Crown ethers, mobile carriers

Several attempts to perform enantioselective separations using membranes of a chiral mobile carrier have been reported and have been extensively discussed in a recent review [185]. Various chiral carriers, mainly crown ethers, were used for this purpose but poor enantioselectivity was usually obtained and no preparative application has been described. 

Due to their pronounced selectivity in metal ion ccmplexation (6), crown ethers (macrocyclic polyethers) and related macrocyclic multidentate ligands are attractive mobile carriers for metal ion transport across liquid membranes. As summarized in recent reviews of macrocycle-facil itated transport of ions in liquid membrane systems (7,8), most studies have been conducted with macrocyclic carriers which do not possess ionizable groups. For such carriers, metal ions can only be transported down their concentration gradients unless some type of auxiliary complexing agent is present in the receiving aqueous phase. 

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