Computer simulation and the dielectric constant of polarizable polar systems

A theory presented recently, allowing the static and frequency-dependent dielectric constant of polar systems to be calculated in computer simulations with arbitrary boundary conditions, is extended to the polarizable case. For modified dipolar interactions not explicitly depending upon time, the resulting equations are found to be of the generalized Fröhlich-Glarum type.     

The effect of dielectric constant on the electrical conductivity in double liquid systems

The effect of dielectric constant on the electrical conductivity of double systems containing chemically noninteracting components is considered. An equation has been obtained by means of which it is possible to evaluate the isotherm for electrical conductivity of such systems, the equation being confirmed by all available experimental material.     

Accurate calculation of the dielectric constant of water from simulations of a microscopic droplet in vacuum

We present a successful calculatioon of the static dielectric constant of water based on simulations of microscopic droplets in vacuum. A 1.5 ns simulation of a 24 Å radius sphere of TIP3P water gives ε = 82 ± 5 at 292 K; a 2 ns simulation of a 14 Å sphere gives ε = 82 ± 5 at 294 K. Inclusion of all electrostatic interactions in the droplet was critical to success. Polar fluctuations in the inner half of the droplet are bulk-like, due to efficient screening by the outer half, despite the surrounding vacuum. This suggests that droplet simulations of more complex systems, such as proteins in water droplets, can successfully account for solvent screening of charges near the center of the simulation sphere.     

Broadband dielectric studies of weakly polar and non-polar liquid crystals

Complex dielectric permittivities, for two orientations of the director n, parallel (E || n) and perpendicular (E ⊥ n) to the probing electric field E, of the weakly polar liquid crystals (LCs) 4,4'-dihexylazoxybenzene (D6AOB) and 4,4'-diheptylazoxybenzene (D7AOB) as well as the non-polar LC diheptylazobenzene (D7AB) have been measured in the frequency range 75 kHz to 1 GHz. The measurements were performed in the nematic, smectic and isotropic phases of the LCs. The dielectric anisotropies Δε (=ε_||-ε_⊥) obtained from the values of dielectric permittivities at 100 kHz in the nematic phase were found to increase with decreasing temperature. However, for the DnAOBs, the Δε values are somewhat smaller than that for D7AB which does not have a permanent dipole moment. In the nematic phase two molecular relaxation processes were observed for both DnAOBs in each of the orientations—parallel and perpendicular. The four processes merge into two separate processes in the isotropic phase. For D7AB no orientational relaxations were observed in the experimental frequency range.     

New 1,3-dioxan-type ionic liquid crystal compounds having a large dielectric constant

New pyridinium-type thermotropic ionic liquid crystal materials having a 1,3-dioxan ring in its central core: N-substituted-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides (6) were synthesized. The mesomorphic behaviour of these compounds and dielectric constant perpendicular to the molecular axis were measured. The principal features of these compounds are that they exhibit a smectic A phase over a wide temperature range including room temperature - for example 6f: g -9 SmA 181 I (°C)-and they have a large dielectric constant perpendicular to the molecular axis.     

New 1,3-dioxan-type ionic liquid crystal compounds having a large dielectric constant

New pyridinium-type thermotropic ionic liquid crystal materials having a 1,3-dioxan ring in its central core: N-substituted-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides (6) were synthesized. The mesomorphic behaviour of these compounds and dielectric constant perpendicular to the molecular axis were measured. The principal features of these compounds are that they exhibit a smectic A phase over a wide temperature range including room temperature - for example 6f: g -9 SmA 181 I (°C)-and they have a large dielectric constant perpendicular to the molecular axis.     

Impact of ferroelectric nanoparticles on the dielectric constant of nematic liquid crystals

ABSTRACT

The influence of ferroelectric BaTiO₃ nanoparticles on the dielectric constant in the isotropic phase of the isotropic-nematic phase transition is studied within Landau phenomenological theory. The ferroelectric BaTiO₃ nanoparticles dependence of transition temperature and dielectric constant in the isotropic phase is calculated. We compare our theoretical results with experimental data available in the literature.     

Superdipole liquid scenario for the dielectric primary relaxation in supercooled polar liquids

We propose a dynamic structure of coupled dynamic molecular strings for supercooled small polar molecule liquids and accordingly we obtain the Hamiltonian of the rotational degrees of freedom of the system. From the Hamiltonian, the strongly correlated supercooled polar liquid state is renormalized to a normal superdipole liquid state. This scenario describes the following main features of the primary or alpha-relaxation dynamics in supercooled polar liquids: (1) the average relaxation time evolves from a high temperature Arrhenius to a low temperature non-Arrhenius or super-Arrhenius behavior; (2) the relaxation function crosses over from the high temperature exponential to low temperature nonexponential form; and (3) the temperature dependence of the relaxation strength shows non-Curie features. According to the present model, the crossover phenomena of the first two characteristics arise from the transition between the superdipole gas and the superdipole liquid. The model predictions are quantitatively compared with the experimental results of glycerol, a typical glass former.     

