A Brief Review of Water Anomalies

Numerous anomalies of water have been reported in the literature. Anomalous behaviour of liquid water is so striking when it is supercooled below the melting temperature of ice, Tm. Several physical properties have been found to be diverging in the supercooled liquid phase, including isobaric heat capacity, isothermal compressibility, relaxation time and thermal expansion coefficient. Interestingly hydrogen bond life times show a divergence at this temperature, indicating its connection to these singularities. Liquid water exhibits both density maximum and minimum, the latter has been discovered by a recent Small Angle Neutron Scattering (SANS) experiments, considered to be two of its most notable thermodynamic anomalies. Unlike other liquids, translational and diffusive motions in water exhibit contrasting behaviour and product of these two diffusive constants is found to be insensitive to temperature and density. Formation of water clusters of varying sizes dictates the nature of diffusion in supercooled water. Several propositions have been made in order to account for water’s anomalies, which include Liquid−Liquid Critical Point theory, Singularity Free hypothesis, Critical Point−Free hypothesis and Stability Limit conjecture. In bulk phase, water shows its most of the anomalies. In addition, it exhibits several other anomalous characters when confined to nanoscale geometries and is near to macromolecular surface. It has to be noted that in the vicinity of non−polar solutes the strength and lifetimes of water network increases.

Keywords: Boltzmann constant, Critical point, Entropy, Hydrophobic hydration, Isochore, Isotherm, Life times, Short range, Singularity−free, Spinodal, TMD, Well depth, Widom line.