Selectivity of calix[4]arene-bis(benzocrown-6) in the complexation and transport of francium ion

It is shown for the first time that a representative member of the "cesium-selective" calix[4]arene-crown-6 family exhibits a high affinity for Fr⁺ ion. In the investigation, the transport of ²²¹Fr⁺ and Cs⁺ ions by calix[4]arene-bis(benzocrown-6) from an aqueous sodium nitrate solution into the water-immiscible diluent 1,2-dichloroethane was measured and compared to address the question of selectivity of the calix-crown-6 cavity toward alkali metal ions of increasing size. Selective separation of ²²¹Fr⁺ from its parent ²²⁵Ac and from the matrix Na⁺ ions was demonstrated. Higher distribution ratios were obtained than those for the Cs⁺ ion. The extraction equilibria were determined for the case of the Cs⁺ ion, and the same equilibria were shown to be applicable to the case of Fr⁺ with inclusion of additional competitive effects.