Field and structure behaviour of electrolytes

Abstract

Measurements of the thermodynamic properties of electrolytes in hot water have a widespread practical and theoretical interest. The chemistry of many solutes changes rapidly as a solution is heated but the reasons for such changes have not been understood in terms of electrolyte theory. The temperature behaviour of the equilibrium thermodynamic properties is best seen by examining the ionic heat capacities of representative electrolytes, and we report here measurements just completed of C̄°_p2 of NaCl(aq) and BaCl₂ (aq) up to 300 °C. The values are so negative that we now believe that the properties of electrolytes at higher temperatures are determined primarily by the electrostatic field of the ions and the bulk dielectric constant of water, and not by structural arrangements of ions and water molecules which characterize solutions at lower temperatures. These new phenomena are summarized in Fig. 1, which not only illustrates the large values at 300 °C for C̄°_p2 for NaCl (aq) and BaCl₂ (aq) of −339 and −1,011 cal mol⁻¹ deg⁻¹, but also indicates a continued rapid change above 300 °C. The observations also suggest a need for reexamination of the theory of aqueous electrolytes at 25 °C, a temperature in the transition region between the two types of behaviour.