KINETIC VERSUS DIFFUSION CONTROL IN CARRIER-MEDIATED COTRANSPORT OF ALKALI CATIONS THROUGH SUPPORTED LIQUID MEMBRANES

Usually, the rate-limiting step of alkali cation transport through supported liquid membranes (SLM) mediated by neutral carriers is the diffusion of the carrier cation complex through the membrane. However, in this study, it is demonstrated that some macrocyclic carriers like calix[4]arene crown ethers and calixspherands show such slow kinetics of decomplexation that the release of salt from the membrane strongly affects the transport rates. The influence of rates of decomplexation on alkali cation transport can be described by a simple mathematical model. Data for the rate-limiting steps in the transport process have been obtained from maximum fluxes, lag-time measurements, temperature variation, and membrane permeation experiments with dyes. The relative contributions of diffusive and kinetic resistances inside the SLM were determined by variation of the membrane thickness. When kinetics of release are relatively slow, the transport selectivity (K+/Na+) is lower than in the transport regime of diffusion limitation.