Electrical potential due to the ionic atmosphere

We have agreed (expression [4.1]) that the interaction energy could be considered to be the sum of an energy due to the electrical charges and an energy due to the molecular interactions in proximity. We now suppose that, if the molecular interactions were the only forces at play, the solution would be accurately represented by the model of an ideal dilute solution (see section 3.1.3) and that the deviation from this model is due merely to the long-distance interactions caused by the electrical 

Because our solution is dilute, the energy is, in fact, due only to 

The Helmholtz energy of the real solution with the charged ions would exceed that of the hj jothetical solution, wherein the ions are not charged with an amount such that  

In addition, the interaction energy due to the electrical charges can, itself, be broken down into two terms  

We can therefore write the excess Helmholtz energy due only to the electrical interactions in the form  

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In calculating the activity coefficient of the species a, we shall need only the electric potential due to the ionic atmosphere, which we denoted by y/ iR/a). First we need to solve for the total potential function i//(R/of). To evaluate if/(R/a), we use the Poisson equation from electrostatics, which reads... 

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