Interfacial ion transport

However, the interplay between electrolyte and polymer layer needs to be considered when optimizing the performances of polymer-modified electrodes. Structural factors will influence the interfacial ion transport and this will have a direct effect on the mechanism and location within the polymer layer of the mediation process. The following discussion will show that the nature of the mediation process can be changed dramatically by changing the electrolyte, from a situation where an electrochemical sensor with good sensitivity is obtained, to a situation where the sensitivity obtained is not much better than that observed for the bare electrode. 

The stochastic model of ion transport in liquids emphasizes the role of fast-fluctuating forces arising from short (compared to the ion transition time), random interactions with many neighboring particles. Langevin s analysis of this model was reviewed by Buck [126] with a focus on aspects important for macroscopic transport theories, namely those based on the Nernst-Planck equation. However, from a microscopic point of view, application of the Fokker-Planck equation is more fruitful [127]. In particular, only the latter equation can account for local friction anisotropy in the interfacial region, and thereby provide a better understanding of the difference between the solution and interfacial ion transport. 

The interface between two immiscible liquids is used as a characteristic boundary for study of charge equilibrium, adsorption, and transport. Interfacial potential differences across the liquid-liquid boundary are explained theoretically and documented in experimental studies with fluorescent, potential-sensitive dyes. The results show that the presence of an inert salt or a physiological electrolyte is essential for the function of the dyes. Impedance measurements are used for studies of bovine serum albumin (BSA) adsorption on the interface. Methods for determination of liquid-liquid capacitance influenced by the presence of BSA are shown. The potential of zero charge of the interface was obtained for 0-200 ppm of BSA. The impedance behavior is also discussed as a function of pH. A recent new approach, using a microinterface for interfacial ion transport, is outlined. 

A number of techniques have been applied to the interface in recent years. For the detailed list and explanations, several reviews are suggested (4-11). Typical evidence of interfacial ion transport is illustrated by Figure 3, where the transport of acetylcholine cation is observed between water and... 

At constant polymer charge c, an increase in the bulk electrolyte concentration corresponds to a proportional variation in so that the region of equal participation of cations and anions in the process of interfacial ion transport becomes wider. 

Armstrong FA, Cox PA, Hill HAO, Lowe VJ, Ohver BN. 1987. Metal-ions and complexes as modulators of protein interfacial electron-transport at graphite-electrodes. J Electroanal Chem217 331-366. 

The automatic measurement of the extracellular and intracellular electrical potential difference can be effectively used in plant electrophysiology for studying the molecular interfacial mechanisms of ion transport, the influence of external stimuli on plants, and for investigating the bioelectrochemical aspects of the interaction between insects and plants. 

In addition to the criticisms from Anderman, a further challenge to the application of SPEs comes from their interfacial contact with the electrode materials, which presents a far more severe problem to the ion transport than the bulk ion conduction does. In liquid electrolytes, the electrodes are well wetted and soaked, so that the electrode/electrolyte interface is well extended into the porosity structure of the electrode hence, the ion path is little affected by the tortuosity of the electrode materials. However, the solid nature of the polymer would make it impossible to fill these voids with SPEs that would have been accessible to the liquid electrolytes, even if the polymer film is cast on the electrode surface from a solution. Hence, the actual area of the interface could be close to the geometric area of the electrode, that is, only a fraction of the actual surface area. The high interfacial impedance frequently encountered in the electrochemical characterization of SPEs should originate at least partially from this reduced surface contact between electrode and electrolyte. Since the porous structure is present in both electrodes in a lithium ion cell, the effect of interfacial impedances associated with SPEs would become more pronounced as compared with the case of lithium cells in which only the cathode material is porous. 

In volume 1 (Chapters 4 and 5) a fairly detailed treatment of the movement and transport of ions was presented qualitative pictures and quantitative accounts were given of the diffusion and electrical migration of ions in the bulk of the electrolyte. In the treatment of electrodic processes, no mention was made at first of a connection between the transport in solution and processes at electrodes. It was then realized that this neglect of ion transport in solution (ionics) was tantamount to assuming that at no stage in the course of a charge-transfer reaction did the interfacial concentrations of electron acceptors and donors depart from their bulk values. 

On drawing current from a passivated lithium anode, polarization may be at first severe, but the voltage recovers fairly rapidly (Fig. 4.6). Initially, charge transfer at the anode is limited by lithium ion transport through a thin or imperfect section of the interfacial film. This process progressively... 

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