Abstract
Water clusters are ubiquitously formed in aqueous solutions by hydrogen bonding, which is quite sensitive to various environment factors such as temperature, pressure, electrolytes, and pH. Investigation of how the environment has impact on water structure is important for further understanding of the nature of water and the interactions between water and solutes. In this work, pH-dependent water structure changes were studied by monitoring the changes for the size distribution of protonated water clusters by in-situ liquid ToF-SIMS. In combination with a light illumination system, in-situ liquid ToF-SIMS was used to real-time measure the changes of a light-activated organic photoacid under different light illumination conditions. Thus, the proton transfer and pH-mediated water cluster changes were analyzed in real-time. It was found that higher concentration of free protons could lead to a strengthened local hydrogen bonding network as well as relatively larger protonated water clusters in both organic acid and inorganic acid. Besides, the accumulation of protons at the liquid-vacuum interface under light illumination was observed owing to the affinity of organic molecules to the low-pressure gas phase. The application of in-situ liquid ToF-SIMS analysis in combination with in-situ light illumination system opened up an avenue to real-time investigate light-activated reactions. Besides, the results regarding water structure changes in acidic solutions showed important insights in related atmospheric and physiochemical processes.
Highlights
Proton transfer (PT) along hydrogen-bond (HB) network in water plays a key role in multiple physical, chemical, and biological processes, such as acid-base reactions (Mohammed et al, 2005), organic synthesis (Guo et al, 2016), electro-catalysis (Badalyan and Stahl, 2016), and biological redox processes (Di Luca et al, 2017)
The result demonstrated the capability of in-situ liquid ToF-SIMS for real-time monitoring of proton transfer process mediated by light
The effects of inorganic and organic acids on water structure and hydrogen bonding networks in aqueous solutions were investigated by in-situ liquid ToF-SIMS
Summary
Proton transfer (PT) along hydrogen-bond (HB) network in water plays a key role in multiple physical, chemical, and biological processes, such as acid-base reactions (Mohammed et al, 2005), organic synthesis (Guo et al, 2016), electro-catalysis (Badalyan and Stahl, 2016), and biological redox processes (Di Luca et al, 2017). Water Cluster Analysis by ToF-SIMS transfer in the form of hydronium (H3O+) or hydroxide (OH−) by forming HBs with adjacent water molecules in aqueous solutions, resulting in the formation of protonated or hydroxide water clusters with different size and structure (Agmon, 1995). This mechanism explained the anomalously high mobility of hydronium than other ions (Miyazaki et al, 2004; Headrick et al, 2005; Natarajan et al, 2015; Chen et al, 2018; Daldrop et al, 2018). Identifying the environmental effects on the dynamic and structural properties of water clusters at molecular level is of significant importance for better understanding the nature of water and the interactions between water and solutes
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