Thermal Properties of the Water Confined within Nanopores of Silica MCM-41

Abstract

Adiabatic calorimetry was carried out of the water confined within the pores of silica MCM‐41 with diameters of 1.2, 1.6, and 1.8 nm. Glass transitions were found at Tg = 115 and 165 K in the 1.2 nm case, 117, 165, and 205 K in the 1.6 nm, and 118 and 210 K in the 1.8 nm, respectively. The transition at Tg = 115–118 K was interpreted as caused by the freezing of the rearrangement of the water molecules located on the pore wall and interacting with silanol groups, and those at Tg = 165 and 205–210 K of the water molecules located in the center of the pores. It was noticed that the Tg increased discretely with increasing the pore diameter from 115 to 165 to 210 K. This indicates that the Tg and therefore the activation energy for the rearrangement are strongly connected with the development of the hydrogen‐bond network and furthermore the number of the hydrogen bonds formed by each water molecule. It was suggested that the bulk water undergoes the glass transition at Tg = 210 K rather than at 136 and 165 K debated hitherto and shows a change from fragile to strong behaviors in the relaxation times with cooling down to 210 K.