Dielectric observations on polystyrene microcapsules and the theoretical analysis with reference to interfacial polarization

Dielectric measurements were carried out for polystyrene microcapsules which were prepared by means of an interfacial polymer deposition technique. The microcapsules showed a couple of dielectric relaxations termedP for lower andQ for higher frequencies. The frequency profiles were characteristic of the structure that shelled spheres were dispersed in a continuous medium. Gelatin aqueous solutions, cationic polyelectrolyte solutions and distilled water were loaded in the capsule interior to examine the effect of conductivities of the constituent aqueous phases on the dielectric properties. Relaxation frequencies of the relaxationsP andQ observed were directly proportional to the conductivities of the continuous medium and of the capsule interior, respectively. A dielectric theory was proposed for a suspension of shelled spheres in a continuous phase in order to analyze the relaxation data observed for the microcapsules. Volume fraction of the capsules, relative permittivity and conductivity of the capsule interior and thickness of the capsule wall are evaluated from the dielectric observations by use of the theoretical formulas derived. The dielectric behavior observed for the polystyrene microcapsules are interpreted quantitatively in terms of the dielectric theory proposed.     

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.     

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 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.     

Application of boundary element method to calculation of the complex permittivity of suspensions of cells in shape of Dinfinityh symmetry

A numerical method using the boundary element method was developed to calculate the complex permittivity of suspensions of particles in the shape of Dinfinityh symmetry covered with a shell phase. It was an extension of the analytical methods based on Maxwell-Wagner-Sillars' effects in suspensions of shelled ellipsoids. This method was applied to particles, which were relevant to budding yeast cells and erythrocytes, to examine the effects of the shape on frequency-dependence of the permittivity and conductivity of their suspensions. Results of the calculations showed that the permittivity and conductivity at high frequencies were insensitive to the change in the shape. The change in shape affected the permittivity and conductivity at low frequencies and their frequency-dependence in the intermediate frequency region. This behavior could not be imitated by the calculation using analytical methods with shelled spheroid models.     

The influence of polarizability on the dielectric spectrum of the ionic liquid 1-ethyl-3-methylimidazolium triflate

This work reports for the first time the computational, frequency-dependent dielectric spectrum of the polarizable molecular ionic liquid 1-ethyl-3-methylimidazolium triflate as well as its experimental analogue. In the frequency range from 500 MHz up to 20 GHz the agreement between the computational and the experimental spectrum is quantitative. For higher frequencies up to 10 THz the agreement is still remarkably good. The experimental asymptotic limit ε(∞) is 2.3. The difference in the computational value of 1.9 comes solely from the neglect of polarizability of the hydrogen atoms. For reasons of efficiency the simulations are based on the Lagrangian algorithm for the Drude oscillator model which cannot handle polarizable hydrogens. In the computational analysis the complete spectrum of the generalized dielectric constant ∑(0)*(ν) is splitted into its translational and non-translational components, called dielectric conductivity ?(0)(ν) and dielectric permittivity ε(ν). For 1-ethyl-3-methylimidazolium triflate both components contribute with equal weight and overlap in the complete frequency range. The inclusion of polarization forces, however, is quite different for the two components: the collective non-translational dynamics is accelerated and hence the dielectric permittivity is shifted to higher frequencies. The low frequency region of the dielectric conductivity is also affected while its high frequency part remains almost unchanged. Inductive effects are not only visible at high frequencies but also contribute in the sub-GHz region. The computational peak found in this region correlates with the experimental OKE-spectrum. It may be interpreted as the correlation between the induced dipole moment of the cations and the local electric field exerted by the anionic cage.     

Relaxations in thermosets. XVI. Dielectric studies of negative feedback during curing of an epoxide-ethylene-diamine thermoset

The complex dielectric permittivity of thermosets of diglycidly ether of bisphenol-A cured with ethylene diamine has been studied during their isothermal curing at several temperatures. As cross-linking progresses, the dc conductivity decreases. At the beginning of the cure the dc conductivity can be fitted to both the scaling laws with a critical exponent of about 4 and an equation indicating approach toward a singularity. In the later stage of the cure, the change in permittivity corresponds to dipolar relaxation of an infinitely connected network, and the Argand diagram for the complex permittivity measured at a fixed frequency obtained as the curing process proceeds at 305 K is similar to that for the complex permittivity as frequency is varied for a time-invariant system which obeys a stretched exponential relaxation function with the curing parameter or exponent, γ = 0.29. Increase in the temperature of isothermal curing lowers both γ and the net decrease in the equilibrium permittivity on curing. A plot of the calculated relaxation time with curing time is sigmoidal and shifts to shorter times on increasing the curing temperature. Measurement of the dielectric properties during the cure but for different frequencies show that the various parameters for the curing kinetics are independent of the frequency of measurement. These observations confirm the development of our concepts of thermoset curing in terms of a phenomenon of negative feedback between molecular diffusion and chemical reactions.     

