We found that a calix[4]arene-bearing pyridine is soluble in a typical room-temperature ionic liquid (RTIL), 1-alkyl-3-methylimidazolium hexafluorophosphate. Pyridinocalix[4]arene showed a high extraction ability and selectivity for silver ions. The extraction performance of the calix[4]arene was greatly enhanced by dissolution in RTILs compared to in chloroform. In a competitive extraction test using five different metal ions (Ag+, Cu2+, Zn2+, Co2+, Ni2+), only silver ions were transferred by the calix[4]arene from the aqueous feed phase into the RTILs, through a cation-exchange mechanism. The pyridinocalix[4]arene was found to form a stable 1:1 complex with silver ions, both by slope analysis and by Job's method. Since it is easy to strip silver ions from RTILs by controlling the aqueous-phase pH, the extraction performance of calix[4]arene in RTILs was maintained after five repeated uses.
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Shimojo et al. (2004) studied this question.
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