Selective Ion Permeation Across Membranes Prepared from Steroidal Crown Ethers

Five crown ethers covalently‐linked to the cholesterol skeleton were synthesized. Thermal analysis established that four of them have a cholesteric liquid‐crystal phase. In particular, the monobenzo‐13–crown‐4 covalently‐linked through an ethylene spacer to the cholesterol skeleton shows a stable cholesteric mesophase even at room temperature. The stable cholesteric mesophase at room temperature was also obtained by mixing two steroidal crown compounds. These mixed systems were embedded in a polymer matrix and the polymer/liquid‐crystal composite membranes were subjected to ion‐permeation experiments. It was found that the permeation selectivity is basically explained by the hole‐size selectivity in the crown‐metal interaction. In particular, composite membranes containing a monobenzo‐13–crown‐4 unit showed the remarkably high Li⁺ selectivity. It was concluded on the basis of examination of activation parameters that ion permeation occurs across channel‐like stacks of crown ether rings.