Dielectric approach to polystyrene microcapsule analysis and the application to the capsule permeability to potassium chloride

A method of numerical analysis is proposed to determine the phase parameters from dielectric observations of polystyrene microcapsules by using the conductivity at intermediate frequencies between two dielectric relaxations observed. The method was more feasible for the analysis of the microcapsule observations than the method by which the calculation is performed by use of the permittivity at lower frequencies. The method was applied to the dielectric observations of the KCl-loaded microcapsules to obtain the permittivity and the conductivity of the capsule interior, the thickness of the capsule wall, the volume fraction of the suspended capsules and the conductivity of the surrounding aqueous phase. The conductivity of capsule interior was found to be unchanged with time for as long as 3 months. From the results it is concluded that the microcapsules are impermeable to KCl.     

Dielectric study of poly(methyl methacrylate) microcapsules in particular reference to the effect of the frequency dependence of the permittivity of the capsule wall

The dielectric behavior of poly(methyl methacrylate) (PMMA) microcapsules was analyzed theoretically by taking account of the frequency dependence of the relative permittivity _s and the electrical conductivity _s of the capsule wall. For comparison, similar analysis was applied to polystyrene (PS) microcapsules which have frequency-independent _s and _s . The behavior of _s and _s of the PMMA microcapsules was assumed to be represented by a dielectric relaxation of them-th power type on the basis of dielectric data on underwater PMMA films. By means of a theory of the interfacial polarization, some consideration was carried out regarding the effect of the frequency dependence of _s and _s on the dielectric behavior of the PMMA microcapsules. The consideration led to predictions that the frequency dependence of _s and _s causes (i) the variation in the relaxation intensity with the change in the conductivity of suspending medium, (ii) the frequency dependence of the permittivity at low frequencies and (iii) the frequency dependence of the conductivity at high frequencies. The theoretical prediction (i) was verified by observed data, (ii) and (iii) remaining unverified owing to the experimental difficulty.     

Open-mouthed metallic microcapsules: exploring performance improvements at agglomeration-free interiors

Although artificial capsule structures have been thoroughly investigated, functionality at the surfaces of their interiors has been surprisingly overlooked. In order to exploit this aspect of capsular structure, we here report the breakthrough fabrication of metallic (platinum) microcapsules with sufficient accessibility and electroactivity at both interior and exterior surfaces (open-mouthed platinum microcapsules), and also we demonstrate improvements in electrochemical and catalytic functions to emphasize the practical importance of our concept. The open-mouthed platinum microcapsules were prepared by template synthesis using polystyrene spheres, where surface-fused crystalline nanoparticles formed a capsule shell. Subsequent removal of the polystyrene spheres induced formation of mouth-like openings. The open-mouthed platinum microcapsules exhibit a substantial increase of their electrode capability for methanol oxidation and catalytic activities for carbon monoxide (CO) oxidation. Notably, activity loss during CO oxidation due to undesirable particle agglomeration can be drastically suppressed using the open-mouthed microcapsules.     

Theory of dielectric relaxations due to the interfacial polarization for two-component suspensions of spheres

A theoretical formula of dielectric relaxation in a form of complex relative permittivity is derived for dilute suspensions of spherical particles of two kinds on the basis of the Maxwell-Wagner theory of interfacial polarization. Another theoretical formula is derived further for concentrated suspensions of spheres of two kinds on condition that the formula derived above holds for the infinitesimally increasing process in concentration of the dispersed spheres. Furthermore a theoretical formula is derived for concentrated suspensions of shelled spheres of two kinds as the extension of the formula for concentrated suspensions. By use of the theoretical formulas proposed, values of the permittivities and the conductivities of the two-component suspensions were calculated for some examples with different sets of phase parameters. Results of the numerical calculation demonstrates dielectric relaxation profiles full of variety and characteristic of the suspensions containing two kinds of spheres covered with or without shells.     

Theoretical study of dielectric behavior of microcapsule suspensions with distributed drug release rate

Theoretical consideration is made concerning the dielectric behavior of aqueous suspensions of W/O-type microcapsules containing aqueous electrolyte solutions, especially concerning distribution effects of the electrolyte release rate on the dielectric behavior.It is shown at first that the distribution of the release rate causes electrical conductivity of the capsule interior. Next, it is deduced, in the light of a theory of the interfacial polarization, that the distribution characteristics of the capsule interior's electrical conductivity are closely related to those of the relaxation time for a dielectric relaxation exhibited by the suspensions. Several formulas are derived to describe the relation between the distribution characteristics of the release rate and those of the relaxation time.     

