Thermodynamic properties of ionic fluids over wide ranges of temperature

The principle of corresponding states is valid in reasonable approximation for ionic fluids but the quantitative behavior differs considerably from that for neutral—molecule fluids. The role of a dielectric is such as to make the cT product the effective temperature. Thus aqueous electrolytes near room temperature have very high reduced temperatures on a corresponding states basis. The status of knowledge for various aqueous solutes is summarized. The critical region for an ionic system is difficult to treat theoretically and is out—of—range experimentally for simple substances such as NaC1. Current best approxi— nations are discussed. A model, two—component ionic system with an experimentally convenient critical temperature has been found and its properties investigated. The role of long—range fluctuations on properties in the critical region is considered for ionic systems. Fused salts (without solvent) have low reduced temperatures and they follow corresponding states quite well as has been shown by Reiss and others. Fused salts have a remarkably large coefficient of expansion; its cause is discussed. At low reduced temperature an ionic—particle vapor is largely associated to neutral clusters —— pairs, quartets, etc. An interesting system of considerable engineering and geological importance is NaC1 in steam which has been treated in terms of successive hydration equilibria for the ion—pair.