The effect of HO concentration on the change of H-bonds at a water/quartz interface was systematically examined by surface-specific sum-frequency generation (SFG) spectroscopy. Molecular dynamics (MD) simulation was further utilized to interpret the specific molecular dynamics as well as the configuration and evolution of water and HO molecules at the interface. The results from this study demonstrated the important role of surface H-bonds on determination of the stability of adsorbed HO at solvated, silica, xerogel surfaces. It was revealed that prior to reaching the surface saturation with HO molecules (less than 20% in bulk solution), multiple H-bonds were formed with silanols at relatively short interactive distances. These H-bonds proved to be strong enough to enable the overall stability of adsorbed HO. However, once saturated, the HO molecules would be adsorbed at longer distances away from the surface, and could easily migrate to the bulk solution; therefore, in this case, the bonds failed to support stable HO adsorption. These new findings explained the detailed molecular mechanism of the relationship between HO concentration and HO stability in HO-silica xerogels. This solves the current challenge of effective HO storage, and provides fundamental insight for predicting the adsorption behavior of HO at the silica surface.

BACKGROUND In comparison with ordinary bubbles, microbubbles exhibit low floating rate and high surface charge density. More importantly, microbubbles shrink gradually and the gas–liquid interface charges congest significantly, generating large surface potential and energy, which promotes the formation of reactive oxygen species (ROS) when microbubbles eventually collapse. In this study, size‐controllable microbubbles were introduced to alkaline solutions by titanium microporous filters, and the effects of microbubble diameter, alkaline concentration and temperature on the formation of ROS were examined. RESULTS Maximum ROS concentration was obtained by using a microporous filter with average pore size of 25 µm and 40 µm in NaOH and KOH solutions, respectively. Further, with increase in alkaline concentration and temperature, the formation of ROS both exhibited parabolic trends, and optimal temperature for ROS formation was determined to be 65 °C. The maximum ROS concentration was 1.05 mmol L−1 and 3.51 mmol L−1 in NaOH and KOH solutions, respectively, after aerating for 60 min. CONCLUSION The formation of ROS is dependent on the bubble size and solution composition and is influenced by both temperature and alkaline concentration. The results further extend the range of ROS application and provide references for ROS regulations in alkaline media. © 2016 Society of Chemical Industry

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