X-ray diffraction study of water confined in activated carbon pores over a temperature range of 228–298 K

The structure and properties of water confined in activated carbon (AC) pores of an average diameter 20 Å have been investigated over a temperature range between 298 K and 228 K by adsorption isotherms, differential scanning calorimetry (DSC), and large-angle X-ray diffraction. The adsorption isotherm at 298 K has shown the V-type one, in which the adsorption of water takes place at a relative pressure (p/p0) ∼ 0.4, increasing rapidly with an increase in the relative pressure to a saturated value at p/p0 = 1. The DSC data of water in AC at p/p0 = 1.0 have shown three steps of vitrification of water, suggesting different pore sizes of AC. The X-ray diffraction data on confined water at p/p0 = 1.0 have revealed that the ice-like tetrahedral network of water is slightly perturbed from bulk water structure, but not to such an extent as found for water confined in hydrophilic pores of MCM-41 previously reported [P. Smirnov, T. Yamaguchi, S. Kittaka, S. Takahara, Y. Kuroda, J. Phys. Chem. B 104 (2000) 5498]. With decreasing temperature, the hydrogen bonded network of water was enhanced, and at 243 K hexagonal ice Ih was partially formed in the AC pores, in contrast with cubic ice Ic formed in hydrophilic pores (diameter 100 Å) of silica. The microscopic structure of supercooled water confined in hydrophobic AC pores is compared with those in hydrophilic MCM-41 pores.