Lipophilic diaza crown ethers in supported liquid membranes; influence of pH on transport rates and membrane stability

Lipophilic diaza‐18‐crown‐6 derivatives are used to transport potassium cations through supported liquid membranes. The influence of the pH of the aqueous phases on the rate of transport and membrane stability has been studied. At pH values lower than 10, the flux is reduced significantly because the ionophores are protonated and leach out of the membrane phase. With these carriers, proton‐driven transport is possible. The length of the alkyl chain attached to the carrier has a pronounced effect on membrane stability. Carriers having two n‐pentyl chains leach out to the aqueous phases at pH 10, whereas carriers with two n‐decyl or n‐tetradecyl chains result in a stable membrane system. Model calculations show that di‐n‐decyldiaza‐18‐crown‐6 forms a 1:1 complex with potassium cations (K 3.1 · 10⁶ l ṁ mol⁻¹) in 2‐nitrophenyl n‐octyl ether (NPOE). The calculated diffusion coefficient of the complex in NPOE is the same as the diffusion coefficient of the dibenzo‐18‐crown‐6 potassium cation complex in NPOE (1.1 · 10⁻³ cm2·h⁻¹). This means that the higher potassium fluxes for diaza crown ethers compared to dibenzo crown ethers are due to stronger complexation in the membrane phase with potassium cations. Very high carrier concentrations (and, therefore, very high fluxes) can be obtained using these lipophilic diaza crown ethers.