Global Phase Diagram of Water Confined on the Nanometer Scale

Water confined on the nanometer scale exhibits unusual behavior that cannot be observed in the bulk. The freezing/melting temperature T m of liquids is one of the most prominent phenomena that strongly depend on geometrical confinement. In this work, we report the phase behaviors of water in a cylindrical pore of single-walled carbon nanotubes (SWCNTs). Previous studies [Maniwa et al. : Chem. Phys. Lett. 401 (2005) 534] of SWCNTs with diameters D from 1.17 to 1.45 nm clarified that water shows a liquid-to-solid like transition with decreasing temperature. In the present work, we increased the diameter to D =2.4 nm to establish the global phase diagram of water. Employing a combination of X-ray diffraction and NMR experiments, we found that water within an SWCNT undergoes a wet–dry transition instead of ice formation once D exceeds about 1.6 nm.