Nuclear size and shape effect in chemical isotope effect of gadolinium using dicyclohexano-18-crown-6

Gadolinium isotopes were fractionated in a liquid-liquid extraction system using dicyclohexano-18-crown-6. The isotope enrichment factors showed break downs of the conventional Bigeleisen-Mayer approximation for every conditio n of the initial phases; the concentrations of gadolinium and hydrochloric acid and the type of organic solvents. The nuclear mass effect and the fiel d shift effect were estimated by use of the isotope pairs Gd-152-Gd-160 and Gd-154-Gd-160. Since both isotopes,Gd-152 and Gd-154, have the unique char acteristics of the nuclear charge radii, < r(2)>'s, the unusual isotope eff ects due to the field shifts took place, while the mass effects of the othe r isotopes canceled the field shift effects. The maximum isotope effect was observed to be epsilon(160,152) = 0.0252 +/- 0.0024 whose initial aqueous phase was 0.023 M GdCl3 in 12 M HCl, and its organic phase was 0.2 M DC18C6 in chloroform: this was 0.00315 +/- 0.00030 in terms of the enrichment fac tor for unit mass.