Dielectric analysis of PMMA microcapsules containing an aqueous KCl solution to derive a distribution function of the release rate of KCl

Release of KCl from a batch of PMMA microcapsules containing an aqueous 3 mM KCl solution was studied with the following two methods: 1) dielectric measurements of suspensions of the microcapsules in distilled water. The electrical conductivity of capsule interior was estimated with this method. 2) Measurements of the conductivity of suspending medium of the microcapsule suspensions. These two kinds of experiments provided us with information about the amount of KCl remaining in the capsules and that diffused outside. Results of these experiments showed that the microcapsules varied in their release rate of KCl. A distribution function of the release rates was derived from the results of the dielectric measurements. That distribution function was successful in explaining the results of the measurements of the conductivity of suspending medium. Furrther examination revealed that the release rate increased with the increase in the thickness of capsule wall.     

Dielectric measurement of a single sub-millimeter size microcapsule

A new technique is described for measuring dielectric dispersion of a single microcapsule in suspension over a frequency range from 1 kHz to 10 MHz. It was applied to polystyrene microcapsules which showed a two-step dielectric dispersion, that is, a superposition of two Debye type dispersions. The dielectric dispersion was analyzed by an electrical model in which a spherical core covered with a shell is immersed in a continuous phase, yielding the phase parameters related to the microcapsule: the wall thickness, the permittivity and conductivity of the core phase. The advantage of this technique is that it can characterize individual microcapsules, whereas the conventional method provides average properties of many microcapsules. Hence, the technique enables us to directly determine the distributions of the phase parameters and to exactly examine the relationship between the dielectric behavior and the microcapsule structure simultaneously observed by microscopy.     

Analysis of dielectric observations of KCl-charged poly(methyl methacrylate) microcapsules using a two-component model consisting of KCl-permeable and KCl-impermeable capsules

Dielectric measurements were carried out on aqueous suspensions of poly(methyl methacrylate) microcapsules charged with KCl solutions to examine the KCl permeability of the microcapsules. The specimens exhibited three kinds of dielectric relaxation. Two kinds of dielectric relaxation were observed immediately after washing the specimens with distilled water. These overlapped each other when the specimen was left standing in the measuring cell for a long time. Their relaxation frequencies were affected by the KCl concentration of the suspending medium in which the specimen was kept before washing. Another relaxation was observed distinctly when the spepcimens were washed thoroughly with distilled water. Its relaxation frequency was affected by the KCl concentration of the charged solution. These results were explained by assuming that the specimens were mixtures of KCl-permeable and KCl-impermeable capsules. The theoretical analysis procedure was devised in the light of dielectric theory of interfacial polarization to estimate the parameters which characterize the structure of the specimens.     

Dielectric analysis of polystyrene microcapsules using scanning dielectric microscope

 A dielectric imaging technique with a scanning dielectric microscope was applied to polystyrene microcapsules in an aqueous environment to study the electrical properties of individual ones. The dielectric images obtained over a frequency range from 10 kHz to 10 MHz showed frequency dependence, which indicated dielectric dispersion (or relaxation) due to interfacial polarization or the build up of charge on the boundaries between the microcapsule shell and the aqueous phases. The dielectric dispersion was analyzed based on an equivalent electrical circuit model and a shell-sphere model in which a spherical core is covered with an insulating shell. Received: 26 September 1997 Accepted: 26 December 1997     

Dielectric imaging of biological cells

A dielectric technique that can image local permittivity and conductivity has been applied to living biological cells in an aqueous environment. The local permittivity and conductivity were measured between 10 kHz and 10 MHz with a fine probe electrode, which was laterally scanned over cells on a plate electrode. The dielectric images of the cells depended on frequency, indicating dielectric relaxation that is due to interfacial polarization. The low-frequency image (at 10 KHz) in which the cells have high permittivity and low conductivity compared with the medium results from the presence of the plasma membrane with high resistivity. The dielectric image of the cell interior is obtainable at high frequencies (10 MHz), where the plasma membrane is short-circuited.     

Formation of Triple‐Shelled Molybdenum–Polydopamine Hollow Spheres and Their Conversion into MoO2/Carbon Composite Hollow Spheres for Lithium‐Ion Batteries

Unique triple‐shelled Mo‐polydopamine (Mo‐PDA) hollow spheres are synthesized through a facile solvothermal process. A sequential self‐templating mechanism for the multi‐shell formation is proposed, and the number of shells can be adjusted by tuning the size of the Mo‐glycerate templates. These triple‐shelled Mo‐PDA hollow spheres can be converted to triple‐shelled MoO₂/carbon composite hollow spheres by thermal treatment. Owing to the unique multi‐shells and hollow interior, the as‐prepared MoO₂/carbon composite hollow spheres exhibit appealing performance as an anode material for lithium‐ion batteries, delivering a high capacity of ca. 580 mAh g^?1 at 0.5 A g^?1 with good rate capability and long cycle life.     

