Electrokinetic actuation of an uncharged polarizable dielectric droplet in charged hydrogel medium

Electrokinetic transport of an uncharged nonconducting microsized liquid droplet in a charged hydrogel medium is studied. Dielectric polarization of the liquid drop under the action of an externally imposed electric field induces a non-homogeneous charge density at the droplet surface. The interactions of the induced surface charge of the droplet with the immobile charges of the hydrogel medium generates an electric force to the droplet, which actuates the drop through the charged hydrogel medium. A numerical study based on the first principle of electrokinetics is adopted. Dependence of the droplet velocity on its dielectric permittivity, bulk ionic concentration, and immobile charge density of the gel is analyzed. The surface conduction is significant in presence of charged gel, which creates a concentration polarization. The impact of the counterion saturation in the Debye layer due to the dielectric decrement of the medium is addressed. The modified Nernst–Planck equation for ion transport and the Poisson equation for the electric field is considered to take into account the dielectric polarization. A quadrupolar vortex around the uncharged droplet is observed when the gel medium is considered to be uncharged, which is similar to the induced charge electroosmosis around an uncharged dielectric colloid in free-solution. We find that the induced charge electrokinetic mechanism creates a strong recirculation of liquid within the droplet and the translational velocity of the droplet strongly depends on its size for the dielectric droplet embedded in a charged gel medium.     

Investigation of MWS polarization and dc conductivity in polyamide 610 using dielectric relaxation spectroscopy

Dielectric relaxation spectroscopy technique was employed to study the Maxwell–Wagner–Sillars (MWS) polarization and dc conductivity in polyamide 610. The experimental dielectric data were analyzed within the formalisms of complex permittivity and electric modulus. The results were discussed in terms of ac conductivity, MWS polarization, electrode polarization and dc conductivity. In the frequency spectra of polyamide 610, charge carriers movement resulted in high values of the dielectric permittivity. The results revealed that the motion of the polymer chains governs the charge carrier transport. Two different mechanisms for charge carrier movement showed a transition temperature located between 110 and 120 °C. The change of charge carrier movement mechanisms was resulted from the onset of the polymeric chains in the interphase between amorphous and crystalline phases.     

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.     

The impact of highly paramagnetic Gd3+ cations on structural,spectral, magnetic and dielectric properties of spinel nickel ferrite nanoparticles

In this work, fabrication of Gd³⁺ substituted nickel spinel ferrite (NiGd_xFe_2-xO₄) nanoparticles was carried out via co-precipitation route. X-ray powder diffraction (XRD) confirmed the spinel cubic structure of NiGd_xFe_2-xO₄ nanoparticles. XRD data also facilitated to determine the divalent and trivalent metal cations distribution at both A and B sites of the ferrite lattice. Site radii, hopping and bond lengths were also calculated from XRD data. The spectral studies elucidated the formation of cubic spinel ferrite structure as well as stretching vibrations of M–O (metal–oxygen) bond at A and B sites of ferrites, represented by two major bands υ₁ and υ₂ respectively. FESEM analysis confirmed the irregular morphology of NiGd_xFe_2-xO₄ nanoparticles. EDX spectrographs estimated the elemental compositions. The dielectric attributes were explained on the basis of the Debye-relaxation theory and Koop’s phenomenological model. At higher applied frequencies (AC) no prominent dielectric loss was observed. Magnetic parameter variations can be attributed to the substitution of the rare earth cations having larger ionic radii as compared to the radii of Fe³⁺ ions. Moreover, spin canting, magneto-crystalline anisotropy and exchange energy of electrons also helped in magnetic evaluation. Due to small coercivity values NiGd_xFe_2-xO₄ nanoparticles can be employed significantly in high-frequency data storage devices.     

