Dielectric properties of aqueous zwitterionic liposome suspensions

The dielectric spectra of aqueous suspensions of unilamellar liposomial vesicles built up by zwitterionic phospholipids (dipalmitoylphosphatidyl-choline, DPPC) were measured over the frequency range extending from 1 kHz to 10 MHz, where the interfacial polarization effects, due to the highly heterogeneous properties of the system, prevail. The dielectric parameters, i.e., the permittivity epsilon'(omega) and the electrical conductivity sigma(omega), have been analyzed in terms of dielectric models based on the effective medium approximation theory, considering the contribution associated with the bulk ion diffusion on both sides of the aqueous interfaces. The zwitterionic character of the lipidic bilayer has been modeled by introducing an "apparent" surface charge density at both the inner and outer aqueous interface, which causes a tangential ion diffusion similar to the one occurring in charged colloidal particle suspensions. A good agreement with the experimental results has been found for all the liposomes investigated, with size ranging from 100 to 1000 nm in diameter, and the most relevant parameters have briefly discussed in the light of the effective medium approximation theory.     

Solvent effects on the dielectric dispersion of poly(vinyl pyrrolidone)-poly(ethylene glycol) blends

The complex relative dielectric function , loss tangent , complex electric modulus and alternating current electrical conductivity dispersion behaviour of liquid poly(vinyl pyrrolidone)-poly(ethylene glycol) (PVP–PEG) blends in water, ethyl alcohol and 1,4–dioxane solvent over the entire volume mixture concentration range has been investigated in the frequency range of 20 Hz to 1 MHz at 25 °C. The PVP–PEG blends show the dielectric dispersion corresponding to the micro-Brownian motion of the PVP chain in the upper frequency region, whereas in the lower frequency region, dielectric dispersion is due to ionic conduction and electrode polarization phenomena. Results show that the conductivity values of these blends in water and dioxane can be monitored with the change in the solvent concentration, whereas it has a small variation with ethyl alcohol concentration. The comparative dielectric dispersion shape study confirms that the chain dynamics of polymers blend is influenced by heterogeneous interactions and solvent polar strength.     

AC Electrical Properties of Plasma Polymerized <Emphasis Type="Italic">o</Emphasis>-Methoxyaniline Thin Films

Alternate current (ac) electrical properties of the plasma polymerized o-methoxyaniline (PPOMA) thin films synthesized in the glow discharge plasma using a capacitively coupled reactor are studied. Measurement revealed that the ac electrical conductivity varies with frequency ω as ω ⁿ , where the exponent n is less than unity in the range 0.1 to 2.0 kHz, indicating the Debye type conduction mechanism in the PPOMA thin films, while above this frequency range the exponent is become greater than unity indicating non-Debye type conduction. At low frequencies the conduction is considered to be due to hopping of carriers between the localized states. The PPOMA thin films of thicknesses 100?250 nm possesses dielectric constant <10, which remains static in the range 0.1?10 kHz, and decreases at higher temperature due to the orientation polarization. The dielectric loss increases with the increase in frequency having a peak around or above 10 kHz for all the PPOMA films of different thicknesses. Cole-Cole plot between the real and imaginary dielectric constant exhibits single relaxation mechanism in the PPOMA thin films.     

Hydrothermal Synthesis and Dielectric Characterization of a Double Perovskite Ba2FeSbO6

A double perovskite oxide Ba₂FeSbO₆ was hydrothermally synthesized and structurally characterized by X-ray diffraction. This solid compound shows a single phase and has a trigonal structure with space group R m and cell parameters of a=0.57261 nm and c=1.40244 nm. The dielectric constant and loss tangent of the solid measured in a frequency range from 100 Hz to 1 MHz at temperatures from 313 K to 513 K reveal a relaxation process of frequency dependence of the real part(ε') of dielectric constant and dielectric loss tand. The frequency dependence of electrical property led to the framework of conductivity and electric modulus formalisms. The scaling behavior of imaginary part of electric modulus suggests that the relaxation describes the single mechanism at various temperatures. The variation tendency of the alternating current impedance indicates the thermally activated conduction process follows Jonsche’s power law.     

