Effect of Macrocycle Type on Pb2+ Transport through an Emulsion Liquid Membrane

The relative effectiveness of 14 different macro-cycles in transporting Pb(NO3)2 has been determined at 25 °C using a water-toluene-water emulsion membrane system. The largest amount of Pb2+ transport was found with dideeyl-1,1O-diaza-18-crown-6 (91%), followed by dicyclohexano-18-crown-6 (81%), di-tert-butyl-dicyclohexano-l8-crown-6 (77%), 1,10-diaza-18-crown-6 (27%), and cryptand 2.2.1 (4,7,13,16,21,24-pentoxa-1,10-diazabicyclo(8, 8, 5)-tricosane) (16%). The use of the other macrocycles produced little Pb2+ transport. Analysis of the transport results shows that, for most effective transport, the macrocycle should distribute preferentially to the organic phase and the log K value for the binding of the macrocycle with Pb2+ must be large enough for quantitative extraction of the Pb2 + into the membrane. However, this log K value must be sufficiently smaller than that for interaction of Pb2+ with P2O7 4−, the receiving phase complexing agent, to allow a large Pb2+ concentration gradient to be established. These features provide information which should be useful in designing systems for cation separations using emulsion membranes.