Boundary-element calculations for dielectric relaxation of water-in-oil-in-water emulsions consisting of spherical droplets with a spheroidal core

Using the boundary-element method, theoretical calculations were made for the complex permittivity of water-in-oil-in-water emulsions consisting of spherical droplets. A spheroidal inner phase was placed at the center of the droplet. To examine the effects of the shape of the inner phase, the axial ratio of the inner phase was changed by keeping its volume unchanged. The results showed that the dielectric relaxation of the emulsions was markedly affected by the shape of the inner phase. As a detailed analysis, effects of external electric fields directed in the mirror plane and those along the rotation axis of the droplets were examined separately. This showed that the relaxation consisted of two kinds of relaxation terms. The first related to the structure of the droplet parallel to the external field and had similar character to the low-frequency relaxation of emulsions of shelled spheres. The second, found in restricted cases, related to the structure perpendicular to the external field, having character similar to the high-frequency relaxation for the shelled spheres. Received: 8 May 2001 Accepted: 29 August 2001     

Microwave dielectric properties of aqueous potassium iodide solutions as a function of temperature

The complex dielectric permittivity of aqueous KI solutions was studied for molalities of 0.50–4.01 m and temperatures of 288–323 K in the region of water dielectric permittivity dispersion. The values of high-frequency of dielectric permittivity (ε) and dielectric losses (ε″) were obtained at seven frequencies ranging between 7.5 and 25 GHz. The low-frequency electrical conductivity of the aforementioned solutions was measured for calculating ionic losses. A single relaxation process is observed in these solutions, fitted by the Debye or Cole-Cole equation with small distribution parameters. The static dielectric constant and dielectric relaxation time were studied as functions of temperature and concentration; the activation enthalpy of dielectric relaxation was calculated. The temperature dependence of the static dielectric constant was found to disappear in highly concentrated solutions. The structure-breaking effect on water caused by K⁺ and I⁻ ions was affirmed, this effect disappearing in going to elevated temperatures.     

Viscoelastic studies of some phenols from dielectric measurements

The variation of dielectric relaxation time with the viscosity of the medium is being exploited in drawing certain quantitative conclusions regarding molecular motion and intermolecular forces in liquids, liquid mixtures, dilute solutions and multi-component polar solutes in dilute solution. With no perfect empirical or theoretical equation in place for the variation of dielectric relaxation time with viscosity, the experimental investigations on different systems only can give an insight. In the present study the results of dielectric measurements carried out on pure samples of o-ethyl phenol, 2-n-butylphenol, 4-n-butylphenol, and 2,6-butylphenol in dilute solutions in different mixed solvents (benzene?+?paraffin) and on binary mixtures (1?:?1) of p-2-n-butylphenol?+?4-n-butylphenol and p-bromonitrobenzene?+?2,6-butylphenol, are reported. For comparison, the results on p-bromonitrobenzene?+?m-bromonitrobenzene as an example of mixture of non-associative liquids was also carried out and the results are presented. Different parameters determined using these dielectric measurements are also presented using different models and these studies indicate that the dielectric behavior at microwave frequencies favor the concept of dynamic viscosity and a single viscoelastic relaxation time for the system under study.     

非导电微球悬浊系的介电谱——弛豫机制的考察和界面信息的解析

研究了不同浓度电解质溶液中聚苯乙烯微球悬浊液的介电谱, 发现在40 Hz～110 MHz频率范围内出现了两个明显的弛豫. 在介电模型基础上对弛豫原因的理论分析结果表明, 千赫兹频域出现的弛豫是由粒子附近双电层中对离子的扩散所致, 兆赫兹附近出现的弛豫源于空间电荷在固/液界面的累积. 应用Hanai方法计算出体系内部的相参数, 获得了微球/溶液界面的电信息, 并给出了合理解释. 理论计算结果验证了模型和方法的适用性. 实验采用透析法调制样品, 有效地防止了体系内部粒子二次团聚的发生.     

