Ever since values for the differential capacity of the electrical double layer have been available there have been speculations about the behaviour of the solvent in the interphase. Measurements using different solvents led Fmmkin I to the conclusion that the dielectric constant in the interphase did not parallel that of the bulk solvent, thus indicating that the properties of the solvent in the interphase were different from those in the bulk. This conclusion has been amply confirmed in later work 2 and at the same time theories have been developed 3-8 to account for the behaviour of the solvent especially in the monolayer adjacent to a metal electrode. Although these theories have been applied to other solvents, particular attention has always been given to water, notably to the questions of the net orientation at the point of zero charge, the 'hump' in the capacity and the temperature coefficient. Since quite detailed models of water in the interphase have now been proposed 6-8, it seemed likely that these could be tested by comparing the capacity of a mercury electrode in H20 solutions with that in D20 solutions. The replacement of D20 for H20 is a unique change of solvent because the qualitative behaviour of the solvent is unchanged. The changes will occur only in the.precise quantitative values of a few properties which change by a small amount. Thus the models of the interphase can be examined without requiring a gross qualitative modification.
Experimental
Double layer capacity measurements were made using the bridge described previously 9. The cell was re-designed so that it used a much smaller volume of working solution. Dropping electrodes were thin-walled and treated internally with dimethyldichlorosilane. They had a drop-time of ~ 7 s at the p.z.c, and the bridge was balanced 4.4 s after the birth of the drop. The reference electrodes were 0.1 M KC1 calomel electrodes in โข H2 O, the junction to the working solution being made in a tap. Potentials of zero charge were measured with a streaming electrode.