The dipole moment of an adsorbed ion

In general a polar bond is formed when an ion is specifically adsorbed on a metal electrode this results in an uneven distribution of charges between the adsorbate and the metal and hence in the formation of a surface dipole moment. So the adsorption of an ion gives rise to a dipole potential drop across the interface in addition to that which exists at the bare metal surface. 

The same effect exists for adsorption on a metal surface from the gas phase. In this case the adsorbate-induced dipole potential changes the work function by an amount A f . If nad is the number of adsorbed molecules per unit area, the component mx of the dipole moment of single adsorbed molecule can be inferred from the relation  

As before, the x direction has been taken normal to the metal surface. In electrochemistry, the dipole moment mx associated with an adsorbate bond can be defined in the following way For simplicity suppose that the electrode has unit area. At the beginning the electrode surface is bare and kept at the pzc. Then a number nad of ions with charge number z are adsorbed simultaneously a counter charge —zeon is allowed to flow onto the metal surface. The change A(j in the electrode potential is related to the dipole moment through  

Note that both before and after the experiment the sum of the charges on the metal surface and in the adsorbate layer is zero, and hence there is no excess charge in the diffuse part of the double layer. However, after the adsorption has occurred, the electrode surface is no longer at the pzc, since it has taken up charge in the process. 

In the gas phase the dipole moment determined through Eq. (4.10) refers to an individual adsorbed particle. This is not so in the electrochemical situation. The dipole moment of an adsorbed species will tend to align neighboring solvent molecules in the opposite direction, thereby reducing the total dipole potential drop (see Fig. 4.3). Only the total change in dipole potential can be measured, and there is no way of dividing this into separate contributions from the adsorbate bond and the reorientation of the solvent. The apparent dipole potential of an ion adsorbed from a solution on a particular metal is often substantially smaller than that of the same ion adsorbed in the vacuum (see Table 4.1), since it contains a contribution from the solvent. For comparison we note that the dipole moments of alkali ions adsorbed from the vacuum are usually of the order of the order of 10 29 C m. 

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