X-ray scattering measurements on water confined in the cylindrical pores of MCM-41 with different pore sizes C10 (diameter ) 21 Å) and C14 (28 Å) have been performed over a temperature range of 223-298 K. Both samples were sealed in glass capillaries at relative water vapor pressures p/p 0 ) 0.3 and 0.6 under monolayer and capillary-condensed adsorption conditions, respectively. The X-ray radial distribution functions showed the presence of a distorted tetrahedral-like hydrogen-bonded network of water in both pores, characterized by peaks at ∼2.8, ∼4.2, and ∼4.9 Å, with non-hydrogen-bonded H 2 O-H 2 O interactions at ∼3.3 Å and H 2 O-Si interactions at ∼3.8 Å between water and the silica wall. With decreasing temperature, the number of the hydrogen-bonded H 2 O-H 2 O interactions at 2.8 Å increases, accompanied by the shifts of the 2.8 and 4.9 Å peaks to shorter distances and of the 4.2 Å peak to larger distances, for the capillarycondensed samples, showing a tendency to form more tetrahedral-like hydrogen-bonded water structure at subzero temperatures in both pores. The amount of the hydrogen-bonded water molecules is larger with less non-hydrogen-bonded H 2 O molecules in the C14 pores than in the C10 ones, showing that decreasing pore size leads to increasing distortion and/or breaking-down of hydrogen bonds in adsorbed water structure. No significant structural change of water was observed for the monolayer sample with decreasing temperature.

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