CALIXCROWNS AS SELECTIVE POTASSIUM CATION CARRIERS IN SUPPORTED LIQUID MEMBRANES

Calixcrown derivatives transport potassium cations selectively through supported liquid membranes (Accurel/omicron-nitrophenyl n-octyl ether, NPOE). These lipophilic carriers give stable fluxes (> 4 weeks) for the membrane system described. Bridging with a tetraethylene glycol unit at the lower rim is essential since simple calix[4]arenes do not transport potassium cations. The potassium/sodium cation selectivity of these carriers was compared with the selectivity of other potassium-selective carriers like valinomycin and dibenzo-18-crown-6 in both single-cation and mixed-cation experiments. In the competition experiments, the order of decreasing K+/Na+ selectivity was as follows: valinomycin > 1,3-dimethoxy-p-tert-butylcalix[4]arene crown-5 > dibenzo-18-crown-6 > 1,3-dihydroxy-p-tert-butylcalix[4]arene crown-5. The transport selectivity is not simply related to the transport rates in single-cation experiments or to association constants. Although the K+/Na+ extraction selectivity is high, the K+/Na+ transport selectivity of valinomycin and 1,3-dimethoxy-p-tert-butylcalix[4]arene crown-5 (5) is low due to a high association constant for K+ in NPOE combined with a low diffusion constant. A mathematical model has been developed that predicts the observed K+/Na+ selectivities in competition experiments from the results of single-cation experiments.