X-ray diffraction study of water confined in mesoporous MCM-41 materials over a temperature range of 223-298 K

X-ray scattering measurements on water confined in the cylindrical pores of MCM-41 with different pore sizes C10 (diameter = 21 Angstrom) and C14 (28 Angstrom) have been performed over a temperature range of 223-298 K. Both s amples were sealed in glass capillaries at relative water vapor pressures p /p(0) = 0.3 and 0.6 under monolayer and capillary-condensed adsorption cond itions, respectively. The X-ray radial distribution functions showed the pr esence of a distorted tetrahedral-like hydrogen-bonded network of water in both pores, characterized by peaks at similar to 2.8, similar to 4.2, and s imilar to 4.9 Angstrom, with non-hydrogen-bonded H2O-H2O interactions at si milar to 3.3 Angstrom and H2O-Si interactions at similar to 3.8 Angstrom be tween water and the silica wall, With decreasing temperature, the number of the hydrogen-bonded H2O-H2O interactions at 2.8 Angstrom increases, accomp anied by the shifts of the 2.8 and 4.9 Angstrom peaks to shorter distances and of the 4.2 Angstrom peak to larger distances, for the capillary-condens ed samples, showing a tendency to form more tetrahedral-like hydrogen-bonde d water structure at subzero temperatures in both pores. The amount of the hydrogen-bonded water molecules is larger with less non-hydrogen-bonded H2O molecules in the C14 pores than in the C10 ones, showing that decreasing p ore size leads to increasing distortion and/or breaking-down of hydrogen bo nds in adsorbed water structure. No significant structural change of water was observed for the monolayer sample with decreasing temperature.