Pore-Blocking-Controlled Freezing of Water in Cagelike Pores of KIT-5

To examine the mechanism of the thermal hysteresis between freezing and melting for water confined in a network of the ink-bottle pores of ordered silica, we measured the freezing and melting behavior of pore water in seven kinds of KIT-5 samples with various neck and cavity sizes by means of differential scanning calorimetry and X-ray diffractometry. The melting temperature of pore ice depended on the cavity size, whereas the freezing behavior of pore water depended on the neck size. When the neck size is smaller than ∼4 nm in diameter, the pore water in the spherical cavities freezes via homogeneous nucleation on cooling. On the other hand, the freezing behavior of the pore water changed drastically when the diameter of the necks was increased beyond this critical size: the freezing temperature was increased remarkably and the freezing became very broad. Such a freezing behavior may be accounted for by a gradual percolation of the ice front that grows by traveling through the connected pore network.