Anna Gutina

Dielectric spectroscopy data treatment: I. Frequency domain

The computational methods for complex dielectric permittivity data treatment are considered in this paper. The dielectric spectroscopy data analysis in the frequency domain can be reduced to the problem of choosing the appropriate model functions and an estimation of their model parameters. To address the latter problem a method has been formulated based on a penalized maximum likelihood approach, for obtaining a smooth estimate for the model parameters expressed as functions of temperature. The use of the Hilbert transform (Kramers?Kronig relation) for dc conductivity evaluation directly from the complex dielectric permittivity data has been explored as well. In this paper a numerical algorithm for this procedure, using the fast Fourier transforms and a suitable interpolation technique, is suggested. Based on these methods, state-of-the-art software for dielectric spectroscopy data analysis in the frequency domain has been developed.

Dielectric relaxation of porous glasses

Abstract The dielectric properties of porous glasses, obtained from sodium borosilicate glass, were investigated in the frequency range 20 Hz to 1 MHz and temperature range −100°C to +300°C for the purpose of inferring the geometric properties of porous materials. The features of the dielectric properties due to the geometrical disorder were analysed by using models describing the non-Debye slow decay dynamics. The dielectric response is affected by the geometrical micro- and mesostructural properties of the porous matrix and the properties of the material filling the pores. It provides information on the hindered dynamics of water molecules, located within the pores and affected by the surfaces. An analysis of the dielectric parameters enables us to describe the porosity of the materials.

DIELECTRIC RELAXATION OF WATER IN CLAY MINERALS

The study of confined water dynamics in clay minerals is a very important topic in aluminosilicate-surface chemistry. Aluminosilicates are among the most technologically versatile materials in industry today. Dielectric spectroscopy is a very useful method for investigating the structure and dynamics of water adsorbed on solid matrix surfaces and water in the vicinity of ions in solutions. Use of this method for the study of clay minerals has been underutilized to date, however. The main goal of the present research was to understand the relaxation mechanisms of water molecules interacting with different hydration centers in clay minerals, with a view to eventually control this interaction. Two types of natural layered aluminosilicates (clay minerals) — montmorillonite with exchangeable K+, Co2+, and Ni2+ cations and kaolinite with exchangeable K+ and Ba2+ cations — were examined by means of dielectric spectroscopy over wide ranges of temperature (from -121°C to +300°C) and frequency (1 Hz–1 MHz). An analysis of the experimental data is provided in terms of four distributed relaxation processes. The low-temperature relaxation was observed only in montmorillonites and could be subdivided into two processes, each related to a specific hydration center. The cooperative behavior of water at the interface was observed in the intermediate temperature region, together with a proton percolation. The dielectric properties of ice-like and confined water structures in the layered clay minerals were compared with the dielectric response observed in porous glasses. The spatial fractal dimensions of the porous aluminosilicates were calculated by two separate methods — from an analysis of the fractality found in photomicrographs and from the dielectric response.

Fast-sol–gel derived silsequioxane glasses embodying glycerol moieties: dielectric properties and morphology

Abstract The fast-sol–gel synthetic route was utilized to prepare glassy matrices of varying porosity. Glycerol doping of these matrices was carried out at various stages of the sol–gel reaction. Dielectric spectroscopy data were collected from all these matrices, and interpreted in terms of various relaxation processes from which the internal morphology of the matrices could be deduced. Glycerol dopant added to the sol–gel-derived glass during preparation is adsorbed and partly chemically bound at the ends of the polymer-like chains of the matrix, acting as a cross-linking agent. It does not form bulk moieties at concentrations up to 2%. Cluster glycerol moieties are formed inside the glass at a concentration of 4%.

Comment on “Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water, and Maxwell–Wagner processes” [J. Chem. Phys.128, 154503 (2008)]

This comment on the experimental methods and the conclusions of the paper “Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water, and Maxwell–Wagner processes” [J. Sjostrom et al., J. Chem. Phys.128, 154503 (2008)] exposes serious shortcomings in the understanding and the interpretation of the observed relaxation behavior of confined water presented in the article. There are contradictions between the suggested theory and the experimental observations that mislead the reader. In this comment we prove that the Maxwell–Wagner relaxation cannot explain the nonmonotonous temperature dependence for the process observed in confined water.

