TRANSPORT PROCESSES IN MEMBRANES CONTAINING NEUTRAL ION CARRIERS, POSITIVE-ION COMPLEXES, NEGATIVE MOBILE SITES, AND ION-PAIRS

The response of membranes, which are prepared with neutral ion carriers and mobile sites, to an externally applied potential step was investigated theoretically and experimentally. Two cases were identified: the first, in which the neutral carriers were in large excess, showed a gradual decrease in the magnitude of transient currents due to the increase in bulk membrane resistance. Using digital simulations, this observation was related to the polarization of the concentration profiles of sites and counter ions. The second case showed a behavior limited by the small amount of carrier. A characteristic break in a current vs time plot in this constant-resistance system corresponded to depletion of the carrier at the interface where the selected ion entered the membrane. Most of the experimental evidence for both processes was obtained from plasticized PVC membranes containing valinomycin and tetraphenylborate-type trapped mobile sites. The results support the presence of a carrier mechanism in these systems and illustrate the transport of ions and ion pairs in the membranes as well.