Temperature Dependence of Ion and Water Diffusion in Crown Ether Loaded Nafion Matrix

Abstract

Temperature dependence study of the self-diffusion coefficient of Cs(+) ion in dibenzo-18-crown-6 (DB18C6) modified Nafion-117 (Cs-Naf-CR) was carried out in the temperature range of 50-65 °C. Temperature dependence of water diffusion in Cs-Naf-CR was also studied to understand the mechanism of cation and water transport in the membrane. Because of the very slow kinetics of isotopic exchange, self-diffusion measurement of Na(+) in Na-Naf-CR was carried out only at 60 °C. The result indicates that self-diffusion behavior is governed by the nature of the cation in which the crown ether was loaded in the membrane matrix. The activation energy of diffusion for Cs(+) ion and water in Cs-Naf-CR was found to be much higher than that in the pure Cs(+) form of Nafion (Cs-Naf). Water uptake of the membrane was also found to have reduced compared to Cs/Na-Naf. The results point to the binding of the ions by DB18C6 and the destruction of the water channels in the crown ether loaded membrane. The differential scanning calorimetry (DSC) data supports these observations.

Electrodriven Ion Transport through Crown Ether–Nafion Composite Membrane: Enhanced Selectivity of Cs+ over Na+ by Ion Gating at the Surface

“…This shows that there are two different types of Cs + ionic sites in these Cs-Naf-CR membranes: one is associated with crown ether, and the other is free of crown ether. As seen from the profiles in Figure a, with increasing N , the crown ether associated sites are increasingly difficult for Cs + sol ⇆ Cs + mem exchange because of changes in membrane morphology …”

“…The Cs + sol ⇆ Cs + mem isotopic exchange and Na + sol ⇆ Cs + mem ion-exchange measurements in the partially crownether-loaded Cs-Naf-CR membranes have been carried out at room temperature following the standard radiotracer method as described elsewhere. 21 The details of the method have been given in Supporting Information.…”

“…As seen from the profiles in Figure 4a, with increasing N, the crown ether associated sites are increasingly difficult for Cs + sol ⇆ Cs + mem exchange because of changes in membrane morphology. 21 The time-dependent relative cation transport (% Cs + /% Na + ) for different membranes (Cs-Naf, Cs-Naf-CR (N = 0.36), and Cs-H-Naf-CR) are shown in Table 1. The data shown in the table are for the experiments that have been carried out at 4 V with the feed solution having 5 mmol Cs + and 5 mmol Na + .…”

“…The amount of crown ether in the DMF solution is in a stoichiometric ratio of 1:1 to the total ion-exchange sites in the membrane. The annealing of the crown-ether-loaded membrane has been done by heating the membrane sample at 70 °C under vacuum . As calculated from gravimetric and UV–visible spectrophotometric measurement, the value of n CR / n IES ( n IES is the number of ion-exchange sites in the membrane) in the Cs-H-Naf-CR membrane has been found to be 0.32.…”

“…In our earlier attempt, , dibenzo-18-crown-6 (DB18C6) was incorporated in Nafion-117 cation-exchange membrane to prepare an alkali metal ion-selective membrane. It has been observed that the transport behavior of the alkali metal ions in those membranes is strongly dependent on the cationic form of the membrane in which the crown ether is loaded.…”