The influence of macrocyclic ligand structure on carrier-facilitated cation transport rates and selectivities through liquid membranes

Abstract Rates of transport of the nitrate salts of Li + , Na + , K + , Rb + , Cs + , Ag + , Mg 2+ , Ca 2+ Sr + , Ba 2+ , and Pb 2+ through stirred bulk chloroform membranes have been determined using a large number of macrocyclic ligand carriers. The effects of various ligand structural features on cation transport rates and on cation transport selectivities have been demonstrated. Among crown ethers and cryptands with cavities no larger than that of 21-crown-7, transport selectivity was observed for those cations which best fit the ligand cavity as long as the complex stability constant did not exceed certain values. Open chain analogs of crown ethers were ineffective as cation carriers. The effects on cation transport rates and' selectivity of substituting sulfur, nitrogen, and pyridine nitrogen for ether oxygen donor atoms in 18-crown-6 were shown. It was found that benzo-substituted analogs of crown ethers gave lower transport rates than cyclohexano substituted or non-substituted crowns. Crown ether—diesters were in general not as effective in transporting cations as were their crown ether analogs. Addition of large aliphatic groups to crown ethers and cryptands, which serve to minimize loss of carrier to the water phases, did not significantly alter transport rates or selectivities. Rates and selectivities of cation transport of several of the synthetic macrocycles rivalled those of the antibiotic valinomycin.

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