Synergistic extractions of alkali ions byp-tert-butylcalix[4]arene and crown ether mixtures

Mixtures of macrocyclic crown ethers (L′=DC18C6, DB18C6, 18C6; L″=B15C5) andp-tert-butylcalix4]arene (LH₄) in dichloroethane exhibit synergistic effects in the extration of alkali ions (M⁺). These extractions are described by two independent reactions: —a two phase ion exchange: $$LH_{4org} + M_{aq}^ + \rightleftharpoons MLH_{3org} + H_{aq}^ + , (K_{ex} )$$ —the formation of an adduct in the organic phase: $$MLH_{3org} + nL_{org}^\prime (or nL_{org}^{''} ) \rightleftharpoons LH_3 ML_{norg}^\prime (or LH_3 ML_{norg}^{''} )(K_{fn} ).$$ It is shown thatn=1 for all the systems including 18 membered crown ethers (M=Na, K, Rb, Cs) and for the Na⁺-B15C5 system; whereasn=2 in the case of the K⁺-B15C5 and Rb⁺-B15C5 systems. Ion size effects on the stability constant of the adducts reveal strong interactions between the crown ether and the cation in the above mentioned systems. The corresponding adduct in the Cs⁺—B15C5 system has a very low stability constant in comparison with the others. This seems to show that B15C5 is unable to remove the Cs⁺ ion from the calixarene ‘cup’ in the cesium calixarenate complex.