Hydrogen-bonded structures and low temperature transitions of the confined water in subnano channels

The interfacial and confined water have long been attractive objects due to their crucial roles in biological, geological processes, etc. In this paper, we investigate the hydrogen-bonded structures of water and their low temperature transitions in the subnano channels of AlPO₄-11 for the first time on the basis of infrared spectroscopy. The number of the adsorbed water molecules is estimated to be 8.45 per channel in one unit cell by thermogravimetric analysis. It is found that the confined water molecules are involved in saturated and unsaturated coordination with different hydrogen bond strengths at ambient temperature. The former refers to ice-like four-coordinated water and the latter includes liquid-like structures, Al-coordinated and relatively free water molecules. Unique coordination between water molecules and framework Al sites is responsible for the ice-like structures in the channels above the ice melting point. The appearance of liquid-like structures is closely related to the strong channel confinement, which does not allow the formation of extensive tetrahedral hydrogen-bonded configuration. As temperature decreases, a structural transformation of confined water happens in the channels of AlPO₄-11. Isolated small water oligomers and two new components with stronger hydrogen bonds, such as low-density amorphous ice-like structures and a kind of low-density liquid-like structures are preferred. Our results provide important insights into the structural organizations and thermal-dynamic behaviors of confined water in extreme narrow channels.