Phase transitions in the antiferroelectric liquid crystal C8 tolane determined by dielectric and electro-optical measurements

Simultaneous dielectric permittivity measurements and analysis of the first and the second harmonic electro-optical responses have been performed on a 26 mum thick planar oriented sample of the antiferroelectric liquid crystal tolane C8, which has a polymorphism similar to that of MHPOBC. Comparison of these measurements for different frequencies and geometries (in the case of the electro-optical experiment) allows one to determine all the transition temperatures. The high frequency dielectric permittivity exhibits its maximum (depending on the frequency) inside the SmA phase, whereas the electro-optical coefficient is a maximum at the transition SmA-SmC_alpha* independent of the frequency. A new method for obtaining the temperature dependence of the relaxation frequency in the SmA phase is proposed.     

Phase transitions in the antiferroelectric liquid crystal C8 tolane determined by dielectric and electro-optical measurements

Simultaneous dielectric permittivity measurements and analysis of the first and the second harmonic electro-optical responses have been performed on a 26 mum thick planar oriented sample of the antiferroelectric liquid crystal tolane C8, which has a polymorphism similar to that of MHPOBC. Comparison of these measurements for different frequencies and geometries (in the case of the electro-optical experiment) allows one to determine all the transition temperatures. The high frequency dielectric permittivity exhibits its maximum (depending on the frequency) inside the SmA phase, whereas the electro-optical coefficient is a maximum at the transition SmA-SmC_alpha* independent of the frequency. A new method for obtaining the temperature dependence of the relaxation frequency in the SmA phase is proposed.     

Analysis of dielectric relaxations of w/o emulsions in the light of theories of interfacial polarization

An attempt is made to apply dielectric theories of interfacial polarization to observations of dielectric relaxations for W/O emulsions. Approximate formulas for disperse systems in a W/O type were derived from the two theories: one proposed by Maxwell and Wagner for dilute disperse systems of spherical particles, and the other developed by Hanai for concentrated disperse systems. Dielectric measurements were carried out on concentrated W/O emulsions prepared from kerosene and distilled water or KCl aqueous solutions by minimal use of emulsifiers. Marked dielectric relaxations were observed with the emulsions, the dielectric parameters having been determined to characterize the relaxation data. Phase parameters such as relative permittivity, electric conductivity and volume fraction of the disperse phase were evaluated from the dielectric parameters by use of the approximate formulas of the respective theories. The phase parameters evaluated and the frequency dependence of complex permittivity of the W/O emulsions deduced from the theory for concentrated disperse systems are in excellent agreement with the observed data in comparison to that for dilute disperse systems. It is concluded that the dielectric relaxations due to the interfacial polarization of disperse systems of spheres are explained satisfactorily by the theory for concentrated disperse systems.     

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.     

Dielectric spectroscopy of Anabaena 7120 protoplast suspensions

The dielectric spectroscopy of Anabaena 7120 protoplast suspensions has been investigated over the frequency range of 40 Hz-110 MHz. The protoplast suspensions showed a complicated dielectric dispersion consisting of at least four distinct sub-dispersions with the increasing frequencies due to the Maxwell-Wagner interfacial polarization. The double-shell model, in which an equivalent shell of thylakoid was assumed inside the cytoplasm, was adopted to describe the special morphology of the protoplast. Under the assumption that the conductivity of plasmalemma was negligible and the conductivity of the equivalent shell was 0.1 microS/cm, we attempted to estimate the dielectric properties of various protoplast components by fitting theoretical curve to experimental data. The relative permittivity of the plasmalemma epsilon(mem) was estimated to be 6.5+/-0.5, and the permittivity of the equivalent shell of thylakoid epsilon(thy) was estimated to be about 3.2+/-0.2. The permittivity epsilon(cyt) and conductivity kappa(cyt) of the cytoplasm were estimated to be 60 and 0.88+/-0.11 mS/cm, respectively. The permittivity epsilon(nuc) and conductivity kappa(nuc) of the nucleoplasmic region were determined to be 100 and 0.13+/-0.02 mS/cm, respectively.     

Dielectric analysis of sheep erythrocyte ghost. Examination of applicability of dielectric mixture equations

The purpose of this study is to confirm the applicability of dielectric mixture equations in dielectric analysis of biological cell suspensions. Two dielectric mixture equations, the Pauly-Schwan (P-S) equation and the Hanai-Asami-Koizumi (H-A-K) equation were tested using sheep erythrocyte ghosts whose internal solution is identical with the external solution. Dielectric measurements were carried out for the ghost suspensions over a frequency range 10 kHz to 100 MHz; a single dielectric relaxation was found between 100 kHz and 10 MHz. From the dielectric relaxation, the conductivity and permittivity of the ghost interior and the capacitance of the cell membrane were calculated following the P-S and H-A-K equations. When the H-A-K equation was employed (and as expected from the property of the ghosts), the estimated internal conductivity was in good agreement with the external conductivity at volume fractions up to about 0.7. With the P-S equation, on the other hand, the same results as above were obtained but only at low volume fractions below about 0.3. In addition, the H-A-K equation provided a better simulation for the observed relaxation curves than did the P-S equation, especially at high volume fractions. It is, therefore, concluded that the H-A-K equation is applicable to a wider range of volume fraction than is the P-S equation.     

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.     

