(507g) Limits on Crystallization/melting of Water in the Pores of Mcm-41 Silica

The limits of crystallization/melting of water in the cylindrical mesopores of high-quality MCM-41 silicas with pore width in the range 2.6 to 4.6 nm was studied by a combination of differential scanning calorimetry (DSC) and NMR cryoporometry. In addition, the local density of water in the pores as a function of temperature is being measured by neutron small-angle diffraction, by following the intensity of the leading Bragg peak of the pore lattice of water-filled MCM-41 samples under contrast-matching conditions. By combining the results of these studies we can locate the limiting pore width below which water does not crystallize due to geometrical confinement. The melting point depression as obtained by DSC can be represented by a modified Gibbs-Thomson equation of the form ΔT = C/(R - t), where R is the pore radius and the parameter t accounts for a boundary layer of nonfrozen water, for which we find a value corresponding to 1-2 layers of water molecules. The melting enthalpy Δh decreases with decreasing pore radius and approaches zero at R < 1.5 nm. A simple model indicates that this cannot be explained solely by the increasing surface-to-volume ratio of the crystals, but must be due in part to other factors, such as an intrinsic temperature dependence of the melting enthalpy of ice. For the most narrow pore size (2.6 nm), NMR indicates that there is still a transition from solid-like to liquid-like water, although this transition is smeared out over a wider temperature range than for the samples with larger pore width.