Prediction of Thermodynamic Behaviours of Aqueous Electrolytes at all Temperatures

Central Electrochemical Research Institute, Karaikudi-623 006 Based upon structurally modified continuum theory, two simple approaches have been described for theoretical prediction of the free energy, entropy and heat capacity of hydration of ions and electrolytes at all temperatures. The treatments are con­sistent with the assumptions that the effective size of an ion in water does not change with temperature and that the Born relationship and its various derivations accurately describe the electrical part of the hydration thermodynamic functions. In the first approach, the ionic radii of an electrolyte are fixed by the experimental hydration energies at 25°. Consequently we are able to calculate the entropy of hydra­tion over a wide interval of temperature with only one arbitrary structural entropy parameter. This structural parameter can then be used with the continuum relation­ships to predict the change of the free energy of hydration with temperature to a high degree of accuracy. Even the prediction of the heat capacity of hydration, which is given directly from the second derivative of the Born equation, is accurate for electro­lytes at higher temperatures. In the second approach, even the single arbitrary parameter used in the first approach has been done away with Here one sees the solution process of a gaseous solute ion as the combination of the two steps : (i) the formation of a cavity of a suitable size in the solvent to accommodate the solute ion and (ii) the introduction into the cavity of the solute ion which interacts with the solvent. In this approach, the thermodynamic functions associated with the cavity formation have been calculated using the scaled particle theory and it has been assumed that the specific interaction free energy occurring in the near vicinity of the ion in solution, which the traditional Born equation can not recognise, is sufficiently strong and relatively temperature insensitive. As in the earlier approach, the thermodynamic prediction through this simple approach is highly accurate for free energy, entropy as well as heat capacity at a wide range of temperature