Effect of Pore Shape on Freezing and Melting Temperatures of Water

To examine the effect of pore shape on freezing and melting temperatures of water, we measured X-ray diffraction patterns from water confined in four kinds of ordered mesoporous silicas with thin carbon films on the pore wall and three kinds of the inverse carbon replicas during freezing and melting processes. The melting temperature of the pore ice revealed a good correlation with the capillary condensation pressure of nitrogen at 77 K, whereas the freezing temperature of the pore water did not. This indicates that the experimental melting point represents an equilibrium solid−liquid phase transition temperature, whereas the experimental freezing point is controlled by a kinetic factor. The curvature effect of the solid−liquid interface adjacent to the pore wall did not appreciably affect the melting behavior of the pore ice. The melting point depression of the pore ice revealed an almost linear relationship with ln(p₀/p) of the capillary condensation pressure of nitrogen at 77 K for various pores of different sizes and shapes. This strongly suggests that the reciprocal of ln(p₀/p) for capillary condensation of nitrogen at 77 K gives a good measure of pore size, irrespective of the pore shapes, and the melting-point depression of the pore ice is almost proportional to the surface-to-volume ratio (S/V) of the pores confining it.