KINETICS OF THE IODINE-MEDIATED AND BROMINE-MEDIATED TRANSPORT OF HALIDE-IONS - DEMONSTRATION OF AN INTERFACIAL COMPLEXATION MECHANISM

Stationary and kinetic experiments were performed on lipid bilayer mem branes to study the mechanism of iodine- and bromine-mediated halide t ransport in detail. The stationary conductance data suggested that fou r different 1:1 complexes between I2 and Br2 and the halides I and Br- were responsible for the observed conductance increase by iodine and bromine (I3-, I2Br-, Br2I-, and Br3-). Charge pulse experiments allowe d the further elucidation of the transport mechanism. Only two of thre e exponential voltage relaxations predicted by the Lauger model could be resolved under all experimental conditions. This means that either the heterogeneous complexation reactions k(R) (association) and k(D) ( dissociation) were too fast to be resolved or that the neutral carrier s were always in equilibrium within the membrane. Experiments at diffe rent carrier and halide concentrations suggested that the translocatio n of the neutral carrier is much faster than the other processes invol ved in carrier-mediated ion transport. The model was modified accordin gly. From the charge pulse data at different halide concentrations, th e translocation rate constant of the complexed carriers, k(AS), the di ssociation constant, k(D), and the total surface concentration of char ged carriers, N(AS), could be evaluated from one single charge pulse e xperiment. The association rate of the complex, k(R), could be obtaine d in some cases from the plot of the stationary conductance data as a function of the halide concentration in the aqueous phase. The translo cation rate constant, k(AS), of the different complexes is a function of the image force and of the Born charging energy. It increases 5000- fold from Br3- to I3- because of an enlarged ion radius.