Membrane potential of bipolar membranes

Tunable Current Rectification and Selectivity Demonstrated in Nanofluidic Diodes through Kinetic Functionalization

The possibility of tuning the current rectification and selectivity in nanofluidic diodes is demonstrated both experimentally and theoretically through dynamically functionalizing a conical nanopore with poly-l-lysine. We identified an optimum functionalization time equivalent to optimum modification depth that assures the highest rectification degrees. Results showed that the functionalization time-dependent rectification behavior of nanofluidic diodes is dominated by the properties of current at positive voltages that in our electrode configuration indicate the "on" state of the diode and accumulation of ions in the nanopore. The functionalization time also tunes the ion selectivity of the diode. If the functionalization time is sufficiently short, an unusual depletion of counterions near the bipolar interface results in a cation-selective nanopore. However, a further increase in the duration of functionalization renders a nanopore that is an anion-selective nanopore. The dynamic functionalization presented in this Letter enables tuning ion selectivity of nanopores.

Electrical coupling in ensembles of nonexcitable cells: modeling the spatial map of single cell potentials

We analyze the coupling of model nonexcitable (non-neural) cells assuming that the cell membrane potential is the basic individual property. We obtain this potential on the basis of the inward and outward rectifying voltage-gated channels characteristic of cell membranes. We concentrate on the electrical coupling of a cell ensemble rather than on the biochemical and mechanical characteristics of the individual cells, obtain the map of single cell potentials using simple assumptions, and suggest procedures to collectively modify this spatial map. The response of the cell ensemble to an external perturbation and the consequences of cell isolation, heterogeneity, and ensemble size are also analyzed. The results suggest that simple coupling mechanisms can be significant for the biophysical chemistry of model biomolecular ensembles. In particular, the spatiotemporal map of single cell potentials should be relevant for the uptake and distribution of charged nanoparticles over model cell ensembles and the collective properties of droplet networks incorporating protein ion channels inserted in lipid bilayers.

Assessing the utility of bipolar membranes for use in photoelectrochemical water-splitting cells

Membranes are important in water-splitting solar cells because they prevent crossover of hydrogen and oxygen. Here, bipolar membranes (BPMs) were tested as separators in water electrolysis cells. Steady-state membrane and solution resistances, electrode overpotentials, and pH gradients were measured at current densities relevant to solar photoelectrolysis. Under forward bias conditions, electrodialysis of phosphate buffer ions creates a pH gradient across a BPM. Under reverse bias, the BPM can maintain a constant buffer pH on both sides of the cell, but a large membrane potential develops. Thus, the BPM does not present a viable solution for electrolysis in buffered electrolytes. However, the membrane potential is minimized when the anode and cathode compartments of the cell contain strongly basic and acidic electrolytes, respectively.

Dielectric Relaxation on the Intermediate Layer in a Bipolar Membrane under the Water Splitting Phenomenon

In this study, we examined the dielectric properties of an intermediate layer in a bipolar membrane, which is composed of a negatively charged layer and a positively charged layer joined in series. As a result of the time-dependent impedance measurements of charged membranes, the negative increment in electric conductivity and the positive increment in electric capacity were observed only in the case of a bipolar membrane under the application of reverse-biased voltages, which were quite different from the behavior of both monopolar membranes and of a bipolar membrane under forward-biased voltages. Further, the observed shifts showed a nearly constant value against the reverse-biased voltage. It is concluded that these characteristics coincide with the process of ion exclusion in the intermediate layer and are attributed to the water splitting mechanism.

Ionic Environmental Effect on the Time-Dependent Characteristics of Membrane Potential in a Bipolar Membrane

The membrane potential characteristics of a bipolar membrane are discussed in an examination of the contribution of an intermediate phase to time-dependent behavior. Bipolar membranes, which consist of a poly(sulfone) base polymer with a quaternary amino group and a poly(styrene-co-divinylbenzene) cation-exchange membrane, were prepared in this study. The membrane potentials in various external concentration differences, the facing directions of the membrane, and the external electrolyte solutions were measured as a function of time. In this study, the time course of the membrane potential was simulated by using the equation based on the Teorell-Meyer-Sievers theory and by assuming the concentration in the intermediate phase, which is much higher or lower than that in the external solution. The membrane facing direction and the intermediate phase condition will alter the direction of the membrane potential change. The intermediate phase in a bipolar membrane seems to act as an alteration barrier for the membrane potential according to the membrane facing direction. Copyright 2001 Academic Press.

Membrane Potential of Composite Bipolar Membrane in Ethanol-Water Solutions: The Role of the Membrane Interface

The membrane potential across a composite bipolar membrane (CBM) composed of a cation-exchange membrane with an anion-exchange membrane is theoretically and experimentally analyzed for LiCl ethanol-water solutions. The theoretical approach is based on an extension of the Donnan equilibrium and the Nernst-Planck equation of monopolar charged membranes for the case of two ion-exchange layers by considering the effect of electrolyte ion pairing in the external solution. The experimental results show that the effective membrane charge densities of the two ion-exchange layers will become smaller than those which are separately estimated for each layer. We have introduced a contact factor, zeta, into the theoretical approach to clarify this phenomenon in this study, and the theoretical predictions were in good agreement with the experimental data. The membrane potential measurements show that CBM has the characteristics of a bipolar membrane and can significantly contribute to a better electrochemical characterization of the CBMs. Copyright 1999 Academic Press.

Effect of the Interface Component on Current-voltage Curves of a Composite Bipolar Membrane for Water and Methanol Solutions

The current-voltage curves of a composite bipolar membrane (CBM) were experimentally measured by varying the interface component between cation- and anion-exchange membranes for water and methanol solutions. In each solution system, 0.05 mol/l LiCl was used as the electrolyte. The interface component was varied by pasting the polymers or installing the thin membranes in the intermediate region of the CBM. The measured results show that the functional groups of the polymers and thin membranes enhanced the water and methanol splitting effect. This phenomenon can be explained by the protonation-deprotonation reactions occurring between these functional groups and the water or methanol molecules in the intermediate region of the CBM. The effect of transition metal compounds existing in the intermediate region of the CBM was also experimentally examined in this study. It was found that the effect of transition metal compounds on water or methanol splitting was not obvious. Copyright 1999 Academic Press.