The shape of water - how cluster formation explains the hydrophobic effect

The hydrophobic effect is a particular characteristic of water, which in its pure form exhibits more anomalous properties than any other known liquid. A unifying molecular scale origin of these observations has been sought for decades. Here, we show that thermodynamically driven formation of water clusters in water simultaneously explains the hydrophobic effect and water's anomalous properties. Our unifying first-principles based model for water cluster formation predicts the correct thermodynamics of organic molecule solvation in water as well as surfactant self-assembly. Our model provides an intuitive explanation that water as a whole becomes more hydrophobic as temperature decreases, because higher concentrations of clusters favour internal hydrogen bonds, which leads to fewer opportunities for interactions with solutes.