Novel frequency dependence of dielectric biaxiality of surface stabilized ferroelectric liquid crystals

The frequency dependence of the dielectric biaxiality of surface stabilized ferroelectric liquid crystals (SSFLCs) was studied. The principal values of the dielectric tensor ε₁, ε₂ and ε₃ were measured by the MOM (molecular orientational model) method. Three dielectric permittivities were measured for each of two samples. These were the permittivity of the homeotropic cell and the permittivity of the planar homogeneous cell with and without the DC bias. Then the dielectric tensor components were calculated based on the molecular orientational models. We present the theory and experimental procedure of the MOM method. Measurements have been performed on Merck FLC compound SCE-8. The following novel dielectric behaviour was observed, as the DC bias voltage was increased the dielectric permittivity of the planar homogeneous cell decreased at the low frequencies (∼ 1 kHz) while increased at the high frequencies (10kHz ∼). The sign of the dielectric biaxiality ∂εε (= ε₂ - ε₁) inverted around 1 kHz, being negative at low frequencies and positive at high frequencies. The roles of the biaxiality on the dielectric behaviour of SSFLC cells are discussed.     

The Dynamic Mobility of Colloidal Semiconducting Antimony-Doped Tin(IV) Oxide Particles

The dynamic mobility spectra of suspensions of semiconducting tin(IV) oxide particles doped with antimony have been measured with the technique of electroacoustics. The magnitude of the complex mobility decreases essentially monotonically with increasing frequency, just as for a nonconducting (dielectric) particle under the same conditions. Unlike the case for a dielectric particle, however, the magnitudes at low frequency increase with increasing conductivity. The phase angle behavior is also different from that of a normal dielectric particle. The change in the phase angle behavior is most obvious at low suspension conductivity and high frequency where the phase angles showed a much smaller phase lag than at high conductivities. Reasonable agreement was found between the experimental mobility and the theoretical dynamic mobility spectra obtained with O'Brien's theory for the enhanced permittivity of semiconductors. Copyright 2001 Academic Press.     

Dielectric Dispersion and Electric Relaxation Processes Induced by Ionic Conduction in Formamide, 2-Aminoethanol and Their Binary Mixtures

The complex dielectric permittivity, ionic conductivity, electric modulus and impedance spectra of the dipolar molecules formamide (FA), 2-aminoethanol (AE) and their binary mixtures were investigated in the frequency range from 20 Hz to 1 MHz at 303.15 K. Debye-type distributions of the frequency dependent electric modulus and complex impedance were found, corresponding to an ionic conduction relaxation process in the upper frequency regime of the spectra, whereas a spike in the impedance spectra at low frequencies confirms the contribution of an electrode polarization (EP) relaxation process induced by ionic conduction. Due to the high static permittivity of FA, its ionic conductivity was found more than one order of magnitude higher than that of the AE, which is also shown by the comparative values of their EP and ionic conductivity relaxation times. The dependences of dc ionic conductivity values of the binary mixtures on their relaxation times and static permittivity were explored. The concentration dependent static permittivity and the relaxation times led us to infer the formation of a 1:1 H-bonded stable complex between FA and AE molecules with reduction in the number of effective parallel-aligned dipoles.     

Conductivity and dielectric properties of the monomer and polymer of the bis(p-toluene sulfonate) of 2,4-hexadiyne-1,6-diol

The electrical conductivity of single crystals of the bis(p-toluene sulfonate) of 2,4-hexadiyne-1,6-diol (TS) and its polymer (PTS) was measured in a temperature range 77–373 K (to 353 K for TS), dielectric properties were measured from 123 to 283 K. Anomalies of the conductivity and the real part of the complex dielectric permittivity of TS and PTS along the molecular stacking direction at the temperature corresponding to the low-temperature phase transitions were observed. The dependence of phase transitions on the polymerization conversion of the monomer TS was examined. The phase diagram of the TS-PTS mixed system was constructed.     

Dielectric analysis of chitosan gel beads suspensions: influence of low crosslinking agent concentration on the dielectric behavior

Dielectric measurements were carried out for the suspensions of chitosan gel beads (CGB) crosslinked with glutaraldehyde (GA), ranging in crosslinking time from 1 to 180 min at six fixed low GA concentrations over a frequency range from 40 to 110 MHz. The distinct dielectric relaxations observed in the frequency range of 10(4)-10(5) Hz were explained by the effects of the interfacial polarization. By fitting the experimental data with Cole-Cole equation the dielectric parameters of the suspensions were obtained, and the phase parameters were calculated using Hanai's method. The influence of crosslinking on the dielectric spectra of CGB was investigated by viewing the crosslinking time dependencies of conductivity increment (Deltakappa), relaxation frequency (f(0)), permittivity of CGB (epsilon(i)), conductivity of CGB (kappa(i)) and conductivity of continuous media (kappa(a)) at six fixed GA concentrations. Some information were gained, such as, thinner electric double layer, particle with rougher surface and more compact gel backbone with increasing crosslinking time. The basic information obtained can provide valuable references for the preparation of CGB and the application to more extensive fields such as controlled-release technology of medicament.