Impedance spectroscopic study on ionic transport in a pH sensitive membrane

The characteristics of ionic transport under the influence of an AC electric field in a pH sensitive membrane made of poly(acrylamidocaproic acid) were studied using impedance spectroscopy. The dielectric spectra were obtained at various temperatures and for a range of pH values of HCl aqueous solutions in which the membrane was equilibrated. The contribution from the ionic transport which can be identified by a power dependency on frequency (i.e. ω⁻ⁿ; where 0 < n < 1) in the dielectric loss spectra, provided a means of determining the tortuosity of the long range ionic transport pathways.     

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.     

Hydrostatic pressure influence on static dielectric permeability of polymers

The generality of baric changes in static dielectric permeability of polymers for different temperatures of the sample is considered in the present work. It is shown that the initial growth of dielectric permeability is followed by its lowering that arises during the glass transformation of the sample. Formulas that reflect the changes of dielectric permeability under isothermal changes of pressure and isobaric changes of temperature of the sample are obtained. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2665–2669, 1998     

Dielectric spectroscopy on aqueous solutions of pyridine and its derivatives

The complex (dielectric) permittivity has been measured as a function of frequency between 1 MHz and 40 GHz for aqueous solutions of pyridine, 2- and 3-methylpyridine, as well as 2,4- and 2,6-dimethylpyridine at various temperatures and solute concentrations. Different relaxation spectral functions are used to analytically represent the data, in particular the Cole-Cole function. The solute contribution to the extrapolated static permittivity has been calculated to show that, in correspondence with other aqueous solutions of organic molecules and ions, the permittivity of the solvent seems to be enhanced with respect to the pure water value. Also in accordance with other aqueous systems it is found that the principal dielectric relaxation time for equimolar solutions of stereo isomers at the same temperature may significantly differ from one another. A further result is the finding of an unusually strong temperature dependence in the relaxation time of the 1 molar solution of 2,6-dimethylpyridine.     

Effect of dipole position on relaxation parameters

Dielectric absorption studies have been made on numerous monobromoalkanes and two,-dibromoalkanes dispersed in polystyrene and polypropylene matrices in the frequency range 10–10⁵ Hz between 80-285 K. All compounds exhibited two clear cut dispersion regions in polystyrene except 3-bromopentane, 4-bromoheptane and 4-bromooctane. Nevertheless, when these three solute molecules are dispersed in polypropylene, they then show two dispersion regions. The low temperature process (LT) has been attributed to -CHzBr group rotation about the C-C bonds and the high temperature (HT) process to molecular rotation. In the case of,-dibromoalkanes, both -CH₂Br groups contribute to the low temperature absorption. For a bromoalkane of a given chain length the relaxation time alters with variation of the main dipole position from the terminal carbon atom to the middle of the chain for the low temperature absorption; this is related to the change in the size of the reorientating unit containing the -CHzBr group. The effect of the same variation of the dipole position on the high temperature absorption is that the relaxation time decreases which is attributed to the decrease in volume swept out on molecular rotation. The ratio,_maxLT/_(max)HT, increases with the variation of the dipole from the terminal to the middle of the chain.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

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     

Electric dipole moments in polar solvents. II. Tetraisoamylammonium nitrate ion pairs in chlorobenzene. Effect of mutual polarization of the ions

The dielectric constant and conductivity of dilute solutions of tetraisoamylammonium nitrate in chlorobenzene are measured between –34.6° and 99.0°C to give association constants for the formation of ion pairs (K _A) and triple ions, and electric dipole moments. The quantityK _A as a function of temperature is reproduced by the Denison-Ramsey-Fuoss treatment for unolarized ion pairs [Eq. (2)] with a distance of closest approach of 4.90 Å. The dielectric data are reproduced by Onsager's equation with an inherent (gas-phase) dipole moment of the ion pairs of 14.2±0.3 D. Other methods of calculation lead to consistent dipole moments, confirming that the mutual polarization of the ions is important. The energetics of ionic association is considered on the basis that the ion pair may be treated as a polarizable dipole in a spherical cavity.     

