Página 1 dos resultados de 2170 itens digitais encontrados em 0.012 segundos

- ELSEVIER SCI LTD
- ELSEVIER SCIENCE SA
- Elsevier; Oxford
- Biblioteca Digitais de Teses e Dissertações da USP
- Elsevier B.V.
- Institute of Physics Publishing Ltd.
- Sociedade Brasileira de Ciência do Solo
- Hindawi Publishing Corporation
- Springer
- Universidade Cornell
- Biophysical Society
- John Wiley & Sons Inc
- Elsevier
- Sociedad Mexicana de Física
- Mais Publicadores...

## Investigation of some dielectric properties of phosphate glasses doped with iron oxides, by a microwave technique

Fonte: ELSEVIER SCI LTD
Publicador: ELSEVIER SCI LTD

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

56.31%

#Microwave#Polarization#Dielectric constant#Glass#PHYSICAL-PROPERTIES#PBO-AS2O3 GLASSES#BORATE GLASSES#IONS#VIBRATIONS#SYSTEM#ESR

The effects of iron ions on dielectric properties of lithium sodium phosphate glasses were studied by non-usual, fast and non-destructive microwave techniques. The dielectric constant (epsilon`). insertion loss (L) and microwave absorption spectra (microwave response) of the selected glass system xFe(2)O(3)center dot(1 - x)(50P(2)O5 center dot 25Li(2)O center dot 25Na(2)O), being x = 0, 3, 6, ....,15 expressed in mol.%, were investigated. The dielectric constant of the samples was investigated at 9.00 GHz using the shorted-line method (SLM) giving the minimum value of epsilon` = 2.10 +/- 0.02 at room temperature, and increasing further with x, following a given law. It was observed a gradual increasing slope Of E in the temperature range of 25 <= t <= 330 degrees C, at the frequency of 9.00 GHz. Insertion loss (measured at 9.00 GHz) and measurements of microwave energy attenuation, at frequencies ranging from 8.00 to 12.00 GHz were also studied as a function of iron content in the glass samples. (C) 2009 Elsevier Ltd. All rights reserved.; FAPESP- Fundacao de Amparo a Pesquisa do Estado de Sao Paulo[00/12813-6]

Link permanente para citações:

## Dielectric properties and physical features of phosphate glasses containing iron oxide

Fonte: ELSEVIER SCIENCE SA
Publicador: ELSEVIER SCIENCE SA

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

46.44%

#microwave#permanent dipole#iron#phosphate glasses#dielectric#absorption#PBO-AS2O3 GLASSES#METAL-OXIDES#IONS#RESONANCE#SYSTEM

Microwave techniques were applied to the study of dielectric properties of phosphate glasses on the basis of contributions from permanent and induced dipolar polarization of local structural units interacting with the electrical component of the electromagnetic radiation. The dielectric constant of the selected glass system (100-x)(50P(2)O(5)center dot 25Li(2)O center dot 25Na(2)O)center dot xFe(2)O(3), where 0 <= x <= 21 is in mol%, was measured using a microwave setup assembled to measure the phase shift of the standing wave pattern produced by the insertion of the sample. It is shown that the Fe2+ ions contribute effectively to the dielectric constant, as expected from the interactions of the dipoles of the local charge compensation pairs with the microwave radiation. However, there is the possibility of occurrence of some ions Fe3+, in general, at low iron content, which reinforces the glass structure and, therefore, decreases the dielectric constant. There is a gradual conversion from Fe3+ to Fe2+ as the iron ions increases. This is possibly the reason of the anomaly in the dielectric constant values observed in the results. These assumptions can be checked by results of electronic paramagnetic resonance (EPR) and optical absorption (OA). The dielectric constant of the glasses studied in this work was found to increase with the temperature in the range of 25-330 degrees C. (C) 2007 Elsevier B.V. All rights reserved.

