Kinzi Asaka

Dielectric properties of cellulose acetate reverse osmosis membranes in aqueous salt solutions

Abstract The dielectric properties were studied of cellulose acetate asymmetric membranes in aqueous electrolyte solutions. The value of the membrane capacitance was 395 nF/cm2, which was attributed to the dense layer in the asymmetric membrane. The thickness of the dense layer was calculated to be 27 nm from the value of the membrane capacitance. The membrane conductance depended on the type of electrolyte, while the membrane capacitance remained almost unchanged irrespective of the kind of electrolyte. The dependence of membrane conductance on the type of electrolyte showed that more highly hydrated ions were only capable of permeating the membrane with more difficulty.

The number of interfaces and the associated dielectric relaxations in heterogeneous systems

Abstract In heterogeneous systems composed of some phases, mobile electric charges accumulate on the boundary surfaces between the constituent phase under the electric field applied. This charge accumulation, the so-called interfacial polarization, gives rise to dielectric relaxations. By the use of several examples, it is shown theoretically as well as experimentally that the dielectric relaxations are equal to the interfaces in the number. Typical examples of diphase systems are (i) a series combination of a polyethylene film and an aqueous solution, and (ii) water-in-oil emulsions. These systems contain one kind of interfaces, showing one dielectric relaxation. Practical examples of triphase systems are (i) a composite system of a Teflon film sandwiched between two aqueous solutions, and (ii) polystyrene microcapsules. These systems contain two kinds of interfaces, showing two dielectric relaxations. A pentaphase system composed of shell-spheres are discussed theoretically, showing four dielectric rela...

Dielectric analysis of concentration polarization phenomena at cation-exchange membrane/solution interfaces by frequency variation and d.c. bias application

Dielectric measurements were carried out of Selemion CMV cation-exchange membranes immersed in distilled water under application of d.c. bias voltage. A distinct double dielectric relaxation was observed for the membrane—aqueous phase system in the frequency range from 100 Hz to 13 MHz. The dielectric relaxation at the lower frequencies around 3kHz is caused by a concentration polarization boundary layer developing alongside the membrane. The relaxation at the higher frequencies around 200 kHz is caused by interfacial polarization due to a series combination of the two aqueous phases adjacent to the membrane. On the basis of a model of the series combination of a left aqueous phase, a concentration polarization layer, and a right aqueous phase, the complex capacitance of the whole system is formulated theoretically so that the frequency profile may be computed. Next, curve fittings to the observed data based upon the formula derived were performed by a nonlinear least-squares minimization algorithm to yield the best-fit parameters, yielding the following values: the thichness and the reduced conductivity of the concentration polarization, the capacitances and the conductances of both aqueous phases. The thickness estimated for the concentration polarization is about 1 mm which is conform to the literature values. The reduced conductivity is about 0.04 μS-cm−1, which is comparable with the value for pure water.

Dielectric theory of concentration polarization. Relaxation of capacitance and conductance for electrolyte solutions with locally varying conductivity

Abstract For a dielectric medium with a locally conductivity used as some simulation of a concentration polarization phase, a complex capacitance is formulated to represent the dielectric relaxation behavior. The frequency dependence of capacitance and conductance was calculated numerically for a combined system of the concentration polarization phase and an aqueous phase with homogeneous concentration. Marked dielectric relaxations were found only in systems where the decrease in concentration is formed inside the concentration polarization phase. The theoritical formulas of dielectric relaxation derived are of great help in finding a significant clue to the estimation of the thickness of the polarization layer and the concentration gradient in a system where concentration polarization occurs alongside a membrane which is subjected to bias voltage.

Electrochemical properties of asymmetric cellulose acetate membranes

Abstract Dielectric measurements and membrane potential measurements were carried out for asymmetric cellulose acetate (CA) membranes in aqueous solution. In view of the results of the measurements, it was concluded that the membrane potential properties of asymmetric CA membranes capable of rejecting solutes severely could be attributed to the dense layer in the membrane. This conclusion was supported by the membrane potential theory on the basis of the two-layer model. It was found that the membrane potentials of the membranes capable of rejecting solutes severely showed a minimum. This was explained by the theory including the concentration polarization caused by osmotic flow. The effective fixed charge density of the asymmetric CA membranes increases with the increase in the salt rejection of the membranes. The mobility ratio of a cation to an anion decreases with the increase in the stokes radius of the cation in aqueous solution and the effective fixed charge density of the membranes. From these results, it may be concluded that the ions in the membranes are hydrated and interact with the fixed charge of the membrane.

Dielectric analysis of concentration polarization structure at anion-exchange membrane/solution interface under dc bias voltage application

Abstract Dielectric measurement was carried out on anion-exchange membrane AMV immersed in distilled water and in sucrose solution under applicatio of dc bias voltage. The frequency dependence of the capacitance C and the conductance G observed for the whole system was clearly characterized by a double dielectric relaxation. The dielectric relaxation at lower frequencies around 3 kHz is caused by a concentration polarization boundary layer generated alongside the membrane. The relaxation at higher frequencies around 200–700 kHz is attributed to interfacial polarization due to a series combination of the two aqueous phases adjacent to the membrane. The feature of the double relaxation is very similar to that observed previously for the cation-exchange membrane CMV. The data on double dielectric relaxations were analyzed using an analytical method recently developed to calculate the structure parameters of the boundary layer, the capacitances, and the conductances of the two adjoining aqueous phases. The thickness of the concentration polarization boundary layer obtained by the analysis is about 0.3 mm for the system composed of the AMV membrane and distilled water and about 1.6 mm for the system composed of the AMV membrane and sucrose solution. These values are consistent with those obtained by other methods of observation. In view of the results for sucrose solutions, it is inferred that the thickness of the concentration polarization layer is affected by the viscosity of the aqueous phase.

Theoretical approach and the practice to the evaluation of structural parameters characterizing concentration polarization alongside ion-exchange membranes by means of dielectric measurement

As the application of a dielectric theory proposed previously (J Membrane Sci 64:153–161 (1991)), theoretical formulation and the practical procedure of dielectric analysis are developed to calculate the structural parameters such as the conductivity gradient and the thickness of the concentration polarization layer, the capacitances and the conductances of the two adjoining aqueous phases from the observed dielectric parameters. The procedure of calculation consequent upon the theoretical formulation was applied to double relaxation data observed for cation-exchange membrane systems under application of d.c. bias voltage. As a consequence, the structural parameters of concentration polarization were readily obtained with accuracy.

Determination of Capacitances and Conductances of the Constituent Phases from Dielectric Observations on Terlamellar Composite Systems

A method of numerical calculation was proposed to derive the electrical capacitances and conductances of three constituent phases from observed dielectric parameters characteristic of the double relaxation pattern exhibited by a terlamellar composite system. Secondly, numerical estimation was carried out on the errors of capacitances and conductances of the constituent phases arising from misestimation of the observed dielectric parameters. Finally, dielectric measurements were made on a composite system where a Teflon film was sandwiched between two aqueous solutions. Reasonable values of the capacitances and the conductances of the three constituent phases were obtained by applying the method proposed. The present method may be of use for the evaluation of material separation films.