The Significance of Hydration: Unveiling Water’s Interaction with Oxide Crystals

In a recent study published in the journal Nanoscale, scientists from Kanazawa University and AGC Inc. utilized three-dimensional atomic force microscopy to investigate the hydrated structures of commonly occurring oxide crystals.

Sapphire and quartz, both oxide crystals with various industrial applications, have intricate atomic-scale structures that have remained poorly understood. These crystals consist primarily of aluminum oxide and silicon dioxide, respectively, which exhibit a strong affinity for water, impacting their chemical reactivity. Consequently, a comprehensive understanding of the water-binding characteristics of these oxides is crucial for future innovations.

Traditional microscopic techniques have previously offered only two-dimensional insights into the surface topography of these crystals. However, a research team led by Keisuke Miyazawa from the NanoLSI at Kanazawa University developed a three-dimensional (3D) microscopy technique to thoroughly investigate how these materials interact with water at the atomic level.

The study commenced by examining the surface structures and hydration patterns of sapphire and α-quartz immersed in water. To achieve this, they employed an advanced microscopy technique known as 3D atomic force microscopy (3D-AFM). Oxide crystals typically possess hydroxyl (OH) groups, the primary molecules responsible for binding with water, closely associated with their oxides. Consequently, the team scrutinized these OH groups and their hydration patterns on both crystals when in contact with water.

The researchers discovered that the hydration layer on sapphire exhibited non-uniformity due to uneven local distributions of surface OH groups. Conversely, the hydration layer on α-quartz remained uniform, thanks to the atomically flat distribution of surface OH groups.

Subsequent measurements of the interaction forces between these oxides and water revealed that sapphire required a greater force to break the water-crystal bonds compared to α-quartz. It was also observed that this affinity was significantly higher in regions where the oxides were in close proximity to the OH groups.

This study illuminated the fact that the hydration structures of oxides are influenced by the location and density of OH groups, in addition to the strength of hydrogen bonding between the OH groups and water. Furthermore, it successfully demonstrated the utility of 3D-AFM in unraveling the interactions between water and various surfaces, offering a promising avenue for gaining a deeper understanding of solid-liquid interactions.

The researchers concluded, “This study contributes to the application of 3D-AFM in exploring atomic-scale hydration structures on various surfaces, and hence, to a wide range of solid–liquid interfacial research fields.”

For further information, please refer to the study titled “Three-dimensional ordering of water molecules reflecting hydroxyl groups on sapphire (001) and α-quartz (100) surfaces,” published in Nanoscale (2023). DOI: 10.1039/D3NR02498A.

Source: Kanazawa University

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