Size-dependent melting of ice in mesoporous silica

Melting of ice in silica gel and SBA-15 with pore diameters ranging from 11.5 nm down to 1.7 nm is systematically investigated by differential scanning calorimetry measurement. The pore diameter is characterized by using two different methods, i.e. nitrogen desorption and small-angle X-ray scattering (SAXS). In comparison with the common belief constructed in the past decade that the melting point depression varies non-linearly against the inverse pore diameter (see e.g. Morishige and Kawano, J. Chem. Phys. 110 (1999) p.4867; Kittaka et al. Phys. Chem. Chem. Phys. 8 (2006) p.3223 and Jähnert et al. Phys. Chem. Chem. Phys. 10 (2008) p.6039), our results together with the data reported by Handa et al. (J. Phys. Chem. 96 (1992) p.8594) and Hirama et al. (J. Colloid. Interf. Sci. 184 (1996) p.349) indicate a linear dependence. The discrepancy between the linear and nonlinear dependences is found to come from a principal difference in determining pore sizes from the adsorption isotherm i.e. to use the Desorption branch or the Adsorption branch. With regard to the melting enthalpy depression, our results are consistent with the “excluded volume” analysis (by e.g. Lai et al. Phys. Rev. Lett. 77 (1996) p.99). Accordingly, the thickness of the pre-melting surface liquid-like-layer is estimated to be 3 monolayers of water molecules and the critical diameter for detecting the endothermic melting peak of pore ice is 1.8 nm.