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ION–ION INTERACTIONS 293

thus to get the contribution to the potential arising from interionic forces, and, finally,
by evolving a charging process to get the chemical-potential change due to ion–ion
interactions, they were able to link the chemical-potential change caused by interionic
forces to the experimentally measurable activity coefficient. Without these essential
contributions of Debye and Hückel, a viable theory of ionic solutions would not have
emerged.

Further Reading

Seminal
1. G. Gouy, “About the Electric Charge on the Surface of an Electrolyte,” J. Phys. 9: 457
(1910).
2. S. R. Milner, “The Virial of a Mixture of Ions,” Phil. Mag. 6: 551 (1912).
3. P. Debye and E. Hückel, “The Interionic Attraction Theory of Deviations from Ideal
Behavior in Solution,” Z. Phys. 24: 185 (1923).

Review
1. K. S. Pitzer, “Activity Coefficients in Electrolyte Solutions,” in Activity Coefficients in
Electrolyte Systems, K. S. Pitzer, ed., 2nd ed., CRC Press, Boca Raton, FL (1991).

Papers
1. C. F. Baes, Jr., E. J. Reardon, and B. A. Bloyer, J. Phys. Chem. 97: 12343 (1993).
2. D. Dolar and M. Bester, J. Phys. Chem. 99: 4763 (1995).
3. G. M. Kontogeorgis, A. Saraiva, A. Fredenslund, and D. P. Tassios, Ind. Eng. Chem. Res.
34:1823 (1995).
4. A. H. Meniai and D. M. T. Newsham, Chem. Eng. Res. Des. 73: 842 (1995).
5. M. K. Khoshkbarchi and J. H. Vera, AlChE J. 42: 249 (1996).

3.6. ION–SOLVENT INTERACTIONS AND THE ACTIVITY COEFFICIENT

3.6.1. Effect of Water Bound to Ions on the Theory of Deviations from
Ideality

The theory of behavior in ionic solutions arising from ion–ion interactions has
been seen (Section 3.5) to give rise to expressions in which as the ionic concentration
increases, the activity coefficient decreases. In spite of the excellent numerical agree-
ment between the predictions of the interionic attraction theory and experimental
values of activity coefficients at sufficiently low concentrations (e.g.,
there is a most sharp disagreement at concentrations above about 1 N, when the activity
coefficient begins to increase back toward the values it had in limitingly dilute
solutions. In fact, at sufficiently high concentrations (one might have argued, when
the ionic interactions are greatest), the activity coefficient, instead of continuing to