An inflection point in the solvation number around Sm³⁺ in the mixed system of methanol and water

Abstract

The stability constants (β₁) of the monofluoro complex of Sm(III) have been determined in mixed solvents of methanol and water at a 0.10 mol·dm⁻³ ionic strength using the solvent extraction technique. The values of lnβ₁ increase as the mole fraction of methanol (X _s) in the mixed solvent system increases. The variation in the stability constants can be correlated with both the large effect due to the solvation of F⁻ and the small effect due to both (1) the solvation of cations in connection with the complexation of SmF²⁺ and (2) the electrostatic attraction of Sm³⁺−F⁻. Based on the variation in the sum of (1) and (2) in H₂O and the mixed solvent solutions, it was determined that the coordination number (CN) of Sm(III) varied from a mixture of CN=9 and 8 to CN=8 for about a 0.06 mole fraction of methanol (X _s) in the mixed solvent. TheX _s value of the inflection point of the CN for Sm(III) is slightly higher thanX _s=0.03 for Eu(III) previously determined by us.