The local structure of sub- and supercritical water as studied by FTIR spectroscopy and molecular dynamics simulations

Abstract

The purpose of this paper is to probe by infrared absorption spectroscopy and molecular dynamics simulations the evolution of the local order of sub- and supercritical water along the isobar 22MPa and in the temperature range between 293K and 693K. Infrared absorption measurements were performed along this isobar using an isotopic water mixture of D2O and H2O with the molar ratio of 1:20 leading to a mixture of D2O, HOD and H2O with molar proportions of 1:40:400, respectively. The evolution of the shape of the infrared profiles associated with the O-D stretching mode of HOD and the combinations ν3+ν2 and ν3+ν1 of H2O reveal a progressive weakening upon the temperature increase of the hydrogen bond network existing in liquid water at room temperature. In order to get a molecular description of such experimental results, Molecular Dynamics simulations of water were performed using a cubic simulation box consisting of 864 water molecules modeled with the TIP4P potential in the (N,P,T) ensemble. We have characterized the local structure using the nearest neighbour radial and orientation distributions, the Voronoi polyhedral and the tetrahedral distributions of water molecules. Our results show that with increasing the temperature, the average distance between two water molecules increases, the local density around a water molecule decreases particularly when the temperature is close to the critical one and finally the tetrahedral distributions shift to lower values indicating the distortion of the tetrahedral distribution. These results correlate with the weakening of the hydrogen bond interaction between two water molecules and then to the shift of the OH vibration frequency to lower values in accordance with the infrared results.