Finite-element calculations for dielectric relaxation of one-sphere systems in a parallel-electrode measuring cell

Using the finite-element method (FEM), the frequency dependence of conductivity and relative permittivity was calculated for a heterogeneous specimen in which a sphere was placed in a medium. The FEM calculations were made for the whole system in which the specimen was located in a measuring cell comprising two parallel electrodes and a spacer with a cylindrical sample cavity, as in practical measurements. Values of the conductivity and the permittivity of the sphere and the medium were chosen so as to simulate specimens of water-in-nitrobenzene (W/N) and oil-in-water (O/W) types. When the volume ratio of the sphere to the sample cavity p was below 0.1, the frequency dependence of conductivity and permittivity was in good agreement with that described by Wagner's theory for both the W/N and O/W specimens. When p was greater than 0.1, deviations from Wagner's theory were found for limiting values of the conductivity and permittivity at low and high frequencies for both the W/N and O/W specimens, and for the relaxation time for the W/N specimens. Received: 15 July 1998 Accepted in revised form: 5 November 1998     

Electric polarization effects on the electronic spectral shift of centrosymmetric compounds

The classic dielectric dipolar Onsager model was extended to include quadrupolar interactions between solute molecules and solvents with different polarities. A multiparametric solvatochromic expression, based on the point quadrupole moment inside a spherical cavity embedded in a dielectric continuum, is applied to centrosymmetric sulfonamide porphyrins, zinc tetraphenyl porphyrin, squaraine and 9,10-dicyanoanthracene, in order to account for the quadrupolar polarization effect of solute molecules. The reaction field polarity functions created respectively by dipole and quadrupole moments are compared and found to be linearly correlated.     

The dielectric response of a colloidal spheroid

In this article, we present a theory for the dielectric behavior of a colloidal spheroid, based on an improved version of a previously published analytical theory [C. Chassagne, D. Bedeaux, G.J.M. Koper, Physica A 317 (2003) 321–344]. The theory gives the dipolar coefficient of a dielectric spheroid in an electrolyte solution subjected to an oscillating electric field. In the special case of the sphere, this theory is shown to agree rather satisfactorily with the numerical solutions obtained by a code based on DeLacey and White's [E.H.B. DeLacey, L.R. White, J. Chem. Soc. Faraday Trans. 2 77 (1981) 2007] for all zeta potentials, frequencies and κa1 where κ is the inverse of the Debye length and a is the radius of the sphere. Using the form of the analytical solution for a sphere we were able to derive a formula for the dipolar coefficient of a spheroid for all zeta potentials, frequencies and κa1. The expression we find is simpler and has a more general validity than the analytical expression proposed by O'Brien and Ward [R.W. O'Brien, D.N. Ward, J. Colloid Interface Sci. 121 (1988) 402] which is valid for κa1 and zero frequency.     

Effect of curing on the broadband dielectric spectroscopy of powder coating

The effect of curing process of thermosetting powder coating consists of carboxylated polyester resin cured with triglycidyl isocyanurate has been investigated using broadband dielectric relaxation spectroscopy over a wide range of frequency (10⁻¹-10⁶ Hz) and temperature (70-105 °C) for different constant curing times. The molecular dynamics of the glass relaxation process (α-process) was investigated as a function of curing time, frequency, and temperature. It has been found that, only one common α-relaxation process has been observed for all measured samples of different degree of curing stages, its dynamics and broadness were found to be curing time dependent. In addition, the curing time dependence of the dielectric relaxation strength, Δε, has also been examined for the α and β-relaxation processes. The Δε for the two relaxation processes decreased strongly at the beginning of curing process and then became almost constant at longer curing times. This finding implied that the numbers of reoriented dipoles decrease with curing time as a result of the formation of three-dimensional polymer network. Furthermore, the dislocation energy, ε_s, calculated from the Meander model was found to be increased with increasing the curing time, i.e. the formation of a three-dimensional polymer network produces many structural defects or dislocation points. In addition, the activation energy of the curing process was calculated from the analysis of the calorimetric exothermic peaks of the curing process at different heating rates.     

