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

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 (Xs) 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 (Xs) in the mixed solvent. TheXs value of the inflection point of the CN for Sm(III) is slightly higher thanXs=0.03 for Eu(III) previously determined by us.