Absolute rate theory says that the rate of a reaction is proportional to the change of free energy between the reactants A and B and the activated complex AB^‡. By comparison with the simple rate theory, when the reactant molecules react independently then the free energy changes as the logarithm of the concentration. This is due to the entropy term in the free energy.

The Brønsted-Bjerrum observations of the primary salt effect are explained by the Debye-Hückel theory of the thermodynamics of solvated ions. The presence of other ions in the solution stabilises accumulations of charge, i.e. lowers their free energy. The reaction speeds up at high ionic strength if A and B have the same sign of charge, so that the activated complex has a higher charge than each reactant separately.

According to the Debye-Hückel theory, the electrostatic interaction between solvated ions means that each positive ion tends to be surrounded by a cluster with a net excess of negative ions, and vice versa.

Summary. An electrolyte is a solution of charged particles. The tendency of positive charges to be found near negative particles means that the free energy of each ion increases with concentration more slowly than the logarithmic variation expected for non-interacting solutes.

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