Simulation of volume polarization for the influence of solvation on chemical shielding

The effect of bulk dielectric solvation on chemical shielding at nitrogen in CH₃CN is studied with reaction field theory. A previous work has demonstrated the strong influence on this property from volume polarization, which describes that part of the reaction field arising from solute charge density penetrating outside its cavity. The essentially exact treatment of volume polarization used in that work is computationally demanding, and a more facile method for simulation of the volume polarization has recently been proposed. It is found in the present work that this simulation of the volume polarization yields results in excellent agreement with the essentially exact treatment of the strong volume polarization effects on nitrogen shielding in CH₃CN.Contribution to the Jacopo Tomasi Honorary Issue     

Towards understanding the effect of electrostatic interactions on the density of ionic liquids

In order to have a better understanding on the electrostatic contribution to the thermodynamic property of ionic liquids (ILs), a two-parameter equation of state (EOS) is developed on the basis of hard sphere perturbation theory by accounting for the dispersion interaction with Cotterman et al.’s EOS for L-J fluid and electrostatic interaction with mean spherical approximation (MSA) approach. The EOS is applicable for the density correlation of molecular liquids, and the resulting parameters, viz. Lennard–Jones dispersive parameter ?/k and soft-core diameter σ, can be used to predict the density of molecular mixtures and the corresponding ILs. The results indicate that the density of IL is always about 10% higher than the corresponding stoichiometric molecular mixture with which the IL is produced as an ionic adduct, for example, IL 1-methyl-3-methylimidazolium dimethylphosphate ([MMIM][DMP]) versus equimolar mixture of 1-methylimidazole (MIM) and trimethylphosphate (TMP). Furthermore, the density enhancement of ILs with respect to their corresponding stoichiometric molecular mixtures can be well represented by the electrostatic contribution among ionic species involved.     

A broad-band dielectric study of poly(vinyl chloride): Effect of molar mass and processing conditions on space-charge mechanisms

The dielectric properties of poly(vinyl-chloride) (PVC) were investigated as a function of frequency and temperature. Attention was focused on the low frequency dielectric properties. Samples with different molar masses were studied. Two dipolar retardation processes were observed, one due to a local motion in the polymeric chain and the other at the glass transition. In compression-molded samples of high molar mass an additional low-frequency space-charge polarization mechanism was found at high temperatures. The effect was most pronounced at the highest molar masses. The space-charge mechanism was absent in samples of well-kneaded material. This macroscopic polarization process in compression molded samples is probably due to discontinuities in the poorly gelified material because of a residual grain particle structure. The detected effects of air inclusions in a PVC matrix can be described by the Maxwell-Wagner theory. The magnitude of the space-charge mechanism is an indication of the effectiveness of the processing method used in destroying the grain structure. © 1994 John Wiley & Sons, Inc.     

The effect of the linear charge density of carrageenan on the ion binding investigated by differential scanning calorimetry, dc conductivity, and kHz dielectric relaxation

The effect of the linear charge density of natural polyelectrolyte, carrageenan, on the ion binding to carrageenan molecules in relation to the gelation was investigated by using the dielectric relaxation spectroscopy, dc conductivity, optical rotation, and differential scanning calorimetry (DSC). Although carrageenan is an anionic polysaccharide, carrageenan molecules in the helix state at low temperatures can bind not only cation, such as potassium and cesium, but also anion, such as iodide. The dc conductivity steeply decreases just below the coil–helix transition temperature, which indicates the binding of ion to the carrageenan molecules in the helix state due to the increase of the linear charge density compared with that in the coil state. The addition of NaI promotes the helix formation, and prevents from aggregation of helices, which was suggested by the results of the dynamic shear modulus and the DSC, and resulted in an increase of the relaxation amplitude of the lowest frequency relaxation (kHz) attributed to the fluctuation of the tightly bound counter ions along the high charge density region (helix). It is concluded that binding of iodide induces (1) the increase in the amount of tightly bound counterions to carrageenan molecules and (2) the formation of non-aggregated helix.