Analysis of the dielectric permittivity of suspensions by means of the logarithmic derivative of its real part

Measurement of the dielectric permittivity of colloidal suspensions in the kilohertz frequency range (the so-called low-frequency dielectric dispersion) is a promising tool for the electrokinetic characterization of colloids. However, this technique is less used than would be desirable because of the difficulties associated with the measurements, the most important of which is the electrode polarization (EP). Recently (M. Wübbenhorst and J. Van Turnhout, Dielectrics Newsl. November (2000)) a method was proposed that appears capable of separating the unwanted electrode effects from the double-layer relaxation that we are interested in. The method, based on the logarithmic derivative of raw epsilon'(omega) data (epsilon'(omega) is the real part of the permittivity of the suspension for a frequency omega of the applied AC field), is first checked against the well-known theory of the AC permittivity of colloidal suspensions developed by DeLacey and White (E. H. B. DeLacey and L. R. White, J. Chem. Soc. Faraday Trans. 277, 2007 (1981)). We show that the derivative epsilon'(D)(omega)=-(pi/2)(partial differential epsilon'/partial differential ln omega) gives an excellent representation of the true imaginary part of the permittivity, epsilon'(omega). The technique is then applied to experimental data of the dielectric constant of polystyrene and ethylcellulose suspensions. We found that epsilon'(D) displays two separated behaviors when plotted against log omega in the frequency range 100 Hz-1 MHz: a monotonous decrease (associated with EP) followed by an absorption peak (associated with the double-layer relaxation, or alpha-relaxation). Interestingly, they are separated enough to make it possible to easily find the characteristic frequency of the alpha-relaxation. Fitting a relaxation function to epsilon'(D)(omega) after eliminating the part due to EP, we could calculate the real part epsilon'(omega) and compare it to the DeLacey and White (DW) theoretical predictions. A significantly better agreement between DW calculations and experimental epsilon'(omega) data is obtained when the logarithmic derivative method is used, as compared to the classical electrode-separation techniques.     

Dielectric response of polar liquids in narrow slit pores

Based on molecular dynamics (MD) simulations and a simple (Stockmayer) model we investigate the static and dynamic dielectric response of polar liquids confined to narrow slit pores. The MD simulations are used to calculate the time-dependent polarization fluctuations along directions parallel and perpendicular to the walls, from which the components of the frequency-dependent dielectric tensor can be derived via linear response theory. Our numerical results reveal that the system's response is strongly anisotropic. The parallel dielectric function, epsilonparallel(omega), has Debye-like character very similar to the corresponding isotropic bulk function, epsilonbulk(omega), at the same chemical potential. Indeed, the main confinement effect on epsilonparallel(omega) consists in a shift toward smaller values relative to the bulk function. On the other hand, in the perpendicular direction we observe a characteristic peak in the absorption part of the dielectric function, epsilonperpendicular(omega). This peak is absent in the bulk system and reflects strongly pronounced, damped oscillations in the polarization fluctuations normal to the walls. We discuss two possible origins of the oscillations (and the resulting absorption peak), that is collective oscillations of dipoles in clusters formed parallel to the walls, and the existence of a "dipolaron mode" previously observed in MD simulations of bulk polar fluids.     

Dielectric and shear mechanical relaxations in glass-forming liquids: a test of the Gemant-DiMarzio-Bishop model

The Gemant-DiMarzio-Bishop model, which connects the frequency-dependent shear modulus to the frequency-dependent dielectric constant, is reviewed and a new consistent macroscopic formulation is derived. It is moreover shown that this version of the model can be tested without fitting parameters. The reformulated version of the model is analyzed and experimentally tested. It is demonstrated that the model has several nontrivial qualitative predictions: the existence of an elastic contribution to the high-frequency limit of the dielectric constant, a shift of the shear modulus loss peak frequency to higher frequencies compared with the loss peak frequency of the dielectric constant, a broader alpha peak, and a more pronounced beta peak in the shear modulus when compared with the dielectric constant. It is shown that these predictions generally agree with experimental findings and it is therefore suggested that the Gemant-DiMarzio-Bishop model is correct on a qualitative level. The quantitative agreement between the model and the data is on the other hand moderate to poor. It is discussed if a model-free comparison between the dielectric and shear mechanical relaxations is relevant, and it is concluded that the shear modulus should be compared with the rotational dielectric modulus, 1(epsilon(omega)-n2), which is extracted from the Gemant-DiMarzio-Bishop model, rather than to the dielectric susceptibility or the conventional dielectric modulus M=1epsilon(omega).     

