A first modification to the simple Debye-Hiickel model

3 A first modification to the simple Debye-Hiickel model 

This is where the Gurney co-sphere concept makes its first appearance in the theory. An extra term is added to the potential energy, and this discusses what happens between r = a and an upper limit. This upper limit is r,- + rj + d, where d is twice the thickness of a co-sphere, taken by some workers to be equal to the diameter of a molecule of solvent, here H20(l), and r, and / are the crystallographic radii of the ions. 

This then modifies the equation for the potential energy, giving for the potential energy of 

The constant in the Gurney term can be positive or negative, and corresponds to the following  

3) could also give an attraction, and this can happen whether the charges are like or opposite. 

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