Surface structure of water and ice : II. A revised model

Abstract

The author's 1962 treatment of the surface structure of water and ice is reconsidered in the light of new theoretical information on the quadrupole moment of the water molecule and on the structure of liquid water. Electrostatic effects are explicitly taken into account and it is concluded that the principal mechanism driving molecular orientation at a liquid water surface is the interaction between molecular dipole and quadrupole moments in the partially oriented region near the surface. The preferred molecular orientation is that with protons directed outwards from the liquid and the dipole layer so formed, modified by the presence of an equilibrium ionic atmosphere, leads to a calculated surface potential jump of about 0·1 v, the surface being positive with respect to the bulk liquid. In the case of an ice surface, the free energy available from surface polarization is sufficient to drive a surface phase change at temperatures a few degrees below the melting point. It is concluded that, at temperatures above about -(5 ± 3)°C the surface of ice is covered by a quasi-liquid layer whose thickness is of order 10 to 40 Å, increasing as the temperature approaches 0°C. The equilibrium ionic concentration in this layer accounts qualitatively for the large surface electrical conductivity of ice.