Conductivity measurement and dielectric,impedance and modulus spectroscopic studies on bis (P-nitrophenol) melaminium monohydrate

The present work is aimed to study the dielectric and electrical characteristics of bis (P-nitrophenol) melaminium monohydrate (BNPM) over the temperature and frequency ranges of 333–453 K and 50 Hz–5 MHz, respectively. The dielectric properties such as dielectric constant (ε′), dielectric loss (ε″), ac-conductivity (σ_ac), real (M′) and imaginary part (M″) of dielectric modulus were investigated as a function of temperature and frequency. The impedance analysis has been carried out using Cole-Cole plots to elucidate the electrical conduction mechanism. Further, observations indicated that the grain boundaries are more resistive and capacitive than the grains, inducing inhomogeneity in the material, which, in turn, causes broad frequency-dependent dielectric anomalies. The observed frequency-dependence of the AC conductivity validated the Jonscher power law. Further, an asymmetric dispersion peak in the imaginary part of the electrical modulus (M”) was noticed, indicating that the fabricated material exhibits non-Debye relaxation behavior.     

Mechanical and electrical properties of Ni0.65Zn0.35CuxFe2−xO4 ferrite

A series of samples in the system Ni_0.65Zn_0.35Cu_xFe_2?xO₄ (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. X-ray analysis showed that they were cubic spinel (single phase). Young's modulus, the dielectric loss and the change in capacitance under mechanical stress were measured for the samples. Young's modulus decreased with increasing Cu content. This is due to the fact that Cu²⁺ ions entered the lattice substitutionally for Fe³⁺ ions at the octahedral sites, creating lattice vacancies gave rise to lattice strain. The minimum value of the dielectric loss corresponding tox=0.3 may be due to the formation of lattice vacancies retarding the jump frequency to be far from the frequency of the applied a.c. field. The increase in capacitance of the samples with mechanical stress may be explained via the mechanism of dielectric polarization.     

Dielectric relaxation in a water-oil-triton X-100 microemulsion near phase inversion

A mixture of water (10 mM KCl), toluene and Triton X-100 (40:40:20 wt %) shows temperature-dependent phase inversion. The phase inversion has been studied by dielectric spectroscopy over a frequency range of 10 Hz to 1 GHz. At temperatures above about 37 degrees C, dielectric relaxation appeared around 10 MHz, which was due to interfacial polarization in a water-in-oil type emulsion. The dielectric relaxation drastically changed between 30 and 25 degrees C. With decreasing temperature, the intensity of dielectric relaxation increased steeply below 30 degrees C to attain a peak at 27 degrees C, where that change was associated with an increase in low-frequency conductivity by about three orders between 30 and 26 degrees C. The dielectric behavior has been interpreted in terms of interfacial polarization with a percolation model in which spherical water droplets, arranged in array in a continuous oil phase, are randomly connected with their nearest neighbors using water bonds.     

Structural dependence of phase transition and dielectric relaxation in ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s

A series of ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s, P(VDF-CTFE-TrFE), with systematically varied chemical compositions have been synthesized via a two-step approach consisting of copolymerization and dechlorination. The effect of polymer structure on polarization responses and dielectric properties has been investigated over a broad frequency and temperature range. As shown in the X-ray diffraction patterns, multiple phases coexist within the terpolymers as a result of the gauche conformation induced by the CTFE unit. The polarization hysteresis loops reveal the variation of remanent polarization and coercive electric field with the CTFE content due to the changes of crystallinity and crystalline phase. The observed broad dielectric constant peak with Vogel-Fulcher dielectric dispersion behavior suggests a transformation from a normal ferroelectric to a ferroelectric relaxor of the polymers. The relationship between the local relaxation process and relaxor ferroelectric behavior has been examined on the basis of the dielectric and mechanical loss tangents as a function of temperature.     

