On the size variation of the primary solvation sphere of Nd3+ and Tm3+ in mixed system of methanol and water

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The stability constants, α1, of each monochloride complex of Nd(III) and Tm(III) have been determined in the mixed system of methanol and water with 1.0 mol·dm−1 ionic strength using a solvent extraction technique. The values of α1 of Nd(III) and Tm(III) increase as the mole fraction of methanol in the mixed solvent system (Xs) increases. However, the variation mode of α1 againstXs in the region of 0.00≤Xs≤0.40 differs from each other, a concave curve for the Nd(III) and a convex curve for the Tm(III). The LnCl2+ formed is present as a solvent-shared ion-pair. Since Cl− is a structure breaking ion, it was assumed that the primary solvation sphere of Ln3+ directly contacted with Cl−. Calculation of Ln3+−Cl− distance using Bom-type equation revealed the followings: (1) for Tm3+ with coordination number 8, the estimated distance between Tm3+ and Cl− increases linearly withXs in 0.00≤Xs≤0.40. The results mean an increase of the primary solvation sphere size of Tm3+ withXs. (2) For Nd3+, the distance between Nd3+ and Cl− decreases linearly withXs in 0.00≤Xs<0.13, where both coordination numbers of 9 and 8 coexist, while it increases withXs in 0.13<Xs≤0.40. The results mean a decrease of the primary solvation-sphere size of Nd3+ withXs in 0.00≤Xs<0.13 and an increase of that withXs in 0.13<Xs≤0.40.