ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY 201 Elsevier Sequoia S.A., Lausanne Printed in The Netherlands

Comment on the paper by Damaskin and Frumkin*

The BockribDevanathan-Muller (BDM) theory of the potential dependence of (undissociated) organic molecules on surfaces on which there are negligible H and O is best explained briefly by saying that the model is Butler's competition between water and organic model with one important difference. Butler assumed that the water molecules sat in one direction. He got reasonable results by using a continuous dielec- tric model for the dielectric constant in the double layer : hence his 1~X2 (e = polaris- ability and X = field strength) term is much higher than that which is reasonable if one takes real values of ct. Such values suggest this term may play a small part in influencing adsorption. However, it is the quadratic term in X which, in Butler's theory, reproduced the parabolic shape of coverage versus potential curves often experimentally obtained. What Bockris, Devanathan and Muller did, therefore, was to suggest that the water dipoles turned round at a point which bore some relation to the potential of zero charge. The symmetry of the O-V curves becomes thus inter- preted. As a first approximation, and of course for simplicity, they chose to illustrate the model by treating a situation in which the organic's dipole had an interaction with the electrode negligible in comparison with that of water.

Bockris, Gileadi and Muller (BGM) then, in a subsequent publication, showed the effect of taking into account the electrical interaction of the molecule with the electrode. Depending upon the detailed parameters of the case studied, either water, or the organic, will dominate the situation, as, qualitatively, with Butler's model.

Thus, in stating (in the above paper) that organic interactions with the electrode are more important than those of water, there is disagreement with the first approxi- mation of the BDM model; necessarily so, for part of its approximation neglected such interactions. The conclusion by Damaskin and Frumkin that in certain cases organic electrode interactions are involved is consistent with the second approxi- mation of the (Butler)-BDM model, published by BGM.

It is clear that BGM is only a second approximation, and neglects factors (e.9., ½ eX 2 terms) which perhaps are not always negligible. It is a matter of the orthogonal component of the water dipole. Further, the BGM (and BDM models) assume that the water dipole is either "up" or "down". Clearly, a better approximation would be to take into account all possible positions. I welcome, therefore, the Damaskin and Frumkin contribution. It is part of the evidence from which the model for water in the double layer will continue to be built up.

Electrochemistry Laboratory, John Harrison Laboratory of Chemistry, University of Pennsylvania, Philadelphia, Pa. 19104 (U.S.A.)

J. O'M. BOCKRIS

Received 23rd July 1971

* J. Electroanal. Chem., 34 (1972) 191.

J. Electroanal. Chem., 34 (1972)