Ice formation on metal surfaces plays a fundamental role in fields as diverse as the atmospheric sciences, geology, and biology. A prerequisite to understanding these wide and varied phenomena is establishing how the water molecules are arranged at the water-metal interface.
Density functional theory (DFT) represents the state-of-the-art theoretical methodology to tackle efficiently this problem. In DFT, the complex motion of many electrons is represented in a simple way using an effective, but approximate, ‘exchange-correlation functional'. However, if adsorption energies with the most commonly used exchange-correlation functionals are to be believed, none of the low temperature experimentally characterized ice-like wetting layers are thermodynamically stable. The typical density functionals used to date do not account properly for van der Waals (vdW) dispersion forces; these are relatively weak forces that act between atoms that are not chemically bonded to one another, but which nonetheless play an important role in the packing of many types of molecule. It is therefore both timely and important to know what role vdW forces play in water adsorption on metals.
In a recent paper, Carrasco et al. (Phys. Rev. Lett. 106, 026101 (2011)) show that when vdW interactions are accounted for, this discrepancy between experiment and theory can be reconciled. The resolution of this long-standing anomaly is demonstrated on one of the most well-characterized wetting layer structures (water on Cu(110)) and on the most widely investigated (water on Ru(0001)).
Figure: Standard density functionals predict that water-ice should not wet metals, but rather form three dimensional non-wetting ice crystals. Accounting for dispersion forces rectifies this problem and produces a result in agreement with experiment.
Journal Link: http://prl.aps.org/abstract/PRL/v106/i2/e026101