Enrichment of zinc isotopes by a liquid membrane system using a crown ether

A liquid membrane system to enrich zinc isotope was investigated. The liquid membrane was composed of a solution of dicyclohexano-18-crown-6 in chloroform. Zinc chloride dissolved in diluted hydrochloric acid was used as a feed phase. A receiving phase was made of a 0.001 M HCl solution. The zinc isotopes were enriched (1) at the interface of the liquid-membrane/feed-phase and (2) at that of the liquid-membrane/receiving-phase. The separation factor attained in the system was alpha = 1.049 for the separation of Zn-64 and Zn-66. The enrichment factor of Zn-66 relative to Zn-67 was epsilon = -0.053, which is in the inverse direction of enrichment by a liquid-liquid extraction. The enrichment factor of Zn-64 to Zn-66 showed a different value from that of Zn-66 to Zn-68, regardless of their similar mass difference. The isotope effects in the Zn-crown complex were dominated by the field shift of electronic energy in the isotope atom, This is in contrast to the Bigeleisen-Mayer approximation for a chemical exchange reaction.