The structure of water; from ambient to deeply supercooled

Here we discuss the structure of water in terms of a temperature-dependent balance between two classes of hydrogen-bonded structures. At high and down to mildly supercooled temperatures most molecules favor a closer packing than tetrahedral, with strongly distorted hydrogen bonds. This allows the quantized librational modes to be excited and contribute to the entropy while the loss of enthalpy due to breaking hydrogen bonds is compensated by enhanced van der Waals interactions. Tetrahedral hydrogen bonding is of lower enthalpy resulting in tetrahedrally bonded water patches appearing, but only as fluctuations with size and life-time increasing at lower temperatures. Measurements of the structure at deeply supercooled conditions show a continuous increase in tetrahedrality which becomes accelerated below the temperature of homogeneous ice nucleation. The two local structures are connected to the liquid-liquid critical point (LLCP) hypothesis in supercooled water and correspond to high density liquid (HDL) and low density liquid (LDL). We propose that both HDL and LDL behave as normal liquids and that the anomalous properties of water result from the transition between them, which occurs over a wide temperature range at ambient pressure. The key issue is the competition between incompatible conditions for maximizing the entropy, favored in HDL, and minimizing the enthalpy, favored in LDL, which leads to the instability in the liquid and is the fundamental origin of the proposed LLCP.