Enthalpy and interfacial free energy changes of water capillary condensed in mesoporous silica, MCM-41 and SBA-15

Abstract

The effect of confinement on the solid-liquid phase transitions of water was studied by using DSC and FT-IR measurements. Enthalpy changes upon melting of frozen water in MCM-41 and SBA-15 were determined as a function of pore size and found to decrease with decreasing pore size. The melting point also decreased almost monotonically with a decrease in pore size. Analysis of the Gibbs-Thomson relation on the basis of the thermodynamic data showed that there were two stages of interfacial free energy change after the constant region, i.e., below a pore size of 6.0 nm: a gradual decrease down to 3.4 nm and another decrease after a small jump upward. This fact demonstrates that the simple Gibbs-Thomson relation, i.e., a linear relation between the melting point change and the inverse pore size, is limited to the range not far from the melting point of bulk water. FT-IR measurements suggest that the decrease in enthalpy change and interfacial free energy change with decreasing pore size reflect the similarity of the structures of both liquid and solid phases of water in smaller pores at lower temperatures.

Melting and freezing of water in cylindrical silica nanopores

“…A melting point depression DT m up to 50 K was found, and the dependence of DT m on the pore radius R could be represented within experimental error by a relation DT m = C/(R À t), with temperature independent values of the parameters C and t E 0.4 nm, corresponding to 1-2 monolayers of nonfreezing water at the pore wall. These findings were in agreement with an earlier NMR cryoporometry study by Schmidt et al, 2 while a recent study of Kittaka et al 5 suggests that such a relation does not apply for the full series of MCM-41 materials.…”

“…Theoretical studies have indicated, however, that for curved substrates all wetting transitions are suppressed and, for cylinders and spheres, the prewetting transition is smeared by finite-size effects. 14 The freezing and melting of water confined in mesoporous silica materials of different pore morphology and width has been studied by a variety of techniques, including calorimetry, 3,5,[15][16][17] NMR spectroscopy, 2,16-20 X-ray 4 and neutron diffraction. [21][22][23] Studies of interfacial premelting in narrow pores are rare and have been hampered by the ill-defined porosity of most substrates used in the past for such studies.…”

Dielectric study on two dynamic phases of 1-propanol confined in mesopores of MCM-41

“…31 The MCM-41 samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and N 2 adsorption measurements. 20 Honeycomb structures in the samples were confirmed by TEM and XRD observations. Figure 1 shows adsorption and desorption isotherms of N 2 for the MCM-41 samples.…”

“…These surface water molecules are not frozen at low temperatures but inner ones freeze. 20 In the present study, the dynamics of 1-propanol molecules confined in MCM-41 were investigated.…”

“…After finding fundamental properties of the liquid in the pores, i.e., Gibbs-Thomson relation in freezing and melting processes, new fine structures and characteristic behaviors of pore water have become clarified. [18][19][20][21][22][23][24][25][26] In relation with the study on water in confinement, present authors have been aiming to study the effect of adding hydrophobic nature to water, i.e., alcohol molecules. [27][28][29] In the previous work, we made dielectric measurements to study the dynamics of methanol and ethanol molecules confined in mesopores of MCM-41, and found that the motions of these molecules in MCM-41, especially near the pore wall, become slower compared with those in bulk liquid.…”