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

Abstract

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 (X _s) increases. However, the variation mode of α₁ againstX _s in the region of 0.00≤X _s≤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 withX _s in 0.00≤X _s≤0.40. The results mean an increase of the primary solvation sphere size of Tm³⁺ withX _s. (2) For Nd³⁺, the distance between Nd³⁺ and Cl⁻ decreases linearly withX _s in 0.00≤X _s<0.13, where both coordination numbers of 9 and 8 coexist, while it increases withX _s in 0.13<X _s≤0.40. The results mean a decrease of the primary solvation-sphere size of Nd³⁺ withX _s in 0.00≤X _s<0.13 and an increase of that withX _s in 0.13<X _s≤0.40.