Brownian dynamics simulation of a model simple electrolyte in solvents of low dielectric constant

Brownian dynamics simulations are performed to investigate the ionic transport of model simple electrolytes, in which ions are interacting with each other through the repulsive core and Coulombic interactions. The equivalent conductivity and self-diffusion coefficient show minima as the function of the number density of ions when the dielectric constant of the solvent is low. Although the minimum of the former is in harmony with various experiments, no experiment has ever been reported on that of the latter. The analysis of time-dependent transport coefficients reveals that the presence of the minima is ascribed to the slow dynamics, rather than to static association models. The inclusion of a model function that resembles the short-range part of the potential of mean force induced by solvent affects the transport coefficients qualitatively, which suggests the importance of solvent-induced potential of mean force in the conduction mechanism of electrolytes in solvents of low dielectric constant.     

The determination of the critical concentration of a binary mixture containing a polar component by dielectric measurements

The step in the dielectric constant that occurs upon cooling a binary liquid mixture below its phase separation is further investigated and explained. It is shown that this phenomenon can be used to determine the critical concentration (x_c).     

Electrical conductivity and dielectric constant measurements of liquid crystal-gold nanoparticle composites

The behaviour of the anisotropic electrical conductivity of liquid crystal-gold nanoparticle (LC-GNP) composites consisting of a commercially available room temperature nematic compound doped with alkylthiol-capped GNPs has been investigated. The nematic-isotropic transition of the composite decreases nearly linearly with increasing X, the concentration of GNP (in weight %) at a rate of about 1°C /weight %. The inclusion of GNPs increases the electrical conductivity of the system with the value increasing by more than two orders of magnitude for X = 5%. However, the anisotropy in conductivity, defined as the ratio of the conductivity along (σ_∥) and orthogonal (σ_⊥) to the director shows a much smaller but definite decrease as X increases.     

Phthalocyanine with a giant dielectric constant

Compound 1 has been prepared by the reaction of 4-nitrophthalonitrile and trans-2-methoxy-4-(2-nitrovinil)phenol by the common method of nucleophilic substitution of an activated nitro group in an aromatic ring. The metallophthalocyanines 2, 3 were prepared by the reaction of a dinitrile derivative with Co(OAc)(2) or Zn(OAc)(2) in DMSO. The lutetium bis-(phthalocyaninato) complex 4 was obtained by treating the dinitrile derivative with lutetium acetate and DBU in 1-hexanol. The new compounds were characterized by elemental analyses, FT-IR, (1)H-NMR, MALDI-TOF MS and UV/Vis spectral data. The spectroscopic data of the new compounds were in accordance with the structures. The temperature and frequency dependence of dielectric and conduction properties of the spin coated film of compounds (2-4) have been studied by fabricating metal-Pc-metal structures. The results show that compound 2 has giant dielectric constant. At a low range of frequency and room temperature, ε' is found to be equal to 2.33 × 10(6), 1.53 × 10(4) and 1.03 × 10(4) for 2, 3 and 4, respectively. The giant dielectric behavior of 2 is mainly attributed to Maxwell-Wagner polarization. The obtained results also indicated that the frequency dependence of the dielectric permittivity, ε'(ω), exhibits non-Debye type relaxation for all temperatures investigated. The ac conductivity results gave a temperature dependent frequency exponent s. The results were compared with the prediction of the Quantum Mechanical Tunneling and Correlated Barrier Hopping models.     

The dielectric constant and oscillometric measurement

Summary New evidence is given that the characteristic conductance and susceptance curves of high frequency measurements, expressed as a function of the log of the electrolyte concentration, depend only on the physical parameters of the measuring cell, and the Debye effect plays only an insignificant role.

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Zusammenfassung Wie neuerlich gezeigt wurde, sind die Konduktanz- und Susceptibilitäts. kurven bei Hochfrequenzmessungen, ausgedrückt als Funktion des Logarithmus der Elektrolytkonzentration nur von den physikalischen Parametern der Meßzelle abhängig. Der Debye-Effekt spielt keine signifikante Rolle.

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Résumé On donne une nouvelle preuve que les courbes caractéristiques de la conductance et de la susceptance des mesures en haute fréquence, exprimées en fonction du log de la concentration de l'électrolyte, ne dépendent que des paramètres physiques de la cellule de mesure, et que l'effet Debye ne joue qu'un rôle minime.

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Presented at the Symposium on Analytical Chemistry, Graz, 29th September–1st October 1965.     