Link permanente para citações:

## A close dielectric spectroscopic analysis of diesel/biodiesel blends and potential dielectric approaches for biodiesel content assessment

Fonte: Elsevier; Oxford
Publicador: Elsevier; Oxford

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

56.38%

#Diesel/biodiesel blend#Quantification#Dielectric/impedance spectroscopy#Conductivity#Dielectric constant#BIODIESEL#BIOCOMBUSTÍVEIS#BLENDAS

Diesel/biodiesel blends have been more and more used as an alternative fuel due to their economic, social and environmental advantages. This is why, in many cases, legislative policies have been or are being implemented to fix a minimal percentage of biodiesel content to be added in diesel fuels. In order to ensure the mandatory use, of course, it is necessary to develop methods able to quantify the amount of biodiesel in such blends. Despite the fact that the methodology based on the Fourier transform midinfrared spectroscopy is the standard one used to determine the biodiesel content, several other techniques have been, in the last years, proposed as alternative methods to overcome some limitations presented by infrared spectroscopy. In particular, impedance spectroscopy has been proposed as a cheaper alternative technique in this quantification process. Owing to the very high resistivity response from diesel/biodiesel blends, nevertheless, development of dielectric-based biodiesel content sensors has been mainly focussed on measuring the corresponding dielectric constant. By using a sample holder with adequate dimensions, it was possible to carry here out a much complete dielectric spectroscopic study of diesel/biodiesel blends...

Link permanente para citações:

## Interpretação dos resultados de ensaios TDR para a determinação do teor de umidade dos solos; Interpretation of TDR test results for determining the moisture content of soil

Fonte: Biblioteca Digitais de Teses e Dissertações da USP
Publicador: Biblioteca Digitais de Teses e Dissertações da USP

Tipo: Dissertação de Mestrado
Formato: application/pdf

Publicado em 20/04/2010
PT

Relevância na Pesquisa

46.4%

#Calibração#Calibration#Constante dielétrica#Dielectric constant#Dielectric mixing model#Física do solo#Modelo dielétrico misto#Reflectometria no domínio do tempo#Soil physics#Soil water content#Teor de umidade

Este trabalho de pesquisa apresenta uma nova abordagem para calibrar uma sonda helicoidal TDR, usada para medir o teor de umidade do solo. Foi empregada uma aproximação tomando em consideração a interpretação física dos valores de constantes dielétricas. As equações de calibração foram determinadas mediante um modelo dielétrico misto, quais relacionam a constante dielétrica da mistura multifase com a constante dielétrica e frações de volumes dessas componentes. Ensaios realizados em laboratório permitirem o desenvolvimento de duas equações de calibração para a avaliação de teor de umidade volumétrico e gravimétrico dos solos. A calibração foi executada com medições de constante dielétrica em diferentes líquidos e solos, usando uma sonda convencional TDR e a sonda helicoidal TDR. As leituras realizadas com as duas sondas foram depois comparadas. O modelo dielétrico das duas fases permitiu descrever a contribuição dos materiais dielétricos do corpo da sonda helicoidal e os materiais investigados. Mostrou-se que o conhecimento das características físicas e geométricas da sonda TDR usada nos ensaios é de crucial para garantir a qualidade das constantes dielétricas lidas, e conseqüentemente a determinação do teor de umidade in situ.; The objective of this work was to calibrate a coiled TDR probe used to measure the water content of soil. It was used an approach that takes into account a physical interpretation of the values of the dielectric constants. The calibration equations were determined by means of a mixed dielectric model...

Link permanente para citações:

## Medidas das constantes dielétricas e deslocamento elétrico em dielétricos: desenvolvimento da técnica e metodologia; Measurements of dielectric constant and dielectric displacement: development of technique and methodology

Fonte: Biblioteca Digitais de Teses e Dissertações da USP
Publicador: Biblioteca Digitais de Teses e Dissertações da USP

Tipo: Dissertação de Mestrado
Formato: application/pdf

Publicado em 21/07/1998
PT

Relevância na Pesquisa

66.42%

#Constante dielétrica#Dielectric constant#Efeitos não-lineares#Ferroelectric polymers#Histerese#Hysteresis#Nonlinear dielectric constant#Polímeros ferroelétricos

