Melting and freezing of water in cylindrical silica nanopores

Abstract

Freezing and melting of H 2 O and D 2 O in the cylindrical pores of well-characterized MCM-41 silica materials (pore diameters from 2.5 to 4.4 nm) was studied by differential scanning calorimetry (DSC) and 1 H NMR cryoporometry. Well-resolved DSC melting and freezing peaks were obtained for pore diameters down to 3.0 nm, but not in 2.5 nm pores. The pore size dependence of the melting point depression DT m can be represented by the Gibbs-Thomson equation when the existence of a layer of nonfreezing water at the pore walls is taken into account. The DSC measurements also show that the hysteresis connected with the phase transition, and the melting enthalpy of water in the pores, both vanish near a pore diameter D* E 2.8 nm. It is concluded that D* represents a lower limit for firstorder melting/freezing in the pores. The NMR spin echo measurements show that a transition from low to high mobility of water molecules takes place in all MCM-41 materials, including the one with 2.5 nm pores, but the transition revealed by NMR occurs at a higher temperature than indicated by the DSC melting peaks. The disagreement between the NMR and DSC transition temperatures becomes more pronounced as the pore size decreases. This is attributed to the fact that with decreasing pore size an increasing fraction of the water molecules is situated in the first and second molecular layers next to the pore wall, and these molecules have slower dynamics than the molecules in the core of the pore.

Studies of water and ice in hydrophilic and hydrophobic mesoporous silicas: pore characterisation and phase transformations

“…3). A comparison of different techniques for determining porosity suggests that the k GT calibration constant is, as theoretically expected, dependent on pore geometry, but a full understanding of these factors is still being developed 7,17,18,19,20 . Measurements were made for the modified and non-modified materials using over-filled and partially-filled samples.…”

Ice nucleation at the nanoscale probes no man’s land of water

“…Both theory 18 and experiments 19,20 showed that the melting temperature of ice is decreased by confinement. In particular, recent simulations showed that the equilibrium melting temperature of ice nanoparticles follows the Gibbs-Thomson equation 21,22 .…”

Temperature-dependent macromolecular X-ray crystallography

“…In conclusion, ice formation within the crystal apparently does not occur if the channels are smaller than about 30 Å . Pure water confined in hydrophilic silica materials (Dore, 2000) has also been reported not to form crystalline ice if the channel sizes are below 28 Å (Jahnert et al, 2008).…”