Dielectric properties of a novel colloidal oral matrix drug carrier

Dielectric Spectroscopy (DS) was employed to study a novel oral matrix carrier (OMC), composed from silica nano-particles, polysaccharides, biopolymers and natural oils. This composition self-orders to a multi hierarchal structure and as such is amenable to be studied by techniques such as DS. The dielectric properties were measured in the frequency range 1MHz-1GHz and a temperature range 10°C-45°C. The results were dominated by two relaxation processes following CC relaxation and having relaxation times that are Arrhenius in nature. These processes can be traced to structural elements in the OMC and are influenced by the melting point of coconut oil, an essential element in the formulation. Furthermore, the correlations between dipolar entities in the OMC are investigated using Froelichs B function formulation. The results point to DS as an effective tool for the study of these systems.

Dielectric relaxation of water adsorbed on the pore surface of silica glasses

The dielectric relaxation properties of porous glass obtained from sodium borosilicate glass were studied by Dielectric Spectroscopy in the wide frequency (20 Hz÷1 MHz) and temperature (−100 °C÷+300 °C) ranges. The relaxation process which is well marked in the temperature range: −50 °C to +150 °C and has a specific saddle-like shape was considered. A simple model has been developed in order to explain the related anomalous behavior of the relaxation time as a function of temperature. The mechanism offered is related to a kinetic transition due to the water molecule reorientation in the vicinity of a defect.

Slow Dynamics and Dielectric Relaxation in Water / Glycerol Mixtures

Broadband dielectric spectroscopy (BDS) has been applied for studies of glass forming liquids, such as anhydrous glycerol and its mixtures with water. Usually, glycerol does not undergo crystallization. However, we showed that the BDS with a special protection of water absorption from air to the sample and a special temperature controlling can observe the crystallization while heating of anhydrous glycerol around 263 K and the melting at 293 K. In the case of a sample without the special protection of water absorption, the crystallization and the melting were not observed, and the sample remained in liquid, supercooled liquid or glassy states. These results indicate that very small amount of water absorption can change the dynamic structure of glycerol significantly and disables the crystallization. We also found non‐Arrhenius behaviors of dc‐conductivity and so called “excess wing” as well as the main relaxation process, and discussed changes of dielectric properties of water / glycerol mixtures with inc...

Nonexponential Dynamics in Porous Glasses

The dielectric relaxation properties of porous glasses prepared from sodium borosilicate glasses are studied by dielectric spectroscopy over a wide range of frequencies (20 Hz–1 MHz) and temperatures (–100–300°C). The dielectric behavior reflecting the geometric disorder is analyzed within the models describing the non-Debye slowly damped dynamics. It is found that the dielectric response is very sensitive to microstructural and mesostructural features of the porous matrix and the properties of a material filling pores. The response contains information on the dynamics of water molecules in pores, which accounts for the interaction of these molecules with the pore surface.

Non‐Debye Dielectric Response and non‐Arrhenius Kinetics in Complex Systems at Mesoscale

The paper considers several examples of non‐Debye dielectric response in complex heterogeneous media. The percolation phenomenon and Cole‐Cole relaxation in disordered matter are discussed in detail. The models enable us to establish the relationship between the parameters of dielectric relaxation broadening, structural properties of the media and transport features of charge carriers in the considered systems. In addition, the origins of “strange kinetic” phenomena and the specific features of relaxation kinetics in systems with different kinds of confinements are discussed in the paper. In contrast to the usual Arrhenius or Vogel‐Fulcher‐Tammann patterns, a quite unusual non‐monotonic dependence of relaxation time versus temperature is observed in such systems. Based on the free volume concept, a model for this type of kinetics was illustrated by several particular examples: water confined in porous glasses and doped ferroelectric crystal.