KINETICS OF CARRIER-MEDIATED ALKALI CATION-TRANSPORT THROUGH SUPPORTED LIQUID MEMBRANES - EFFECT OF MEMBRANE SOLVENT, COTRANSPORTED ANION, AND SUPPORT

The rate-limiting step in the transport of alkali cations through supp orted liquid membranes mediated by calix [4] arene carriers can be the diffusion of the carrier cation complex through the membrane and/or t he kinetics of cation release from the complex. The effects of membran e solvent, co-transported anion, and support on the diffusion constant D-m, the extraction constant K-ex and the rate constant k have been s tudied. 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 obt ained from variation of the operating temperature and from lag time me asurements. The diffusion constants (D-m) for 1/NaClO4 and 2/KClO4 lin early increased with reciprocal solvent viscosity. On increasing the s olvent polarity, the extraction constant increased, while the rate of cation release decreased. Both log K-ex and log k correlate linearly w ith the Kirkwood function, (epsilon(r)-1)/(2 epsilon(r)+1). The co-tra nsported anion (ClO4- vs. SCN-) affects the kinetics of release but no t the diffusion constant. The normalized k and D-m values for 2/KClO4 in NPOE/Accurel(R) 1E-PP and NPOE/Celgard(R) 2500 are nearly the same. This means that the transport regime (diffusion or kinetic control) d epends only on the tortuosity (tau) and thickness of the support, irre spective of its morphology.