Introduction

Water is a common substance. Its basic properties are widely known yet it represents a considerable challenge to the scientist who wishes to understand its behaviour on a molecular scale. The apparent familiarity with water in its bulk liquid phase creates a deceptive illusion about the simplicity of the molecular interactions which govern these microscopic properties since it is found that water is an extremely complex material. Although a considerable amount of information has been gathered over several decades of research investigation and presented in numerous individual reviews a clear picture of the detailed behaviour has not yet been unambiguously determined. In this context the interest remains as strong as ever and the development of new techniques for putting together the ‘final’ pieces of the jigsaw remains as a tantalizing challenge to a wide range of the scientific community. The series of articles comprising Water: A Comprehensive Treatise provides a clear indication of the way the subject has developed in recent years, but it is also apparent that some of the work described in the earlier volumes has now been superseded by new measurements and new ideas.

In this article, the most recent developments in neutron diffraction techniques will be described. The wide range of experiments undertaken by various research groups in many countries has been initiated in an attempt to provide a more complete picture of the spatial correlations that exist between molecules in the liquid. The overriding feature affecting the interaction between water molecules in the condensed state centres on the phenomenon of hydrogen-bonding, in which strongly orientation-dependent forces are known to influence the structural configuration. The exact nature of the time-averaged molecular correlations remains to be established.