Interpretation by the Solvophobic Theory on the Linear Additive Representation of the Logarithm of Ion-Pair Extraction Constant with Individual Contributions of Cation, Anion,and Organic Solvent

摘要

A theoretical basis was given for the linear additive representation of the logarithm of ion-pair solvent extraction constant (K_(ex))in terms of logarithmic values of three individual extraction constants (K_(cation)K_(anion),and K_(solvent)),for cations, anions, and orgainc solvents constituting ion-pair extraction systems. The differences in extractability between cations and between anions were quantitatively discussed according to the thermodynamic cycle model of 1:1 ion-pair extraction. The increment of log K_(cation) and log K_(anion)of hydrophobic ions is explained by taking the increment of the free energy change of the hydration (#DELTA#G_(hyd))of the ions into account. The order of the extractability of inorganic cations and anions was also interpreted uwing literature daa of #DELTA#G_(hyd) of each inorganic ion. The free energy change of the solvation of a hypothetical ion-pair having a shape of spherical was calculated on the basis of the solvophobic theory. The difference in extracting powers of some exrtracting orgaic solvents could also quantitatively be interpreted. Ther are a few sets of proposed values of the individual extraction contants representing the extractablility of the cations and anions, which were calculated based on different reference assumptions The conclusions of this study are effective for the interpretation concerning the increment of the extractability of the ions irrespective of the reference assumptions for determining the individual extraction constants. It was also attempted to estimate a bsolute values of K_(ex) by the theoretical approach in this study. The K_(ex) values of a few inorganic ion-pairs thus calculated were of the same order of magnitude as those estimated from the individual extraction constants experimentally determined. It is concluded that the thermodynamic cycle model based on the solvophobic therory pro0vides an essential framework for the theoretical interpretation of the mechanism of the ion-pair extraction, and that the validity of the linear additive representation of log K_(ex) with three individual extraction constants is demonstrated.