A Two-State Picture of Water and the Funnel of Life

Abstract

Here I show that experimental and simulation data on liquid water using vibrational (infrared and Raman) and X-ray (absorption and emission) spectroscopies, as well as recent data from X-ray scattering, are fully consistent with a two-state picture of water. At ambient conditions there are fluctuations between a dominating high-density liquid (HDL) and a low-density form (LDL). These are related to the two forms of amorphous ice at very low temperature, high-density amorphous (HDA) and low-density amorphous (LDA), which interconvert in a first-order-like transition. This transition line is assumed to continue into the so-called No-mans land as a liquid-liquid transition and terminate in a critical point with very large fluctuations between the two liquid forms. These fluctuations extend in a funnel-like region up to ambient temperatures and pressures and give water its unusual properties which are fundamental to life. With this picture we find simple, intuitive explanations of the anomalous properties of water, such as the density maximum at 4 C, why ice floats, and why the compressibility and heat capacity grow as the liquid is cooled. We summarize by noting that in this picture, water is not a complicated liquid, but two normal liquids with a complicated relationship.