Surface characterization of clay particles via dielectric spectroscopy

Dedicated to Professor Emil Chibowski on the occasion of his 65th birthday.This work deals with the high frequency dielectric relaxation of clay (sodium montmorillonite, or NaMt) suspensions. By high frequency it is meant that the permittivity will be determined in the region where the Maxwell-Wagner-O’Konski relaxation takes place, roughly, the MHz frequency range. The applicability of dielectric determinations for the characterization of the electrical properties of these complex systems is demonstrated. In fact, standard electrophoresis measurements only allow to detect that the charge of the particles becomes slightly more negative upon increasing pH. Much more information is obtained from the high frequency electric permittivity for different concentrations of solids and pHs. From the characteristic frequencies of the relaxation it is possible to detect separate processes for parallel and perpendicular orientations of the clay platelets, modelled as oblate spheroids with a high aspect ratio. In addition, using a suitable model the surface conductivity of the clay particles can be estimated. Our data indicate that this quantity is minimum around pH 7, which is admitted as representative of the isoelectric point of the edges of the clay platelets. Data are also obtained on the amplitude (value of the relative permittivity at low frequency minus that at high frequency) of the relaxation, and it is found that it depends linearly on the volume fraction of solids, and that it is minimum at pH 5. These results are considered to be a manifestation of the fact that particle interactions do not affect the electric conduction inside the electric double layer, while the special behaviour at pH 5 is related to the existence of aggregates at pH 5, which increase the effective size of the particles and provoke a reduction of their effective conductivity.Financial support for this work by MEC (Spain) (Projects FIS2005-06860-C02-01, 02) and Junta de Andalucia (Spain) (Project FQM410) is gratefully acknowledged

Correcting the polarization effect in low frequency Dielectric Spectroscopy

We demonstrate a simple and robust methodology for measuring and analyzing the polarization impedance appearing at interface between electrodes and ionic solutions, in the frequency range from 1 to $10^6$ Hz. The method assumes no particular behavior of the electrode polarization impedance and it only makes use of the fact that the polarization effect dies out with frequency. The method allows a direct and un-biased measurement of the polarization impedance, whose behavior with the applied voltages and ionic concentration is methodically investigated. Furthermore, based on the previous findings, we propose a protocol for correcting the polarization effect in low frequency Dielectric Spectroscopy measurements of colloids. This could potentially lead to the quantitative resolution of the $\alpha$-dispersion regime of live cells in suspension

Impedance-Based Water-Quality Monitoring Using the Parallel-Plate Method

The application of electromagnetic (EM) waves to measure the electrical properties (dielectric constant and loss tangent) of materials is a well-known approach. The electrical properties can be used to indirectly measure several physical properties of solutions in water such as the concentration and chemical composition of contaminants in water, as a representative of the liquid phase in soil. A capacitive method of measuring dielectric properties of solutions is proposed to detect and determine low-concentration chemical and biological contaminations in water. The primary objective of this project is to design a low-cost sensor that would require small volumes of samples to detect low concentrations of dissolved contaminants in water. A forward model was developed using a finite-element method (FEM) to simulate the experimental setup (EXP). A calibration function was also developed to minimize deviations between FEM and EXP results for benchmark/reference solutions with known dielectric properties. The validated, calibrated forward model was then inverted to calculate the electrical properties of unknown solutions using the corresponding EXP results

A new method for the precise multiband microwave dielectric measurement using stepped impedance stub

This article presents a new method of wideband dielectric measurement at microwave frequencies. This method can be used to determine the complex dielectric properties of solid and semisolid materials from 0.9GHz to 4.5GHz, including the ISM bands of 915 MHz and 2450MHz. The new method is based on the scattering parameter measurement of a stepped impedance open circuited micro-strip stub, partly loaded with dielectric test material. Current microwave wideband spectroscopy techniques generally measure dielectric materials over a wide range of frequencies but their accuracy is limited. In contrast, narrowband techniques generally measure dielectric properties to a high accuracy but only at a single frequency. This new technique is capable of measuring dielectric properties over a wide range of frequencies to a high accuracy. The technique has been verified by the empirical characterisation of the dielectric properties of Teflon and Duroid 5880 materials. Empirical results were in good agreement with values in the manufacturer’s data sheets. The complex permittivity data will be useful for further microwave processing of the materials

Monitoring of lung edema by microwave reflectometry during lung ischemia-reperfusion injury in vivo

