The cluster theory for electrolyte solutions. Its extension and its limitations

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Abstract
Cluster theory of electrolytes has been used to study the contribution to the Helmholtz energy, A, of clusters having up to four ions. The population distribution that minimizes A is dominated by neutral clusters. When solutions are sufficiently concentrated, redissociation ensues. We discuss the importance of the particular cluster definition adopted in determining the structure of the electrolyte in the entire concentration range. The comparison of the internal energy calculated by cluster theory to that obtained by simulations, shows a significant improvement over MSA, but the agreement with simulation results is only fair. The osmotic coefficients of two real aqueous electrolytes, NaCl at 573 K and MgSO4 at 373 K, have been used to assess the limitations of cluster theory when applied to real electrolyte systems. Possible extensions of cluster theory are discussed.