X-ray Diffraction Study of Water Confined in Mesoporous MCM-41 Materials over a Temperature Range of 223−298 K

Abstract

X-ray scattering measurements on water confined in the cylindrical pores of MCM-41 with different pore sizes C10 (diameter = 21 A) and C14 (28 A) have been performed over a temperature range of 223−298 K. Both samples were sealed in glass capillaries at relative water vapor pressures p/p0 = 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 A, with non-hydrogen-bonded H2O−H2O interactions at ∼3.3 A and H2O−Si interactions at ∼3.8 A between water and the silica wall. With decreasing temperature, the number of the hydrogen-bonded H2O−H2O interactions at 2.8 A increases, accompanied by the shifts of the 2.8 and 4.9 A peaks to shorter distances and of the 4.2 A peak to larger distances, for the capillary-condensed samples, showing a tendency to form more tetrahedral-like hydrogen-bonded w...