Effect of Cation Driven Loading of Dibenzo-18-Crown-6 in Nafion-117 Membrane on the Diffusion and Transport Behavior of Alkali Metal Ions

The possibility of enhancing the selectivity to separate the alkali metal ions was studied by loading dibenzo-18-crown-6 (DB18C6) in Li+, Na+, K+, and Cs+ form of cation exchange membrane, Nafion-117 (M-Naf-Cr where M = Li, Na, K, and Cs). DB18C6 was incorporated in Nafion-117 in corresponding ionic forms. Presence of DB18C6 in Nafion-117 was confirmed by FTIR. Self-diffusion and ion exchange kinetics of the alkali metal ions were studied in these membranes. It was observed that the diffusion in the membrane slowed down drastically from Li+ to all other monovalent alkali metal ions. Two compartment cell experiments were done with DB18C6 loaded Cs+- form of Nafion-117 (Cs-Naf-Cr) membrane to study the transport of Na+ and Cs+ ions. No transport of ions was observed. When the same experiment were performed by replacing Li+ of Li-Naf-Cr with Cs+ (Cs-Li-Naf-Cr), transport of Cs+ and H+ were observed at much faster time scale compared to Cs-Naf-Cr. The selectivity of Cs+ over Li+ was enhanced by a factor of about 6 when Li-Naf-Cr was used in place Li+ form of Nafion-117 for the transport experiments.