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

The stability constants, beta(1), 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 beta(1) 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 beta(1) against X-s in the region of 0.00 less than or equal to X-s less than or equal to 0.40 differs from each other, a concave curve for the Nd(III) and a convex curve far the Tm(III). The LnCl(2+) formed is present as a solvent-shared ton-parr. Since Cl- is a structure breaking ion, it was assumed that the primary salvation sphere of Ln(3+) directly contacted with Cl-. Calculation of Ln(3+)-Cl- distance using Born-type equation revealed the followings: (1) for Tm3+ with coordination number 8, the estimated distance between Tm3+ and Cl- increases linearly with X-s in 0.00 less than or equal to X-s less than or equal to 0.40. The results mean an increase of the primary salvation sphere size of Tm3+ with X-s (2) For Nd3+, the distance between Nd3+ and Cl- decreases linearly with X-s in 0.00 less than or equal to X-s < 0.13, where both coordination numbers of 9 and 8 coexist, while it increases with X-s in 0.13 < X-s less than or equal to 0.40. The results mean a decrease of the primary salvation-sphere size of Nd3+ with X-s in 0.00 less than or equal to X-s < 0.13 and an increase of that with X-s in 0.13 < X-s less than or equal to 0.40.