Weakly polar liquid crystal dispersed with hydrophobic and hydrophilic aerosils: a broadband dielectric study

Complex dielectric permittivities for parallel orientations of the director n, with the probing electric field E (E || n), of the weakly polar liquid crystal (LC) 4,4'-di-n-hexylazoxybenzene (D6AOB) dispersed with hydrophobic and hydrophilic aerosils, have been measured in the frequency range 20 Hz to 30 MHz. In the nematic phase of D6AOB dispersed with hydrophobic and hydrophilic aerosils, significant changes in the dielectric properties are observed. An additional slow process due to the restricted motion of the molecules in surface layers is observed for hydrophilic samples. The slow process is absent in the hydrophobic samples suggesting that in the LC-hydrophilic aerosils a surface layer is formed at the aerosil interface. The process does not depend on temperature and has a wide distribution of relaxation times. The bulk-like rotation around the short axis for D6AOB dispersed with hydrophilic aerosils has been modified significantly. It seems that this could be due to an overlap between the main bulk-like process and a hindered rotation of molecules around their long axes for molecules that are oriented perpendicular to the probing electric field.     

The dielectric constant of lamellar semicrystalline polymers

The problem of representing the dielectric constant of semicrystalline polymers in terms of the dielectric constants and volume fractions of the constitutent crystalline and amorphous phases is considered. For locally lamellar morphology, bounds based on uniform electric and displacement fields are derived. The equations also include the degree of crystal orientation as a parameter. For unoriented polymers the bounds are considerably tighter than the Hashin–Shtrikman bounds, the latter being the best possible without knowledge of phase geometries. The bounds presented here are sufficiently tight to represent the dielectric constant with practical accuracy for a number of examples of semicrystalline polymers. A treatment is also given of the dielectric constant where the lamellar morphology is further specified as being organized into spherulite-like structures. These bounds are somewhat tighter than the lamellar bounds.     

Low dielectric constant polyimide nanomats by electrospinning

The proper combination of material (i.e. fluorinated polyimides) and processing technique (electrospinning) could lead to the formation of polyimides with low dielectric constant, high thermo‐oxidative stability and glass transition temperature, and high hydrophobicity. The polyimides in this work were based on 4, 4‐bis [3′‐trifluoromethyl‐4′ (4′‐amino benzoxy) benzyl] biphenyl (Q) and various fluorinated and non‐fluorinated dianhydrides namely benzene‐1,2,4,5‐tetracarboxylic dianhydride, 3,3′,4,4′‐biphenyltetracarboxylic dianhydride, benzophenone‐3,3′,4,4′‐tetracarboxylic dianhydride, and 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA). Processing of the polyimides was carried out in poly(amic acid) stage by two different methods—electrospinning and solution casting for comparison purposes. The processing of polyimides by electrospinning led to enhancement in mechanical properties (dianhydride‐structure dependent) and hydrophobicity without sacrificing thermo‐oxidative stability and glass transition temperatures significantly. Also, low dielectric constants (as low as 1.43) could be attained by suitable combination of dianhydride (6FDA) with 4, 4‐bis [3′‐trifluoromethyl‐4′ (4′‐amino benzoxy) benzyl] biphenyl diamine. Copyright © 2011 John Wiley & Sons, Ltd.     

Molecular dynamics simulation of the dielectric constant of water: the effect of bond flexibility

The role of bond flexibility on the dielectric constant of water is investigated via molecular dynamics simulations using a flexible intermolecular potential SPC/Fw [Y. Wu, H. L. Tepper, and G. A. Voth, J. Chem. Phys. 128, 024503 (2006)]. Dielectric constants and densities are reported for the liquid phase at temperatures of 298.15 K and 473.15 K and the supercritical phase at 673.15 K for pressures between 0.1 MPa and 200 MPa. Comparison with both experimental data and other rigid bond intermolecular potentials indicates that introducing bond flexibility significantly improves the prediction of both dielectric constants and pressure-temperature-density behavior. In some cases, the predicted densities and dielectric constants almost exactly coincide with experimental data. The results are analyzed in terms of dipole moments, quadrupole moments, and equilibrium bond angles and lengths. It appears that bond flexibility allows the molecular dipole and quadrupole moment to change with the thermodynamic state point, and thereby mimic the change of the intermolecular interactions in response to the local environment.     

Frequency response of a quartz crystal microbalance loaded by liquid drops

The frequency response of a quartz crystal microbalance (QCM) in contact with a spreading liquid drop is studied in this paper. An improved model describing the frequency change of the QCM with the shape evolution of the liquid drop with time is proposed based on hydrodynamic analysis, which has not been reported in the literature. It is found that the drop spreading shape, including the base radius and height, has a significant influence on the frequency response of the QCM, resulting in an unexpected increase in the resonant frequency of the QCM. The model shows that the combination of the knowledge about the radial sensitivity of the QCM and the dynamic spreading of the liquid drop is potentially important to optimize the interpretation of the experimental results. The predicted results are verified with experimental results obtained with silicone oil.     

Lattice model of a polar liquid

The molecular model of a polar liquid based on the lattice gas theory was used to study the liquid—gas phase equilibrium. In this theory, the key unknown variables are the probabilities of various mutual pair positions of the molecules. The molecules are considered as non-polarizable point dipoles. The intermolecular interaction potential includes a dipole—dipole component depending on the molecular orientation and the Lennard-Jones contribution stabilizing the system with dipole interactions. The theory provides qualitative agreement with experimental data on the water—vapor equilibrium.