中相微乳液的介电弛豫谱解析及弛豫机制研究

对由SDS、CTAB、正庚烷、正丁醇和盐水组成的中相微乳体系进行了介电测量,在不同盐度下低频段出现显著的介弛豫现象,它给出了弛豫时间随盐度的变化关系。通过解析介电谱获得了体系内部结构和电性质等信息,几个介电模型和理论公式被用来解释解析的结果。对弛豫时间的计算结果发现,该弛豫是由多种极化机制参与、界面极化为支配的动力学过程。介电解析和模型计算相互验证,结果暗示着体系盐度变化对中相微乳结构变化的影响和可能的转化机制。     

Dielectric relaxation spectroscopy in poly(hydroxyethyl acrylates)/water hydrogels

The electrical and dielectric properties of poly(hydroxyethyl acrylate), PHEA, hydrogels were studied by means of dielectric relaxation spectroscopy in wide ranges of frequencies (5–2 × 10⁹ Hz), temperatures (173–363 K) and water contents (0.065–0.46, g of water per gram of dry material). The secondary dipolar mechanisms (γ and β_sw) and the dc conductivity mechanism were studied in detail by analyzing the dielectric susceptibility data within the complex permittivity formalism, the modulus formalism, and the complex impedance formalism. For both mechanisms molecular mobility was found to increase with increasing temperature or increasing water content (T-f-h superposition principle). The energy parameters and the shape parameters of the response were determined for both mechanisms at several water contents and temperatures. The temperature dependence of dc conductivity was found to change from Vogel-Tamman-Fulcher (VTF) type to Arrhenius type at water contents of ca. 0.30. At water contents lower than about 0.30 the hydrogels are homogeneous whereas at higher water contents a separate water phase appears. In terms of the strong/fragile classification scheme our results suggest that the PHEA hydrogels are fragile systems. ©1995 John Wiley & Sons, Inc.     

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 Behavior of Middle Phase Microemulsion

l ntroductionMicroemulsiolls are transparent and nuid systems composed of water, oil andamphiphilic molecules. They form spontaneously with simple mixing. Microemulsionsmay be water- or oil- continuous and bicontinuous under some conditions of temperatureor composition. Middle phase microemulsions(MPME) is called the 'bicontinuousstate' which is considered to be a sponge-like random netWork. Owing to its ultralowinterfacial tellsiolls against the excess water and oil phase with which they ar…     

Graphite nanoplatelets/polymer nanocomposites: thermomechanical,dielectric, and functional behavior

Exfoliated graphite nanoplatelets (GNP)/epoxy resin nanocomposites were prepared and tested, varying the amount of the filler content. Systems’ morphology was investigated by means of scanning electron microscopy, while their thermal response was examined via differential scanning calorimetry (DSC). Broadband dielectric spectroscopy and dynamic mechanical thermal analysis were employed in order to characterize the produced systems. Static mechanical tests were also conducted at ambient. Reinforced systems exhibit improved performance under mechanical and electrical excitation. In particular, storage modulus increases systematically with GNP content. DSC results imply that glass transition temperature is not affected by the presence of GNP. Flexural modulus and storage modulus, as determined by static and dynamic mechanical tests, respectively, increased with filler content. Dielectric permittivity increases also systematically with GNP content. Recorded relaxation processes arise from the glass to rubber transition of the polymer matrix (α-mode), re-orientation of polar side groups of the polymer chains (β-mode), and interfacial polarization because of the accumulation of charges at the systems’ interface. Finally, the energy storing efficiency of the nanocomposites enhances with reinforcing phase in the examined frequency and temperature range. Optimum performance corresponds to the nanocomposite with maximum GNP loading.     

Effect of electrolyte concentration on the dynamic surface tension and dilational viscoelasticity of adsorption layers of chitosan and dodecyl chitosan

The effect of an external salt (AcONa) on the kinetics of adsorption and structure formation inside the adsorption layers (ALs) of chitosan (Ch) and dodecyl chitosan (C12Ch) as well as on the frequency dependence of the complex dilational elasticity modulus of these layers has been studied. The complex dilational elasticity modulus of adsorption layers of polymers has been measured on the drop tensiometer (Tracker, IT Concept, France) upon applying a small sinusoidal variation of the drop area with a given frequency, omega, in the range from 10(-2) to 0.63 rad/s and recording the variation of the surface pressure. It has been found that, in the absence of the salt, the dilational storage modulus, E'(omega), of ALs of both Ch and C12Ch is lower with regard to the loss modulus, E' '(omega), in the whole range of frequencies used, testifying for the liquidlike rheological behavior of these layers. With an increase of the salt concentration up to CAcONa > 0.1 M, the ALs become solidlike, as shown when E'(omega) > E' '(omega). Consequently, the characteristic frequency, omega c, corresponding to the intercept between the E'(omega) and E' '(omega) curves, gradually varies from omega c > 1 rad/s to omega c < 0.01 rad/s when the salt concentration is increased from zero to CAcONa = 1 M. Hydrophobically modified C12Ch, having long grafted alkyl chains, exhibited a higher sensitivity to the presence of salt than Ch: the former solidifies more readily and at lower salt concentrations than the latter. It has been found that the experimental E'(omega) and E' '(omega) curves exhibit two characteristic relaxation frequencies, omega 01 approximately 1 rad/s and omega 02 approximately 10(-3)-10(-2) rad/s, whose physical meaning and values were related to the structure of the ALs and to the competitive contribution of electrostatic and hydrophobic interactions between amino and nonpolar groups of Ch and C12Ch to the formation of a gel-like network inside the polymeric film at the interface.     

