Some Observations on the Structure of Water at an Ideal Metal-Solution Interface*

The problem of describing the solvent structure at the solid-solution interface has been the subject of a large number of publications iri recent years13• The majoriity of these models have been based on a simple dipole structure for the solvent and have used classical statistical techniques to calculate the pertinent parameters in the double layer, e. g. inner layer capacity and surface dipole potential. In effect, all are based on the original ideas of Watts-Tobin4 and the differences in these various approaches are in the treatments of the solvent-solvent interaction terms. Some of the weaknesses in this work have been recently pointed out by Cooper and Harrison5• There are, however, a number of other fundamental weaknesses that should be noted; a) It is not possible with these models to introduce, in anything but an arbitrary way, the specific interactions of the solvent molecules with the substrate although there is now a large body of evidence to show the importance and specificity of such interactions.6 b) The orientation of the solvent molecules in these models is chosen at the outset, that is to say the orientational distribution function is not a variable. 'l'l1e only attempt so far to avoid this restriction gave results which agreed poorly with experiment7. c) It has not yet been possible experimentally to show if the interactions of the primary solvent layer with the second solvent layer gives a significant contribution to the observed parameters although in colloid systems such interactions have often been suggested to explain phenomena. These interactions would be difficult to include in current models which already have to fit a large number of parameters to experimental data. These data, unlike spectroscopic data, do not have a high resolution response to the molecular parameters. In this communication a tentative step in a new direction is proposed. One factor these models ignore is the rapidity of recent advances in the theory of the condensed state. If, for example, water is the typical solvent of interest, the recent publications of Stillinger and Rahmans and Frank9