Wide angle X-ray scattering measurements of supercritical water using synchrotron radiation

Wide angle X-ray scattering measurements of sub- and supercritical water have been carried out using synchrotron radiation at SPring-8 in Japan, to study density dependence of the local structure. X-ray diffraction spectra were measured from 300 K to 1023 K at 40 MPa, and up to 1100 K at 100 MPa, corresponding to the density ρ from 1 g cm⁻³ to 0.10 g cm⁻³. The nearest neighbour distance and the nearest neibour coordination number depending on the density indcate that water is uniformly expanded from 1 g cm⁻³ to 0.6 g cm⁻³ with destruction of hydrogen bonding. With further volume expansion, the coordination number becomes approximately one at 0.15 g cm⁻³ while the nearest neighbour distance is approximately 3.3 Å. This experimental fact may be a first direct observation of dimeric molecules in the supercritical water.