Dielectric model and theoretical analysis of cationic reverse micellar solutions in CTAB/isooctane/n-hexanol/water systems

Dielectric relaxation spectra of CTAB reverse micellar solutions, CTAB/isooctane/n-hexanol/water systems with different concentrations of CTAB and different water contents, were investigated in the frequency range from 40 Hz to 110 MHz. Two striking dielectric relaxations were observed at about 10(4) Hz and 10(5) Hz, respectively. Dielectric parameters were obtained by fitting the data using the Cole-Cole equation with two Cole-Cole dispersion terms and the electrode polarization term. These parameters show different variation with the increase of the concentration of CTAB or the water content. In order to explain the two relaxations systematically and obtain detailed information on the systems and the inner surface of the reverse micelles, an electrical model has been constituted. On the basis of this model, the low-frequency dielectric relaxation was interpreted by the radial diffusion of free counterions in the diffuse layer with Grosse model. For the high-frequency dielectric relaxation, Hanai theory and the corresponding analysis method were used to calculate the phase parameters of the constituent phases in these systems. The reasonable analysis results suggest that the high-frequency relaxation probably originated from the interfacial polarization. The structural and electrical information of the present systems were obtained from the phase parameters simultaneously.     

Wide-frequency-range dielectric response of polystyrene latex dispersions

In this work we analyze the dielectric properties of dilute colloidal suspensions of nonconducting spherical particles with a thin electrical double layer from experimental data obtained by performing impedance spectroscopy experiments over a broad frequency range, from 20 Hz to 1 GHz. The electrode polarization correction was made by fitting a circuit model in the complex impedance plane (impedance spectrum) using a constant phase angle (CPA) element to fit the electrode polarization in series with the sample impedance. This simple procedure is found to be effective in eliminating the electrode contribution. The dielectric response shows two different dispersions, the alpha relaxation (counterion relaxation) that occurs at low kilohertz frequencies, and the delta relaxation (Maxwell-Wagner effect) found in the MHz range. These are reasonably well fitted over a broad frequency range by the theoretical expressions given by a simplified standard model (not including anomalous conduction) and a generalized model (including anomalous conduction) for the low-frequency dispersion, plus Maxwell-Wagner-O'Konski theory for the delta relaxation in the mid-frequency range. An analysis was also made of the need to include, for these latices, the effects of ion mobility in the Stern layer in order for the values of the zeta-potential obtained from electrophoretic and dielectric data to be compatible with each other.     

Study on conductivity and dielectric behavior of chemically synthesized polypyrrol dodecylbenzene sulfonic acid blended with poly(methyl methacrylate)

Polypyrrole (PPy) doped with dodecylbenzene sulfonic acid was synthesized and was blended with compatible polymer PMMA in chloroform. Flexible and free-standing films with compositions PPy: PMMA = 10: 90, 20: 80, 30: 70, and 50: 50 were obtained. The percentage of crystallinity and particle size of synthesized polymers were estimated from X-rays diffraction studies. Scanning Electron Micrographs showed that phase separation was observed and compatibility of the mixture decreased with increase of PMMA content. The dielectric measurements were performed in the frequency range 0.1 kHz–1 MHz in temperature range 303–473 K. The frequency dependent conductivity (σ_ac) obeyed a power law of frequency with an exponent s < 1. Electric modulus formalism exhibits a peak in frequency. The peak of conductivity relaxation shifted towards higher frequencies and the magnitude of relaxation decreased with the increase of PMMA content in the composites.