Desenvolveu-se um sistema para medir a constante dielétrica complexa de amostras dielétricas, de construção simples, versátil e de baixo custo. A medida é feita aplicando-se uma tensão senoidal e fazendo-se a aquisição do sinal aplicado e do sinal da resposta elétrica (carga elétrica). Emprega-se uma placa A/D para a aquisição de dados com taxa de amostragem de 100 Ksamples/seg e através da transformada de Fourier discreta destes sinais determina-se a impedância complexa da amostra, a partir da qual calcula-se a constante dielétrica complexa. A placa utilizada e o circuito de medida da carga elétrica introduzem defasagens indesejáveis nos sinais, cujas correções são feitas através de programa no computador usado para aquisição dos sinais. O sistema mostra um desempenho similar àqueles dos equipamentos comerciais para freqüências no intervalo de 0,1H.z a 1KHz, sendo testado com componentes resistivos e capacitivos e posteriormente em amostras de Teflon FEP, poli(fluoreto de vinilideno) e seus copolímeros com trifluoretileno. O sistema também foi utilizado para medir o deslocamento elétrico nos polímeros ferroelétricos sob a aplicação de campos elevados. Neste caso determinaram-se as curvas de histerese dielétrica e os deslocamentos de primeira...

Link permanente para citações:

## Dielectric properties and microstructure of SrTiO3/BaTiO3 multilayer thin films prepared by a chemical route

Fonte: Elsevier B.V.
Publicador: Elsevier B.V.

Tipo: Artigo de Revista Científica
Formato: 260-265

ENG

Relevância na Pesquisa

56.21%

Multilayer thin films with perovskite structures were produced by the polymeric precursor method. SrTiO3/BaTiO3 (STO/BTO) multilayers were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by the spin-coating technique and heated in air at 700 degreesC. The microstructure and crystalline phase of the multilayered thin films were examined by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), resolution-high transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and X-ray diffraction. The SrTiO3/BaTiO3 multilayer thin films consisted of grainy structures with an approximate grain size of 60 nm. The multilayered thin films showed a very clear interface between the components. The SrTiO3/BaTiO3 multilayer thin films revealed dielectric constants of approximately 527 and loss tangents of 0.03 at 100 kHz. The dielectric constant calculated for this multilayer film system is the value of the sum of each individual component of the film, i.e. The total value of the sum of each SrTiO3 (STO) and BaTiO3 (BTO) layer. The multilayer SrTiO3/BaTiO3 obtained by the polymeric precursor method, also showed a ferroelectric behavior with a remanent polarization of 2.5 muC/cm(2) and a coercive field of 30 kV/cm. The multilayer films displayed good fatigue characteristics under bipolar stressing after application of 10(10) switching cycles. (C) 2001 Published by Elsevier B.V. B.V. All rights reserved.

Link permanente para citações:

## The High Dielectric Constant of Staphylococcal Nuclease is Encoded in its Structural Architecture

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

EN

Relevância na Pesquisa

46.5%

The pKa values of Lys-66, Glu-66 and Asp-66 buried in the interior of the staphylococcal nuclease Δ+PHS variant were reported to be shifted by as much as 5 pKa units from their normal values. Reproducing the pKa of these buried ionizable residues using continuum electrostatic calculations required the use of a high protein dielectric constant of 10 or higher. The apparent high dielectric constant has been rationalized as a consequence of a local structural reorganization or increased fluctuations in the microenvironment of the mutation site We have calculated the dielectric constant of Δ+PHS and the Lys-66, Asp-66 and Glu-66 mutants from first principles using the Kirkwood-Fröhlich equation, and discovered that staphylococcal nuclease has a naturally high dielectric constant ranging from 20 to 30. This high dielectric constant does not change significantly with the mutation of residue 66 or with the ionization of the mutated residues. Calculation of the spatial dependence of the dielectric constant for the microenvironment of residue-66 produces a value of about 10, which matches well with the apparent dielectric constant needed to reproduce pKa values from continuum electrostatic calculations. Our results suggest an alternative explanation that the high dielectric constant of staphylococcal nuclease is a property resulting from the intrinsic backbone fluctuations originating from its structural architecture.

