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

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 Dm, the extraction constant Kex and the rate constant k have been studied. These were determined from flux measurements as a function of source phase salt activity (Dm and Kex) and membrane thickness (Dm and k). Additional information about the transport resistances was obtained from variation of the operating temperature and from lag time measurements. The diffusion constants (Dm) for 1/NaClO4 and 2/KClO4 linearly increased with reciprocal solvent viscosity. On increasing the solvent polarity, the extraction constant increased, while the rate of cation release decreased. Both log Kex and log k correlate linearly with the Kirkwood function, (¿r ¿ 1)/(2¿r + 1). The co-transported anion (ClO4¿ vs. SCN¿) affects the kinetics of release but not the diffusion constant. The normalized k and Dm values for 2/KClO4 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 (¿) and thickness of the support, irrespective of its morphology.