Synthetic ionophores for the separation of Li+, Na+, K+, Ca2+, Mg2+ metal ions using liquid membrane technology

Carrier-assisted transport through liquid membranes is one of the important applications of supramolecular chemistry. This work investigates the use of synthetic carrier (ionophore) for the separation of metal ions. We have tested the effect of structure of ionophore on the separation of metal ions. For this purpose, we have used a new series of non-cyclic ionophores having different end groups and chain length (R1–R5). 1,5 bis (2-naphthyloxy)-3-oxapentane (R1), 1,8 bis (2-naphthyloxy)-3,6-di-oxaoctane (R2), 1,11 bis (2-naphthyloxy)-3,6,9-tri-oxaundecane (R3) 1,11-(dianthraquinonyloxy) 3,6,9-trioxaundecane (R4), 1,8 (dianthraquinonyloxy) 3, 6-dioxaoctane (R5) have been used in extraction, bulk liquid membrane (BLM) and supported liquid membrane (SLM) transport of alkali (Li+, Na+, K+) and alkaline earth metal cations (Ca2+, Mg2+). The supported liquid membrane consisted of a porous cellulose nitrate and and an onion membrane support impregnated with ionophore using chloroform as a solvent. The results reveal that ionophores R1, R2 and R3 are better extractants for K+ while R4 and R5 are better extractants for Ca2+. Among these ionophores R3 and R4 are best extractants for K+ and Ca2+ ions. The results of BLM reveal that ionophores R1, R2 and R3 transport Na+ at a greater extent, while R4 and R5 transport Ca2+ and K+ at a greater extent. In SLM experiments using a cellulose nitrate membrane support, it was observed that naphthyl end group bearing ionophores (R1–R3) transports Na+ > K+, and anthraquinone bearing ionophores (R4 and R5) transport K+ > Na+ > Ca2+ respectively. In the onion membrane support R4 transports Ca2+ and Na+ equally and R5 transports K+ selectively. On comparing the membrane support, the cellulose nitrate membrane is found better support for the transport of metal ions. The results suggest that due to the presence of different end groups and chain lengths the selectivity of non-cyclic ionophores towards metal ions is enhanced. Thus selectivity of ionophores may have fruitful application in ion selective electrodes and separation of metal ions.