Link permanente para citações:

## Enhanced dielectric constant resolution of thin insulating films by electrostatic force microscopy

Fonte: Institute of Physics Publishing Ltd.
Publicador: Institute of Physics Publishing Ltd.

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

46.44%

This is the author’s version of a work that was accepted for publication in Journal of Physics: Condensed Matter. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Physics: Condensed Matter 24.15 (2012) 10.1088/0953-8984/24/15/155303; Electrostatic Force Microscopy has been shown to be a useful tool to determine the dielectric constant of insulating films of nanometer thicknesses that play a key role in many electrical, optical and biological phenomena. Previous approaches make use of simple analytical formulae to analyse the experimental data for thin insulating films deposited directly on a metallic substrate. Here we show that the sensitivity of the EFM signal to changes in the dielectric constant of the thin film can be enhanced by using dielectric substrates with low dielectric constants. We present detailed numerical calculations of the tip-sample electrostatic interaction in the following set-up: the insulating thin film, a dielectric substrate (or spacing layer) of known low dielectric constant and a metallic electrode. The EFM sensitivity to the dielectric constant increases with the thickness of the spacing layer and saturates for thicknesses above 100-300 nm...

Link permanente para citações:

## Dielectric constant obtained from TDR and volumetric moisture of soils in southern Brazil

Fonte: Sociedade Brasileira de Ciência do Solo
Publicador: Sociedade Brasileira de Ciência do Solo

Tipo: Artigo de Revista Científica
Formato: text/html

Publicado em 01/06/2010
EN

Relevância na Pesquisa

46.37%

Soil moisture is the property which most greatly influences the soil dielectric constant, which is also influenced by soil mineralogy. The aim of this study was to determine mathematical models for soil moisture and the dielectric constant (Ka) for a Hapludalf, two clayey Hapludox and a very clayey Hapludox and test the reliability of universal models, such as those proposed by Topp and Ledieu and their co-workers in the 80's, and specific models to estimate soil moisture with a TDR. Soil samples were collected from the 0 to 0.30 m layer, sieved through a mesh of 0.002 m diameter and packed in PVC cylinders with a 0.1 m diameter and 0.3 m height. Seven samples of each soil class were saturated by capillarity and a probe composed of two rods was inserted in each one of them. Moisture readings began with the saturated soil and concluded when the soil was near permanent wilting point. In each step, the samples were weighed on a precision scale to calculate volumetric moisture. Linear and polynomial models were adjusted for each soil class and for all soils together between soil moisture and the dielectric constant. Accuracy of the models was evaluated by the coefficient of determination, the standard error of estimate and the 1:1 line. The models proposed by Topp and Ledieu and their co-workers were not adequate for estimating the moisture in the soil classes studied. The adjusted linear and polynomial models for the entire set of data of the four soil classes did not have sufficient accuracy for estimating soil moisture. The greater the soil clay and Fe oxide content...

Link permanente para citações:

## Analysis of the Dielectric Constant of Saline-Alkali Soils and the Effect on Radar Backscattering Coefficient: A Case Study of Soda Alkaline Saline Soils in Western Jilin Province Using RADARSAT-2 Data

Fonte: Hindawi Publishing Corporation
Publicador: Hindawi Publishing Corporation

Tipo: Artigo de Revista Científica

EN

Relevância na Pesquisa

46.42%

Soil salinity is a global problem, especially in developing countries, which affects the environment and productivity of agriculture areas. Salt has a significant effect on the complex dielectric constant of wet soil. However, there is no suitable model to describe the variation in the backscattering coefficient due to changes in soil salinity content. The purpose of this paper is to use backscattering models to understand behaviors of the backscattering coefficient in saline soils based on the analysis of its dielectric constant. The effects of moisture and salinity on the dielectric constant by combined Dobson mixing model and seawater dielectric constant model are analyzed, and the backscattering coefficient is then simulated using the AIEM. Simultaneously, laboratory measurements were performed on ground samples. The frequency effect of the laboratory results was not the same as the simulated results. The frequency dependence of the ionic conductivity of an electrolyte solution is influenced by the ion's components. Finally, the simulated backscattering coefficients measured from the dielectric constant with the AIEM were analyzed using the extracted backscattering coefficient from the RADARSAT-2 image. The results show that RADARSAT-2 is potentially able to measure soil salinity; however...

