New applications of crown ethers. 11. Structural effect of the counter anion on crown-ether mediated cation extraction and transport. A direct examination of the crown ether-cation complex in the organic phase of extraction and transport

Abstract

Structural effects of counter anions on crown-ether mediated cation extraction and transport have been studied systematically by use of a series of trisubstituted phenols. A comparison of the analogous compounds with smaller 2,6-disubstituents shows that the higher extractability and transport rate using triodophenol, 2,6-di(t-butyl)-4-nitrophenol, and picric acid as the cooperative carrier could be attributed to the dehydration effect of the two bulky groups at the 2,6-positions, as well as to the suitable acidity (p K a < 7) and high lipophilicity of the organic acids. Rate of cation uptake ( k u) and release ( k r) at the interfaces between the aqueous and membrane phases, overall cation transport rate ( J) through the membrane and the time course of the concentration of crown ether-cation complex in the organic membrane phase have been compared for several crown ether compounds using 2,6-di(t-butyl)-4-nitrophenol as the cooperative carrier. The cation uptake rate ( k u) increases with the increase of the extraction constant (log K e) of crown ethers, while the cation release rate ( k r) is almost unchanged when log K e < 5, but k r decreases when log K e > 5. In the latter case, decrease of pH of the receiving phase may result in a considerable enhancement of k r and J, but may decrease the complex concentration in the membrane phase. Bis (benzocrown ether)s gives a larger k u and smaller k r compared with their monocyclic analogues in the transport of a cation larger than their hole size. The result can be explained by the formation of an intramolecular sandwich complex in the membrane phase.