Luminescence study on preferential solvation of Europium(III) in water/non-aqueous solvent mixtures

The solvent composition in the first coordination sphere of Eu(III) in water/non-aqueous solvent mixtures was investigated by measuring the luminescence lifetime. The inner-sphere hydration numbers of Eu(III) calculated from the lifetimes gave reasonable values in N,N-dimethylacetamide. N,N-dimethylformamide, dimethyl sulfoxide, formamide, hexamethyl phosphoramide, or N-methylformamide with water system. Eu(IU) was preferentially solvated by the non-aqueous solvent in those systems, over the whole range of the non-aqueous solvent mole fraction in the bulk mixtures, X-s. The degree of the preferential solvation K-PS, defined as the solvent/water mole ratio in the first coordination sphere to that in the bulk solution, varied considerably with the X-s in all the systems. The order of the K-PS for Eu(III) was hexamethyl phosphoramide > dimethyl sulfoxide > N-methylformamide > N,N-dirnethylformamide > formamide > N,N-dimethylacetanlide > water > pyridine > methanol > ethanol > acetone > tetrahydrofuran > acetonitrile at X-s = 0.5. The Gibbs free energy of transfer of Eu(III) from water to non-aqueous solvent was also estimated from the K-PS. (C) 2001 Elsevier Science B.V. All rights reserved.