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 sup ported liquid membranes (SLM) mediated by neutral carriers is the diff usion of the carrier cation complex through the membrane. However, in this study, it is demonstrated that some macrocyclic carriers like cal ix[4]arene crown ethers and calixspherands show such slow kinetics of decomplexation that the release of salt from the membrane strongly aff ects the transport rates. The influence of rates of decomplexation on alkali cation transport can be described by a simple mathematical mode l. Data for the rate-limiting steps in the transport process have been obtained from maximum fluxes, lag-time measurements, temperature vari ation, and membrane permeation experiments with dyes. The relative con tributions of diffusive and kinetic resistances inside the SLM were de termined by variation of the membrane thickness. When kinetics of rele ase are relatively slow, the transport selectivity (K+/Na+) is lower t han in the transport regime of diffusion limitation.