Influence of Pore Wall Hydrophobicity on Freezing and Melting of Confined Water

To examine the influence of pore wall hydrophobicity on freezing and melting behavior of a confined water, we measured the X-ray diffraction pattern of water in the nearly cylindrical pores of ordered mesoporous carbons during cooling and heating processes. The pore walls of the ordered mesoporous carbons are crystalline and cannot form hydrogen bonds with water molecules. Capillary condensation of water in the hydrophobic mesopores takes place very slowly only close to the saturation pressure of a bulk water. Nevertheless, freezing and melting of water confined in the hydrophobic mesopores occurred almost in the same way as those in mesoporous silicas with similar pore sizes, the pore walls of which are amorphous and can form hydrogen bonds with water molecules. This clearly indicates that the pore wall hydrophobicity does not affect appreciably the freezing and melting behavior of the confined water. The existence of a thin water layer between core ice and the pore walls is responsible for it.