Expulsion of Pore Water from MCM-41 and SBA-16 and the Enthalpy Change of Melting of Water

Abstract

Expulsion of pore water from the cylindrical pores of MCM-41 and cage-like pores of SBA-16 was studied for the cooling process. It was observed in MCM-41 just above the capillary full filling. In SBA-16 samples, it only appeared for the fully hydrated sample exposed to the saturated vapour pressure, P 0 . When the pore remained hydrated below P 0 , expulsion of water was not observed. This is explained by the fact that expanded water fills the outermost vacant spherical pores at the particle surface. When an SBA-16 particle was fully wetted with liquid water, the freezing temperature of the pore water decreased slightly. This was caused by the pressure increase derived from occlusion of pore water with surrounding frozen ice. This form of water occlusion should be common in nature at low temperatures (biological cells, liposomes, porous stone, etc.) and was clearly identified in a well-defined porous system.