Recent progress on the structure of liquid water

Abstract

Water is a ubiquitous solvent on earth. Despite numerous efforts for many years, the detailed structure of liquid water is still under debate. This is mainly due to the flexible hydrogen bonding system of water. Thus, in 2005, the structure of water was listed as one of 125 challenging questions to the scientific community in the prestigious journal Science upon the occasion of its 125 anniversary. In this mini-review, we summarized the progress on the understanding of the structure of water in the last decade or so. In comparison with the classical structure model of water which has an ice-like tetrahedron hydrogen bonding network, more disordered models was proposed based on X-ray absorption spectroscopy and X-ray Raman scattering experiments. Disputes on the explanations of the results and beyond, such as the average number of hydrogen bonds per water molecule and the chains or rings molecular arrangements in liquid water, are presented. As to the nature of hydrogen bond, the classical view takes it as short-ranged dipole-dipoleinteraction. A new view of long-ranged angular correlation function of liquid water and its possible implication to the cooperativity of hydrogen bonding network are discussed. Then the quantum nature of hydrogen atoms in water and its implication to hydrogen bonds, the femtosecond-scale formation and breaking dynamics of hydrogen bonds in water, are introduced, followed by a description on the structures of water under extreme conditions, such as high pressure or supercool temperature. The structures of water under special conditions such as on the solid surface and confined space are reviewed. Particularly, water organizations in liquid helium or under other conditions and thus the experimental evidences of water clusters are described. Finally, four suggestions are put forward for future studies on the structures of water, namely the development of ultrafast and high-resolution techniques to collect more reliable experimental results, improvement of quantum chemicaland molecular dynamics methods including development of accurate force field, construction of a full-scale model of water structure which can explain the anomalous properties of water, and investigation of the structures of water in different situations.