It is still unclear whether lung edema can be monitored by microwave reflectometry and whether the measured changes in lung dry matter content (DMC) are accompanied by changes in PaO(2) and in pro-to anti-inflammatory cytokine expression (IFN-gamma and IL-10). Right rat lung hili were cross-clamped at 37 degrees C for 0, 60, 90 or 120 min ischemia followed by 120 min reperfusion. After 90 min (DMC: 15.9 +/- 1.4%; PaO(2): 76.7 +/- 18 mm Hg) and 120 min ischemia (DMC: 12.8 +/- 0.6%; PaO(2): 43 +/- 7 mm Hg), a significant decrease in DMC and PaO(2) throughout reperfusion compared to 0 min ischemia (DMC: 19.5 +/- 1.11%; PaO(2): 247 +/- 33 mm Hg; p < 0.05) was observed. DMC and PaO(2) decreased after 60 min ischemia but recovered during reperfusion (DMC: 18.5 +/- 2.4%; PaO(2) : 173 +/- 30 mm Hg). DMC values reflected changes on the physiological and molecular level. In conclusion, lung edema monitoring by microwave reflectometry might become a tool for the thoracic surgeon. Copyright (c) 2006 S. Karger AG, Basel

Degradation versus self-assembly of block copolymer micelles

The stability of micelles self-assembled from block copolymers can be altered by the degradation of the blocks. Slow degradation shifts the equilibrium size distribution of block copolymer micelles and change their properties. Quasi-equilibrium scaling theory shows that the degradation of hydrophobic blocks in the core of micelles destabilize the micelles reducing their size, while the degradation of hydrophilic blocks forming coronas of micelles favors larger micelles and may, at certain conditions, induce the formation of micelles from individual chains.Comment: Published in Langmuir http://pubs.acs.org/doi/pdf/10.1021/la204625

Temperature Modulation of Electric Fields in Biological Matter

Pulsed electric fields (PEF) have become an important minimally invasive surgical technology for various applications including genetic engineering, electrochemotherapy and tissue ablation. This study explores the hypothesis that temperature dependent electrical parameters of tissue can be used to modulate the outcome of PEF protocols, providing a new means for controlling and optimizing this minimally invasive surgical procedure. This study investigates two different applications of cooling temperatures applied during PEF. The first case utilizes an electrode which simultaneously delivers pulsed electric fields and cooling temperatures. The subsequent results demonstrate that changes in electrical properties due to temperature produced by this configuration can substantially magnify and confine the electric fields in the cooled regions while almost eliminating electric fields in surrounding regions. This method can be used to increase precision in the PEF procedure, and eliminate muscle contractions and damage to adjacent tissues. The second configuration considered introduces a third probe that is not electrically active and only applies cooling boundary conditions. This second study demonstrates that in this probe configuration the temperature induced changes in electrical properties of tissue substantially reduce the electric fields in the cooled regions. This novel treatment can potentially be used to protect sensitive tissues from the effect of the PEF. Perhaps the most important conclusion of this investigation is that temperature is a powerful and accessible mechanism to modulate and control electric fields in biological tissues and can therefore be used to optimize and control PEF treatments

Characterization method of dielectric properties of free falling drops in a microwave processing cavity and its application in microwave internal gelation

[EN] Microwave internal gelation (MIG) is a chemical process proposed for the production of nuclear particle fuel. The internal gelation reaction is triggered by a temperature increase of aqueous droplets falling by gravity by means of non-contact microwave heating. Due to the short residence time of a solution droplet in a microwave heating cavity, a detailed knowledge of the interaction between microwaves and chemical solution (shaped in small drops) is required. This paper describes a procedure that enables the measurement of the dielectric properties of aqueous droplets that freely fall through a microwave cavity. These measurements provide the information to determine the optimal values of the parameters (such as frequency and power) that dictate the heating of such a material under microwaves.This work is a part of the PINE (Platform for Innovative Nuclear FuEls) project which targets the development of an advanced production method for Sphere-Pac fuel and is financed by the Swiss Competence Center for Energy and Mobility. The work has been also financed by the European Commission through contract no 295664 regarding the FP7 PELGRIMM Project, as well as contract no 295825 regarding the FP7-ASGARD Project. MC-S would like to thank the ITACA research team (UPV Valencia, Spain) and the EMPA Thun (Switzerland) for their support in the measurements and Carl Beard (PSI, Switzerland) for the help provided in respect with CST simulations. The work of FLP-F was supported by the Conselleria d'Educacio of the Generalitat Valenciana for economic support (BEST/2012/010).Cabanes Sempere, M.; Catalá Civera, JM.; Penaranda-Foix, FL.; Cozzo, C.; Vaucher, S.; Pouchon, MA. (2013). Characterization method of dielectric properties of free falling drops in a microwave processing cavity and its application in microwave internal gelation. Measurement Science and Technology. 24(9). https://doi.org/10.1088/0957-0233/24/9/095009S24