Raman spectroscopic study on the local structure around O2 in supercritical water

Vibrational Raman spectra of O2 corresponding to the Q-branch of fundamental and hot bands were measured in supercritical water at 380–500 °C and 5.0–39.2 MPa where the corresponding density was ranged in 0.0009–0.031 mol cm−3 so as to investigate the local water structure around O2. The band shapes were asymmetric and looked like the gas phase spectrum at high density that consisted of rotational lines broadened by pressure. They were reasonably reproduced by the band shape calculation using the so called G-matrix, which considers the overlapping effect among the rotational lines. This implies that the rotational relaxation of O2 in SCW is not largely different from the gas phase behavior. In the O2 (10 mol%)–H2O mixture at 380–500 °C, the Raman vibration–rotation shifts decreased linearly dependent on the total density of mixture up to 0.014 mol cm−3 (ρr<0.7), indicating the absence of large clustering or depletion of water around O2 under the experimental conditions. In the O2 (1 mol%)–H2O mixture at 390 °C (Tr=1.02), the Raman shifts also decreased linearly to the total density up to ca. 0.01 mol cm−3. However, in the range from 0.01 to ca. 0.022 mol cm−3 (0.5<ρr<1.2), the decreasing slope of the shifts gradually became smaller and then increased again at the denser region. This anomalous behavior implies the depletion of the local water density around O2 in the neighborhood of the critical point.