activity compared to other water samples, the activity being manifested in sorption properties. Electrochemical properties of water samples - deionized, deaerated and washed with He - are somewhat different. This is primarily due to the different content of dissolved O2 in the samples. Nonempirical simulations helped in clarifying the experimental data. The key factor that predetermines stability and/or collapse of nanobubbles is the organization of the hydrogen-bond network of water that strongly depends on the presence of water ions and foreign particles. In the case of both deionized water and water washed with He, HCO3-anion cannot participate in stabilization of nanobubbles. On the other hand, OH– anions can stabilize nanobubbles.