Link permanente para citações:

## Estimating the dielectric constant of the channel protein and pore

Fonte: Springer
Publicador: Springer

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

56.27%

#Keywords: ion channel#protein#algorithm#article#dielectric constant#electric current#electric potential#ion conductance#ion transport#mathematical analysis#molecular dynamics

When modelling biological ion channels using Brownian dynamics (BD) or Poisson-Nernst-Planck theory, the force encountered by permeant ions is calculated by solving Poisson's equation. Two free parameters needed to solve this equation are the dielectric c

Link permanente para citações:

## Thickness-dependent Dielectric Constant of Few-layer In2Se3 Nano-flakes

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 23/11/2015

Relevância na Pesquisa

46.42%

The dielectric constant or relative permittivity of a dielectric material,
which describes how the net electric field in the medium is reduced with
respect to the external field, is a parameter of critical importance for
charging and screening in electronic devices. Such a fundamental material
property is intimately related to not only the polarizability of individual
atoms, but also the specific atomic arrangement in the crystal lattice. In this
letter, we present both experimental and theoretical investigations on the
dielectric constant of few-layer In2Se3 nano-flakes grown on mica substrates by
van der Waals epitaxy. A nondestructive microwave impedance microscope is
employed to simultaneously quantify the number of layers and local electrical
properties. The measured dielectric constant increases monotonically as a
function of the thickness and saturates to the bulk value at around 6 ~ 8
quintuple layers. The same trend of layer-dependent dielectric constant is also
revealed by first-principle calculations. Our results of the dielectric
response, being ubiquitously applicable to layered 2D semiconductors, are
expected to be significant for this vibrant research field.; Comment: 15 pages, 4 figures, 1 table in Nano letters, 2015 ASAP

Link permanente para citações:

## Dependence of the dielectric constant of electrolyte solutions on ionic concentration

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/08/2012

Relevância na Pesquisa

46.38%

We study the dependence of the static dielectric constant of aqueous
electrolyte solutions upon the concentration of salt in the solution and
temperature. The model takes into account the orientation of the solvent
dipoles due to the electric field created by ions, the ionic response to an
applied field, and the effect of thermal fluctuations. The analysis suggests
that the formation of ion pairs by a small fraction of disassociated ions can
have a significant effect on the static dielectric constant. The model predicts
the dielectric has the functional dependence
$\varepsilon(c)=\varepsilon_w-\beta L(3\alpha c/\beta)$ where $L$ is the
Langevin function, $c$ is the salt concentration, $\varepsilon_w$ is the
dielectric of the pure water, $\alpha$ is the total excess polarization of the
ions and $\beta$ is the relative difference between the water dipole moment and
the effective dipole moment of ion pairs as weighted by the density of ion
pairs and their structural rigidity. The functional form gives an extremely
accurate prediction of the dielectric constant for a variety of salts and a
wide range of concentrations by fitting only the parameter $\beta$.

