The Debye-Huckel theory of Activity coefficients:

The Debye-Huckel theory of Activity coefficients:
The Debey-Huckel theory also attempts to account for the activity coefficient of electrolytes in terms of electrostatic attractions operating between ions in a solution.
The theory starts by assuming that electrically charged particles in solution, like other charges, are subject to the coulomb law of force:
The forces of attraction or repulsion between charges vary directly with the product of the charges, and inversely as the square of the distance r between them.

D: the proportionality constant, the dielectric constant; is determined by the medium in which the charges are immersed (solvent).
As a result of these forces the distribution of ions throughout a solution is not random but is such that any central positive ions is surrounded on an average by an atmosphere of other ions whose net charge is negative, while any central negative ion is surrounded by an atmosphere of net positive charge.
Owing to the presence of an atmosphere about an ion there is a potential ?i established at its surface whose magnitude Debye-Huckel showed to be: