Kunimitsu Morishige

To examine the mechanism of nucleation and growth of stacking disordered ice (ice Isd) formed by freezing of supercooled water, we obtained cubicities (the fraction of cubic stacking sequences of ice crystallite) as a function of pore size and freezing temperature for ice Isd formed from freezing of water confined in various types of mesoporous silicas and carbons. Here, the most important role of mesopores is to control the freezing temperatures of confined water according to their shapes and sizes. Smallness of nanometer-sized water droplets confined to the mesopores and high interfacial thermal conductance of the silica–water interface lead to the suppression of the temperature rise during the freezing process. The cubicities were estimated based on the analysis of X-ray diffraction patterns of ices confined in the mesopores. The cubicities plotted against freezing temperature formed a universal curve irrespective of pore shapes. It is evident that the freezing temperature is the most important factor in controlling the cubicities of ice Isd. A plot of cubicity vs freezing temperature observed is almost identical to the results of molecular simulations in the literature. This gives strong support to the mechanism of nucleation and growth of ice Isd in supercooled water deduced from molecular simulations in the literature: layer-by-layer processes at the ice/liquid interface control the cubicity of freshly grown ice in supercooled water. The structure of ice Isd formed in supercooled water seems to be different from that of ice Isd formed in thin films of ice deposited under cryogenic conditions.

A new method of mesopore size analysis was developed for the cylindrical pores of silica using a developing hysteresis of nitrogen measured at liquid nitrogen temperature. The method is based on the semimacroscopic approach of a modified Bonnet–Wolf model that deals with the grand potential of a vapor bubble in the cylindrical pore. It is capable of assessing correctly the pore structures of mesoporous materials with cylindrical pores using the narrow hysteresis loop of the developing hysteresis. When the mesoporous materials possess cylindrical pores of minor imperfections, two pore size distributions (PSDs) from the adsorption and desorption branches overlap. On the other hand, for cylindrical pores with an enhanced amplitude of pore corrugations and/or constrictions, PSD from the desorption branch is shifted to smaller pore sizes compared to the one from the adsorption branch, and at the same time, both the PSDs evaluated are shifted to lower pore sizes compared to the actual ones. The actual PSD can be assessed from the reversible isotherm measured at a hysteresis critical temperature. In principle, the present method may enable the determination of the PSDs from the adsorption hysteresis measured at any given temperature for mesoporous materials with cylindrical pores.

To obtain insights into the mechanism of the melting-point depression of rare gas solids confined in crystalline carbon pores, we examined the freezing and melting behavior of Xe and Ar confined to the crystalline pores of ordered mesoporous carbons as well as compressed exfoliated graphite compared to the amorphous pores of ordered mesoporous silicas, by means of X-ray diffraction. For the Xe and Ar confined to the crystalline carbon pores, there was no appreciable thermal hysteresis between freezing and melting. Furthermore, the position of the main diffraction peak did not change appreciably on freezing and melting. This strongly suggests that the liquids confined in the carbon pores form a multilayered structure parallel to the smooth walls. For the Xe and Ar confined to the amorphous silica pores, on the other hand, the position of the main diffraction peak shifted into higher scattering angle on freezing suggested that the density of the confined solid is distinctly larger than for the confined liquid. Using compressed exfoliated graphite with carbon walls of higher crystallinity, we observed that three-dimensional (3D) microcrystals of Xe confined in the slit-shaped pores melted to leave the unmelted bilayers on the pore walls below the bulk triple point. The lattice spacing of the 3D microcrystals confined is larger by ∼0.7% than that of the bilayer next to the pore walls in the vicinity of the melting point.

Selective adsorption, so called “molecular sieving”, is one of the significant functions of porous materials because it can play an important role in separation processes to obtain highly pure gases or petroleum. One of the next outstanding challenges of porous materials is the achievement of controllable “molecular sieving” in response to external environments. Here, we show a new temperature responsive 1-dimensional porous compound (CPL-11) which changes its structural uniformity in response to ambient temperature, which gives rise to changes in the effective pore size. As a result of the structural responsiveness, the seeming adsorption properties of CPL-11 are apparently opposite to conventional porous compounds expected from thermodynamic law, resulting in a highly selective adsorption for O₂ compared to Ar. This unexpected adsorption behavior indicates that the adsorption properties can be controlled by changing the channel uniformity of a porous framework depending on the ambient temperature.