Random encounters dominate water-water interactions at supercritical conditions

Supercritical water is widely present in Earth’s crust and has a great potential as an environmentally friendly solvent. Water also serves as the prototype for directional hydrogen bonding at ambient conditions. However, the question of whether supercritical water is still hydrogen-bonded or how water molecules interact en route to the supercritical regime is a matter of controversial discussions. We present terahertz (THz) spectra, which directly probe the intermolecular interactions of water under these extreme conditions. While we spectroscopically detect the liquid-gas phase transition just below the critical point, THz spectra of the high-temperature gas phase are indistinguishable from those of supercritical water at the same density. The accompanying ab initio simulations provide the molecular underpinnings: The water-water contacts at supercritical conditions are essentially orientationally random.