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

Abstract

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.

Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities

“…SBA-16 exhibits cage-like pores in the size range from 6 to 11 nm diameters that are interconnected by channels whose dimensions depend on the hydrothermal treatment temperature during synthesis (Kittaka et al, 2011). The mesoporous KIT-5 silica has spherical cavities (10-20 nm diameters) arranged in a face-centered cubic array and connected through narrow necks (Morishige et al, 2007). Zeolites are microporous crystalline aluminosilicates with pore diameters (D p ) between 0.3 and 1.2 nm.…”

“…They concluded that microcrystals did not grow along the pore length but that many nuclei formed in each cylindrical pore. Freezing temperatures of KIT (Morishige et al, 2007) and SBA-16 samples (Kittaka et al, 2011) that lie between the solid grey and the dotted black lines in Fig. 3 are indicative of heterogeneous freezing within the cage-like pores initiated by ice that propagates through channels connecting the pores.…”

“…For the KIT samples (Morishige et al, 2007) the freezing temperature depended on the hydrothermal treatment during synthesis, which influences the neck size of the pores. For samples that were hydrothermally treated only for one day, the neck size is smaller than 4 nm and the pore water in the spherical cavities of KIT is isolated from nucleation sites by the water remaining unfrozen in the narrow necks, and thus the freezing takes place via homogeneous nucleation at 232 K. For samples with wider necks the external water acts as a nucleation site.…”

“…Jähnert et al (2008) calculated a value of C = 51.9 ± 4 K nm Morishige and Kawano (1999) (X-ray diffraction patterns in ca. 2 K steps), Schreiber et al (2001) (DSC curves at 0.5 K min −1 , evaluation of peak maxima), Kittaka et al (2006) (DSC curves at 5 K min −1 , evaluation of peak onset), Jähnert et al (2008) (DSC curves at 0.5 K min −1 , evaluation of peak maxima), Deschamps et al (2010) (DSC curves of hydrophobic pores at 0.5 K min −1 , evaluation of peak maxima), Kittaka et al (2011) (DSC curves at 5 K min −1 , evaluation of peak onset), Morishige et al (2007) (DSC curves at 0.5 K min −1 , evaluation of peak onset), and Janssen et al (2004) (DSC curves at 0.5 K min −1 , evaluation of peak onset). Black dashed and dotted lines: melting point depression from the Gibbs-Thomson equation (GT).…”

“…Data is taken from Hansen et al (1996) (NMR spectra measured in 2 K steps), Morishige and Kawano (1999) (Xray diffraction patterns in ca. 2 K steps), Schreiber et al (2001) (DSC curves at 0.5 K min −1 , evaluation of peak maxima), Kittaka et al (2006) (DSC curves at 5 K min −1 , evaluation of peak onset), Jähnert et al (2008) (DSC curves at 0.5 K min −1 , evaluation of peak maxima), Kittaka et al (2011) (DSC curves at 5 K min −1 , evaluation of peak onset), Janssen et al (2004) (DSC at 0.5 K min −1 , evaluation of peak onset), Morishige et al (2007) (DSC curves at 0.5 K min −1 , evaluation of peak onset), Liu et al (2007) (DSC curves at 5 K min −1 , DTD (differential temperature difference) curves at 1 K min −1 ). Grey dotted and solid lines: pore diameter needed to incorporate a critical cluster of hexagonal (calculated with values from parameterization by Zobrist et al, 2007) and cubic ice (calculated with values from parameterization by Murray et al (2010a), respectively.…”