Helmholz plane

A simple model of the e.d.l. was first suggested by Helmholz in which the charges at the interface were regarded as the two plates constituting a parallel plate capacitor, e.g. a plate of metal with excess electrons (the inner Helmholz plane I.H.P.) and a plate of excess positively charged ions (the outer Helmholz plane O.H.P.) in the solution adjacent to the metal the... 

The interface region from the electrode to the Helmholz plane (the locus of the centres of the ions in contact with the electrode) across which the potential varies linearly. 

The region from the Helmholz plane into the solution, across which the potential varies exponentially attaining a value of zero at some distance in this region the ions are subjected to both ordering electrical forces and disordering thermal forces. 

Fig. 5.3 Structure of interface at equilibrium. IHP is the inner Helmholz plane through the contact adsorbed, nonsolvated anions and OHP is the outer Helmholz plane...

The inner layer (closest to the electrode), known as the inner Helmholz plane (IHP), contains solvent molecules and specifically adsorbed ions (such as Br or T that are not hydrated in aqueous solutions). It is defined by the locus of points for the specifically adsorbed ions. The next layer, the outer Helmholz plane (OHP), reflects the imaginary plane passing through the... 

Fig.n (a) Effect of distance from electrode on ion concentration. Excess concentration of attracted and repelled ions with respect to bulk concentration in the vicinity of an electrode surface calculated from Gouy-Chapman model [34]. Curves correspond to 0.025, 0.050, and 0.100 V potential differences, (b) Outer Helmholz plane (OHP) and inner Helmholz plane (IHP). Anions adsorbed on the surface have lost part of their solvation layer and are contact adsorbed (IHP) cations remain fully solvated and are water-separated adsorbed (OHP). (Reprinted with permission from Ref. 34, Copyright 2003 by John Wiley Sons Ltd). 

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