Link permanente para citações:

## Dielectric Constant and Charging Energy in Array of Touching Nanocrystals

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 17/12/2015

Relevância na Pesquisa

46.42%

The dielectric constant of an array of nanocrystals (NCs) is important for
many physical properties of the array, for example for the low temperature
electron transport. Here we consider a periodic array of spherical NCs with
dielectric constant $\varepsilon$ immersed in the medium with dielectric
constant $\varepsilon_m$ and separated by spacing $s$ from each other. The
standard way to estimate the dielectric constant of such array is to use the
Maxwell-Garnet formula. This mean field approach fails when NCs are very close
or touch each other ($s=0$). We show that in this case the dielectric constant
of the array $\varepsilon_a = (\pi/2) \varepsilon_m
\ln(\varepsilon/\varepsilon_m)$. On the other hand when NCs touch by small
facets with radius $\rho$ what is equivalent $s<0$, we get $\varepsilon_a =
2\varepsilon (\rho/d)$, where $d$ is the NC diameter. We also also show that in
this case the charging energy $E_c$ of a single NC in an array of touching NCs
has a non-trivial relationship to $\varepsilon_a $, namely $E_c = \alpha
e^2/\varepsilon_a d$, where $\alpha$ varies from 1.59 to 1.95 for studied
three-dimensional lattices.

Link permanente para citações:

## Longitudinal wavevector- and frequency-dependent dielectric constant of the TIP4P water model

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/01/1999

Relevância na Pesquisa

46.38%

A computer adapted theory for self-consistent calculations of the wavevector-
and frequency-dependent dielectric constant for interaction site models of
polar systems is proposed. A longitudinal component of the dielectric constant
is evaluated for the TIP4P water model in a very wide scale of wavenumbers and
frequencies using molecular dynamics simulations. It is shown that values for
the dielectric permittivity, calculated within the exact interaction site
description, differ in a characteristic way from those obtained by the point
dipole approximation which is usually used in computer experiment. It is also
shown that the libration oscillations, existing in the shape of longitudinal
time-dependent polarization fluctuations at small and intermediate wavevector
values, vanish however for bigger wavenumbers. A comparison between the
wavevector and frequency behaviour of the dielectric constant for the TIP4P
water and the Stockmayer model is made. The static screening of external
charges and damping of longitudinal electric excitations in water are
considered as well. A special investigation is devoted to the time dependence
of dielectric quantities in the free motion regime.; Comment: 21 pages, 7 figures

Link permanente para citações:

## Origin of Large Dielectric Constant with Large Remnant Polarization and Evidence of Magnetoelectric Coupling in Multiferroic La modified BiFeO3-PbTiO3 Solid Solution

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/02/2010

Relevância na Pesquisa

46.38%

The presence of superlattice reflections and detailed analyses of the powder
neutron and x-ray diffraction data reveal that La rich
(BF$_{0.50}$-LF$_{0.50}$)$_{0.50}$-(PT)$_{0.50}$ (BF-LF-PT) has ferroelectric
rhombohedral crystal structure with space group \textit{$R3c$} at ambient
conditions. The temperature dependence of lattice parameters, tilt angle,
calculated polarization $(P_{s})$, volume, and integrated intensity of
superlattice and magnetic reflections show an anomaly around 170 K. Impedance
spectroscopy, dielectric and ac conductivity measurements were performed in
temperature range $473K \leq T \leq 573K$ to probe the origin of large remnant
polarization and frequency dependent broad transitions with large dielectric
constant near $T_c^{FE}$. Results of impedance spectroscopy measurements
clearly show contributions of both grain and grain boundaries throughout the
frequency range ($10^{3}$ Hz$\leq f\leq 10^{7} $ Hz). It could be concluded
that the grain boundaries are more resistive and capacitive as compared to the
grains, resulting in inhomogeneities in the sample causing broad frequency
dependent dielectric anomalies. Enhancement in dielectric constant and remnant
polarization values are possibly due to space charge polarization caused by
piling of charges at the interface of grains and grain boundaries. The
imaginary parts of dielectric constant ($\epsilon^{\prime\prime}$) Vs frequency
data were fitted using Maxwell-Wagner model at $T_c^{FE}(\sim 523$K) and model
fits very well with the data up to $10^{5}$ Hz. Magnetodielectric measurements
prove that the sample starts exhibiting magnetoelectric coupling at $\sim 170$
K...

