Using Infrared Spectroscopy to Analyze Confined Water in Subnano Channels

In a recent study published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, a team of authors mostly led by researchers from Bohai University in Jinzhou, China, delved into the intricate world of confined water within subnano channels (1). Through the lens of infrared spectroscopy, they explored the hydrogen-bonded structures and temperature-related transitions of this confined water.

Infrared spectroscopy provides a measurement technique for intense, isolated, and reliable absorption bands of fundamental molecular vibrations from polymers and other organic compounds. This technique allows for univariate calibration with higher signal strength (absorptivity) required for solid-, liquid- or gas-phase measurements.

The researchers focused their attention on water confined within the subnano channels of AlPO₄-11. By employing thermogravimetric analysis, they estimated that approximately 8.45 water molecules reside within each channel of a single unit cell, which revealed a complex interplay of hydrogen bonding and coordination at ambient temperature (1).

At such a temperature, the confined water exhibited two distinct coordination states: saturated and unsaturated. The saturated state resembled ice-like structures, with water molecules forming four-coordinated configurations. In contrast, the unsaturated state encompassed liquid-like structures, where water molecules exhibited coordination with the framework Al sites and a relatively free range of motion. This coordination between water molecules and the framework Al sites was found to be responsible for the ice-like structures observed in the channels above the ice melting point (1). In contrast, the presence of liquid-like structures was attributed to the strong confinement within the channels, preventing the formation of extensive tetrahedral hydrogen-bonded configurations.

When temperature was decreased, the confined water within the AlPO₄-11 channels underwent a significant structural transformation. In this colder regime, isolated small water oligomers emerged, along with two novel components characterized by stronger hydrogen bonds: low-density amorphous ice-like structures and a type of low-density liquid-like structures (1). These findings offer critical insights into the structural organization and thermal dynamics of confined water in extremely narrow channels, shedding light on its behavior under unique conditions.

The research conducted by this team not only contributes to our fundamental understanding of confined water but also has implications for various fields, including biology and geology, where the behavior of water in extreme environments plays a pivotal role. Moreover, this study showcases the power of infrared spectroscopy in unraveling the mysteries of confined water, opening new avenues for further research. The investigation provided a comprehensive examination of the hydrogen-bonded structures and temperature-dependent transitions of confined water within subnano channels.

Reference

(1) Chen, S.; Wang, J.; Li, X.; et al. Hydrogen-Bonded Structures and Low Temperature Transitions of the Confined Water in Subnano Channels. Spectrochim. Acta, Part A 2023, 302, 122912. DOI: 10.1016/j.saa.2023.122912

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