Breaking bonds to evaporate

Print edition : January 22, 2016

Snapshots of an evaporating water molecule. Molecules And B are depicted in bold, while the other molecules are depicted thin. The molecules with gray colour indicate the water molecules 200 fretoseconds earlier. Photo: AMERICAN PHYSICAL SOCIETY/PHYSICAL REVIEW LETTERS

At the molecular level, evaporation of water requires breaking at least one strong intermolecular bond between two water molecules at the interface. The molecular mechanism by which an evaporating water molecule gains sufficient energy to escape from the surface has remained elusive. Molecular dynamics simulations have been used at the water-air interface with polarisable classical force field models to show that the high kinetic energy of the evaporated water molecule is enabled by a well-timed making and breaking of hydrogen bonds involving at least three water molecules at the interface, the recoil of which allows one of them to escape. The evaporation of water is thus enabled by concerted, ultrafast hydrogen-bond dynamics of interfacial water, and follows a specific molecular pathway.

This work has been published in “Physical Review Letters”.

R. Ramachandran

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