Link permanente para citações:

## Role of the Dielectric Constants of Membrane Proteins and Channel Water in Ion Permeation

Fonte: Biophysical Society
Publicador: Biophysical Society

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

56.16%

#Keywords: globular protein#membrane protein#water#article#dielectric constant#energy#ion permeability#mathematical model#membrane channel#membrane permeability#Cell Membrane Permeability

Using both analytical solutions obtained from simplified systems and numerical results from more realistic cases, we investigate the role played by the dielectric constant of membrane proteins εp and pore water εw in permeation of ions across channels.

Link permanente para citações:

## Determination of the effective dielectric constant from the accurate solution of the Poisson equation

Fonte: John Wiley & Sons Inc
Publicador: John Wiley & Sons Inc

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

66.49%

#Keywords: Boundary element method#Conformations#Electrostatics#Functions#Macromolecules#Molecular structure#Poisson equation#Solutions#Constant dielectric function#Distant dependent dielectric function#Effective dielectric constant

Constant dielectric (CD) and distance-dependent dielectric (DDD) functions are the most popular and widespread in the Molecular Mechanism simulations of large molecular systems. In this article, we present a simple procedure to derive an effective dielectric constant, εout,eff, for these two methods based on numerical solutions of the Poisson equation. It was found that because of the very approximate of the CD and DDD models there is no universal εout,eff which will work equally well for all molecular systems. For example, different MD trajectories of the same molecule can produce different optimal εout,effs. The DDD function was found to yield better agreement with the numerical solutions of the Poisson equation than a CD model does. The reason is that a DDD function gives a better description of the electrostatic interactions short distances the atoms. Another interesting finding of this study is that under certain conditions εout,eff can take negative values for a system of two atoms at a limited distance range, However, in principle, there is nothing to prevent the εout,eff from taking negative values for specific conformations of some molecules.

Link permanente para citações:

## Energy of an ion crossing a low dielectric membrane: the role of dispersion self-free energy

Fonte: Elsevier
Publicador: Elsevier

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

56.16%

#Keywords: article#cell membrane#dielectric constant#dispersion#electricity#electrolyte transport#energy transfer#erythrocyte#ion permeability#ion transport#membrane permeability

The Born charging equation predicts that the permeability of a cell membrane to ions by the solubility-diffusion mechanism depends on the ionic radius and on the dielectric constant of the membrane. However, experiments, for example, on red blood cells and on lysosome membranes, show that the permeability depends strongly on the choice of salt anion in a way that cannot be accommodated by differences in ionic size. We demonstrate that one step towards understanding this ion specificity is to take account of the previously ignored dispersion self-free energy of the ion. This is the quantum electrodynamic analogue of the (electrostatic) Born self-energy of an ion. We show that the dispersion self-free energy contribution can be and often is of the same order of magnitude as the Born contribution. To understand the observed specificity, it is essential to take into account of both ionic size and ionic polarizability. In parallel and to reinforce these observations, we also give simple estimates for how self-free energy changes that occur when an ion moves into the air-water interface region (which has a density profile for water molecules) can influence the surface tension of salt solutions. Consistency can be found between the Hofmeister sequences observed in ion permeation and in surface tension of electrolytes when these previously ignored self-free energies are included properly.

Link permanente para citações:

## Non-destructive measurement of the dielectric constant of solid samples

Fonte: Sociedad Mexicana de Física
Publicador: Sociedad Mexicana de Física

Tipo: Artigo de Revista Científica
Formato: text/html

Publicado em 01/12/2009
EN

Relevância na Pesquisa

66.27%

We discuss and analyze a practical methodology for the determination of the dielectric constant of a macroscopic solid sample in a nondestructive way. The technique consists in measuring the capacitance between a pointer electrode and the dielectric surface as a function of the separation distance in a scale comparable to the radius of curvature of the tip's apex. The changes in capacitance that must be measured will commonly be in the atto-farad scale and require specialized instrumentation which we also describe here. The technique requires two calibration standards and the sample needs to have a portion of its surface flat and some minimum dimensions, but otherwise it can have an arbitrary shape. We used a simple model based on the method of images to explain the methodology and present experimental results with the proposed methodology.

Link permanente para citações: