Kinetics of carrier-mediated alkali cation transport through supported liquid membranes: Effect of membrane solvent, co-transported anion, and support

Abstract

The rate-limiting step in the transport of alkali cations through supported liquid membranes mediated by calix [4] arene carriers can be the diffusion of the carrier cation complex through the membrane and/or the kinetics of cation release from the complex. The effects of membrane solvent, co-transported anion, and support on the diffusion constant D m, the extraction constant K ex and the rate constant k have been studied. These were determined from flux measurements as a function of source phase salt activity ( D m and K ex) and membrane thickness ( D m and k). Additional information about the transport resistances was obtained from variation of the operating temperature and from lag time measurements. The diffusion constants ( D m) for 1/NaClO 4 and 2/KClO 4 linearly increased with reciprocal solvent viscosity. On increasing the solvent polarity, the extraction constant increased, while the rate of cation release decreased. Both log K ex and log k correlate linearly with the Kirkwood function, (ϵ r − 1) (2ϵ r + 1) . The co-transported anion (ClO 4 − vs. SCN −) affects the kinetics of release but not the diffusion constant. The normalized k and D m values for 2/KClO 4 in NPOE/Accurel ® 1E-PP and NPOE/Celgard ® 2500 are nearly the same. This means that the transport regime (diffusion or kinetic control) depends only on the tortuosity (τ) and thickness of the support, irrespective of its morphology.