Volume and entropy changes of water in electrolyte solutions below 0°C

Abstract

The effective solute pressure, P_e, has been calculated for aqueous solutions of electrolytes at temperatures below 0°C. For the fully dissociated electrolytes, NaCl and MgCl₂, P_e corresponded (within the uncertainties of the data) to the pressure, P_θ, at which the freezing point, t_θ, of pure water is the same as that of the water in the solution.

For MgSO₄ solutions there is an effective shift in temperature (to below t_θ) due to perturbation of the water structure at constant volume. The entropy change, ΔS_v, associated with these solute–water interactions is close to that expected for a change in the thermal relaxation of pure water for such a temperature shift. Also, ΔS_v is close in value to the constant-volume entropy change for water associated with the conversion of outer-sphere to inner-sphere (contact) ion pairs. These results are consistent with the conclusion that the contact ion pairs interact with water molecules within the water cavities.

It is shown also that for NaCl–MgCl₂ mixed aqueous solutions the partial effective pressures of the solutes are additive and agree with the P_θ for the mixed solution.

Overall the results show that the Chemical Thermodynamic (CT) model, using the extended Tait equation of state, gives reliable estimates of the volumes of the solution components (whether in binary or mixed solutions). In turn, such data enable estimates to be made of the many properties proportional to or derived from the density.