Thermodynamics of Alkali Ion Exchange from Methanolic Solutions on Hydrous Stannic Oxide: Excess Thermodynamic Functions of Mixing

Abstract

The thermodynamics of Li/Na and Li/Cs exchange on hydrous stannic oxide (α-stannic acid) was studied in aqueous and methanolic solutions. With respect to ΔG°, the results in the aqueous medium are qualitatively similar to those for a β-stannic acid sample. For ΔH° and ΔS°, contrary results are obtained, possible reasons for which are discussed. The addition of methanol generally leads to a decrease in ΔG° due to the changed entropy term. When comparison could be made (consistent standard states), the data of transfer thermodynamics showed that the effect of this addition on ion–solvent interactions is the major effect. However, the excess thermodynamic functions of mixing, calculated at different loadings, showed that upon the addition of methanol, the excess enthalpy and entropy of mixing decrease considerably at relatively low loadings and increase at high loadings. As these functions express the departure of the exchange reaction from ideality due to interactions (ion-fixed site and ion–solvent) in the solid phase, it was concluded that alcohol addition affects these interactions. This was not revealed in the transfer data where the whole-exchanger composition range was considered.