Abstract
Freezing and melting of water in the cylindrical pores of ordered mesoporous silica was studied by differential scanning calorimetry. For MCM-41 silica materials wellresolved melting and freezing peaks were obtained for pore diameters down to 3.0 nm, but not in 2.5 nm pores. The enthalpy of melting and the hysteresis connected with the phase transition in the pores both vanish at a pore diameter �� ∗ ≅ 2.8 nm, indicating that �� ∗ represents a lower limit for first-order freezing/melting of water in the pores. A more complex freezing behavior was observed in partially filled pores of SBA-15 materials (pore diameters 6 to 11 nm), where the adsorbed water film undergoes a delayering transition at a temperature below the freezing point of pore water. Our work suggests that studies of the sorption and phase behavior of water in the pores contribute to a better understanding of the pore lattice deformations caused by water in the pores.
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