STABILIZATION OF CROWN ETHER CONTAINING SUPPORTED LIQUID MEMBRANES

Lipophilic ionophores, synthesized by coupling crown ethers 18-crown-6 and 12-crown-4 at the termini of polysiloxane backbones, have been tested as carriers in the transport of potassium perchlorate through a supported (Accurel) liquid membrane that contains o-nitrophenyl n-octyl ether (NPOE) as the membrane phase. The polysiloxane-coupled crown ethers are able to facilitate the ion transport. The stability of the membrane is enhanced in comparison with that of simple crown ethers like dibenzo-18-crown-6 by at least a factor of 25, because of the high partition coefficient (log PCE = 4.7-27) of the carrier for which the contribution of one siloxane moiety to the value of log PCE is 0.6. Experiments with polysiloxane-coupled crown ethers in poly(dimethylsiloxane) as the membrane phase showed a large decrease of the flux, probably due to the unfavorable partition of the salt between water and poly(dimethylsiloxane). The transport through these membranes can be described by a model, assuming thermodynamic equilibrium at the interfaces and linear concentration gradients of both complex and free crown ether. The experimental relation between the flux and the initial carrier concentration can only be explained when the transport takes place via 2:1 (crown ethercation) complexes for both polysiloxane-coupled 12-crown-4 and polysiloxane-coupled 18-crown-6. © 1990, American Chemical Society. All rights reserved.