New applications of crown ethers. 12. 'Double-cycle' transport mechanism for crown ether/ 2,6-di(t-butyl)-4-nitrophenol/alkylammonium salt system. Effect of reversed-micelle like aggregate of alkylammonium salts on cation transport through a liquid membrane

A membrane system containing N-octadecylmonoaza-18-crown-6(C18A18C6) and 2,6-di(t-butyl)-4-nitrophenol (HA) transports cations in a proton-coupled process from a basic aqueous source phase to a neutral aqueous receiving phase. However, replacement of the neutral receiving phase by an acidic aqueous solution results in a remarkable increase in transport rate and disappearance of the anionic form of the cooperative carrier (A−) in the membrane phase. A new transport mechanism, “double-cycle” process, has been proposed and compared with the proton-coupled mechanism (single cycle) to explain the novel cation transport behavior of C18A18C6 cooperated by HA. The new process is distinguished by a counter-anion exchange between the crown ether-metal salt complex and the n-alkylammonium salt in the organic membrane phase; this occurs when the crown-ether mediated cation transport is cooperated with a lipophilic weak acid (pKa ∼ 6) from a basic aqueous source phase to an acidic aqueous receiving phase and in the presence of a higher n-alkylamine. Futher studies show that the new process can proceed only in a system with a monoazacrown ether bearing a long alkyl side chain, but not with a monoazacrown ether bearing a short or bulky group. It is also shown that there is a cmc-like concentration for the crown ether at which the transport system changes its mechanism from the proton-coupled process to the “double-cycle” process. These results indicate that the higher n-alkylammonium salt and crown ether-metal salt complex may form a reversed-micelle like aggregate in the organic membrane phase, and that such an aggregate may favor the extra-exchange and stabilize the metal salt complex in the membrane phase. On the other hand, the study with a single solution did not give evidence for the aggregation of the ammonium salt at the concentration used in the transport. However, at a relative high concentration the higher n-alkylammonium salts do aggregate. It seems that a dynamic equilibrium under the given transport conditions may favor aggregation in the membrane phase.