Extractive Separation of Lithium Isotopes by Crown Ethers

Separative abilities of crown ethers to lithium isotopes were investigated for the number of oxygen atoms composing crown rings and for the substituted groups to 15-crown-5. The separation factors at 0°C were 1.057 for 12-crown-4, 1.042 for benzo-15-crown-5, 1.041 for lauryloxymethyl-15-crown-5, 1.043 for tolyloxymethyl-15-crown-5, and 1.024 for dicyclohexano-18-crown-6. The enthalpy change of the isotopic equilibrium in the absolute value was the greatest for 12-crown-4; ΔH° = −0.78 kJ/mol. In the substituted 15-crown-5s the separation factor was greatest for tolyloxymethyl-15-crown-5, and ΔH° values decreased in the order: benzo- > tolyloxymethyl- > lauryloxymethyl-15-crown-5. For enthalpy changes, benzo-15-crown-5 has the possibility of giving a larger separation factor than the present one, α = 1.042, by choosing more suitable conditions. In spite of its large distribution coefficient, dicyclohexano-18-crown-6 is not superior for isotopic separation of lithium in regard to the small enthalpy change of isotopic equilibrium; ΔH° = −0.15 kJ/mol. The addition of DMSO to the initial solution of LiI caused a remarkable increase in the distribution coefficient without changing the isotopic separation factors.