Synthesis,characterization, optical,thermal and electrical properties of polybenzimidazoles

The variation of dielectric constant and dielectric loss of two novel polybenzimidazole (PBI) were studied at constant temperature with variable frequency. The polymers have shown maximum dielectric constant at low applied frequency 50 Hz at 393 K due to the space charge polarization. The AC conductivity and activation energy of polymers were arrived from dielectric constant and dielectric loss values. PBIs were synthesized by the oxidative polycondensation of benzimidazole monomers, 2-(1H-benzo [d] imidazole-2-yl)-4-bromophenol (BIBP), and 2-(1H-benzo [d] imidazole-2-yl)-6-methoxyphenol (BIMP) in an aqueous alkaline medium using NaOCl as oxidant. The monomers and polymers were characterized by various spectroscopic techniques. Fluorescence spectra of monomers and polymers showed their λ _max emission in the region of 472–479 and 463–472 nm respectively. The electrical conductivities of iodine doped polybenzimidazoles were measured by four-point probe technique and it increases with increase in iodine vapour contact time. The electrical conductivity values were correlated with the charge density on imidazole nitrogen obtained from Huckel calculation method. Both the PBI are having reasonably good thermal stability and are shown by high carbines residues of around 40% at 500°C in thermogravimetric analysis.     

Dielectric properties of ion-exchange beads of sulfonic acid type dispersed in aqueous solutions — effect of fixed charge density

Ion-exchange beads with different densities of fixed charges were prepared by sulfoethylation of dextran gel beads. The relative permittivities and the electrical conductivities of the ion-exchange beads in a sodium form were evaluated by a dielectric technique consisting of the following two procedures proposed in a previous study (2): (a) Dielectric measurements for densely packed sediments of the ion-exchange beads; (b) Analysis of the observed dielectric relaxations by means of a theoretical equation of interfacial polarization for suspensions of spherical particles. The deduced permittivities of the ionexchange beads in equilibrium with water were about 60, which is lower than those of outer aqueous phases. The deduced conductirities were of the order of 1 to 10 mS cm^–1 and were increased reasonably with the fixed charge density. Closer consideration on these deduced values indicates that no specific interaction exists between the counter ions and the fixed charges irrespective of changes in fixed charge density and temperature.     

Dielectric constant and dielectric relaxation in aqueous solutions of K<Subscript>2</Subscript>[PdCl<Subscript>4</Subscript>] and K<Subscript>2</Subscript>[PtCl<Subscript>4</Subscript>]

The MW-dielectric properties of aqueous solutions of K₂[PtCl₄] (I) and K₂[PdCl₄] (II) were studied at 298 and 313 K in the frequency range (12–25 GHz) corresponding to the maximum dielectric constant dispersion for water and aqueous solutions of these salts. The low-frequency conductivities were measured. The static dielectric constant, the dielectric relaxation time, and the enthalpy of activation of the dielectric relaxation of the solutions were determined. Compared to pure water, in solutions of salts I and II, the orientational mobility of water molecules is increased and the network of H-bonds is violated more strongly than that of most other ions with hydrophilic hydration. It was demonstrated for the first time that dielectric spectroscopy can be used for analyzing complexation processes in systems containing aqua and hydroxo chloride complexes of metals.     

Preparation, structure, and magnetic properties of polystyrene coated by Fe3O4 nanoparticles

A novel method of fabricating core-shell structure particles, comprising nearly monodisperse polystyrene (PS) spheres as cores and Fe3O4 as shells, is submitted. In this research, the magnetite (Fe3O4) shell was prepared by seeded growth from the reaction of FeCl2 with diethylene glycol (DEG) in aqueous solutions. The thickness of the shell were controlled in the range of 0-60 nm by using slow injection. The composition and the structure of the shell were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TG), and vibrating-sample magnetometry (VSM). It is found that there are some differences between the magnetic composite spheres shelled with Fe3O4 and pure Fe3O4 particles, such as the size of the magnetites and the ferromagnetic property. Furthermore, the spheres exhibited the superparamagnetic characterization when the thickness of the Fe3O4 shell was less than 15 nm.     

Microwave dielectric properties of potassium hydroxide aqueous solutions

Microwave dielectric properties of potassium hydroxide aqueous solutions were studied at frequencies of 13 to 25 GHz over a wide range of concentrations and temperatures of 288 to 308 K. Experimental electrical conductivity data were used to determine ionic losses at high frequencies. The static dielectric permittivity σ_s, dielectric relaxation time τ, and thermodynamic activation parameters of dielectric relaxation (ΔH _ɛ⁺⁺, ΔG _ɛ⁺⁺, and ΔS _ɛ⁺⁺) were calculated. These quantities decrease in transfer from water to solutions. Thereby we demonstrate that potassium hydroxide has a disturbing effect on the initial hydrogen-bond network of water.