Surface structure of water and ice

Abstract

The state of molecular orientation in a pure water surface is examined and it is concluded that the surface molecules are almost completely oriented with their negative vertices outward. Because of inter-molecular bonding this orientation decays approximately exponentially beneath the surface, the depth of orientation being about ten molecular layers. The driving energy for this orientation is supplied by the electrical asymmetry of the water molecule. When the surface of ice is examined it is found that the free energy of the system can be lowered if the ice surface is covered by a thin liquid layer. Molecules in the liquid surface are oriented, and the orientation decays in the depth of the layer. Thermodynamic calculations based on a molecular model suggest that the equilibrium film thickness is finite above a temperature of about −30°C and that the film thickness increases greatly as the melting point is approached. Typical film thicknesses are of the order of tens of Angstroms.