Ice exhibits a phenomenon known as pre-melting which was first alluded to by Michael Faraday in his ‘regelation' experiments at the Royal Institution in the 1850's. A liquid like layer forms at the surface of ice, but there is dispute about the temperature at which this layer first occurs. Understanding the structure of the layer and its temperature dependence is important in the context of atmospheric heterogeneous catalysis, because the surface of ice particulates facilitate reactions of radicals and trace gases in the atmosphere.
In a recent paper in Nature Materials, researchers from the LCN and Department of Chemistry at UCL (Matt Watkins, Angelos Michaelides and Ben Slater) in collaboration with researchers from University of Zurich, Peking University and the Chinese Academy of Sciences have discovered unexpected properties of ice at the nanoscale that relate to Faraday's experiments. Using density functional theory calculations, they discovered each molecule is bound to the surface by a difference force, unlike most crystalline materials where each surface molecule has an identical binding energy. A fraction of the surface molecules are so weakly bound that they are easily displaced to form an overlayer, leading to less crystalline surface layers. The figure illustrates the variation in binding energy, which arises from the interaction of the water's dipole within a geometrically frustrated array of neighbouring dipole moments.
This work has been published in Nature Materials
Figure: A view of the ice surface illustrating weakly, (red), intermediate (white) and strongly bound water molecules (blue). White molecules are at the external surface, grey lie sub-surface.