Characterization of the State of Two-Dimensionally Condensed Water on Hydroxylated Chromium(III) Oxide Surface through FT-IR, Quasielastic Neutron Scattering, and Dielectric Relaxation Measurements

The dynamic properties of water molecules adsorbed on the hydroxylated chromium(III) oxide surface, whose two-dimensional critical temperature was 303 K, were investigated at around the two-dimensional critical temperature by FT-IR, quasielastic neutron scattering, and dielectric relaxation techniques. The Cr 2 O 3 sample covered with monolayer water gives an IR band having a broad nature at ∼3400 cm -1, due to the mutual hydrogen bonding among the adsorbed water molecules. The quasielastic neutron scattering data have revealed that the motion of the adsorbed water molecules changes across the two-dimensional critical temperature, ranging from the motion of the supercritical fluid to that of a solidified phase with decreasing temperature. The dielectric spectra observed at 298 K and at the coverage of 1.1 monolayers have been interpreted in terms of the presence of two relaxations due to the interfacial and orientational polarizations. The characteristic frequency of the relaxation for the latter polarization was estimated to be ca. 300 Hz. It is concluded from these findings that the two-dimensionally condensed waters adopt the mutually hydrogen-bonded solid state having a commensurate structure with the hydroxylated (001) Cr 2 O 3 surface below the two-dimensional critical temperature of this system. In addition, the interfacial polarization was explained in terms of a hopping model of protons of the adsorbed molecules. The behavior of conductivity arising from this proton hopping also indicates the morphology of the two-dimensionally condensed water below the monolayer coverage: the small clusters of water are formed on the Cr 2 O 3 surface.