Dielectric Relaxation Methods Research Articles

Molecular interaction studies of L-proline in water and ethanol at different temperatures using dielectric relaxation, refractive index and DFT methods

The complex dielectric permittivity of L-Proline in water and ethanol solutions with molar concentrations ranging from 0.025 M to 0.15 M was measured by open-ended coaxial probe technique. The measurements were carried out across a frequency span of 0.02 < ν/GHz < 20 and temperatures varying from 298.15 K to 323.15 K. The densities (ρ) and refractive index (nD ) of the L-proline in aqueous and ethanol solutions were also determined to provide insights into the solute-solvent interactions in the system. The Havriliak-Negami equation was employed to compute the dielectric relaxation time of the mixtures. The relaxation time of L-Proline in an ethanol medium was found to be higher than that of L-Proline in an aqueous medium due to the greater degree of self-association of ethanol molecules. Additionally, the relaxation time of the mixtures lengthened with rising molar concentration, which is attributed to the presence of hydrogen bonds among L-Proline and aqueous/ethanol molecules. The strength of the hydrogen bond interaction of L-Proline in both mediums was calculated using single-point energy calculations employing IEFPCM/PCM solvation models through DFT/B3LYP and MP2 approaches with a 6-311 G ++ (d, p) basis set. The results were correlated with the hydrogen bond strength, Gibbs’ free energy of activation parameter, and dipole-dipole interactions. Communicated by Ramaswamy H. Sarma

Research progress of molecular structure and dynamics of biological water

The specific water molecules that are confined within the solvation shell adjacent to the surface of biological macromolecules (including protein, enzyme, DNA, RNA, cell membrane, etc.) are called biological water molecules. Such water around the biomolecule surface plays a very important role in the structure, stability, dynamics, and function of biological macromolecules. A molecular-level understanding of the structure and dynamics of biological water, as well as the nature of its influence on biological structure and function is the key to revealing the mechanism of the biological functions. However, the researches in this field are still in the initial stage. Here in this paper, we review the relevant researches and recent progress of hydration water from three aspects. The first aspect is about the influence of hydration water on biological structure and function. It is evident that water actively participates in many biological processes such as protein folding, proton donation and migration, ligand binding and drug design, and allosteric effects. For example, water mediates the collapse of the chain and the search for the native topology through a funneled energy landscape. The second aspect is about the structure of water molecules around the biomolecules investigated by nuclear magnetic resonance (NMR), dielectric relaxation, neutron scattering, X-ray diffraction and ultrafast optical spectroscopy. The third aspect is about the dynamic behaviors of biological water, including the relaxation time scale, dynamic property, dynamic coupling between biomolecules and water molecules, and sub-diffusive motion of the water molecules along the protein surfaces. Different techniques measure different timescales for the motion of proteins and their hydration environment. While NMR and dielectric relaxation methods reveal the motion of biological water on a time scale from several tens of picoseconds to nanoseconds, ultrafast optical spectroscopy such as fluorescence and vibrational spectroscopy probes the hydrogen-bonding fluctuations of water on a time scale from the femtosecond to picosecond. It is therefore highly necessary to acquire a real and complete picture of the structure and dynamics of biological water by combining several different techniques. Finally, some unsolved scientific problems are also summarized in this review.

Single-frequency dielectric relaxation used to characterize the glass transition time of polydextrose

Dielectric relaxation methods are applicable to powdery materials such as carbohydrates. These materials have relaxations that occur in the milli-Hz range while samples are held at fixed temperatures and fixed water activities, aw, (relativity humidity). Under proper conditions these materials undergo physical changes where the initially glassy powder transitions to an amorphous equilibrium state at the glass transition temperature, Tg. Determining this transition involves characterizing the boundary curve (Tg versus aw) which determines Tg and aw conditions where materials are stable with long-shelf life or unstable with very a short shelf-life. This paper serves to illustrate multiple methodologies which can be used to characterize glass transition from frequency-spectra. Three methodologies are described: peak-broadening, peak-shift, and single-frequency. The new single frequency method not only provides results that identical to those of the peak-shift method but increases the data acquisition speeds by a factor of 5. This method is illustrated on polydextrose, a common sugar substitute. The information gathered can then be used to construct the boundary curve which is used to characterize the shelf-life of a material at various conditions.

Experimental investigation of insulation parameters affecting power transformer condition assessment using frequency domain spectroscopy

In recent years, dielectric relaxation methods in time and frequency domains have been evolved as a tool for the assessment of transformer insulation. However, it has been observed that the results of these tests are highly influenced by several factors viz. the temperature of the insulation, the magnitude of the test voltage, the geometry of transformer insulation structure. In the present work, a detailed analysis of the influence of the above-mentioned factors on the dielectric response data is presented by performing measurements on a prorated transformer model. A novel experimental procedure is suggested to acquire a database to study the effect of insulation duct geometry on the relaxation measurements. Possible corrective actions are suggested to improve transformer insulation diagnosis.

Interactions between single-walled carbon nanotubes and lysozyme

Dispersions of single-walled and non-associated carbon nanotubes in aqueous lysozyme solution were investigated by analyzing the stabilizing effect of both protein concentration and pH. It was inferred that the medium pH, which significantly modifies the protein net charge and (presumably) conformation, modulates the mutual interactions with carbon nanotubes. At fixed pH, in addition, the formation of protein/nanotube complexes scales with increasing lysozyme concentration. Electrophoretic mobility, dielectric relaxation and circular dichroism were used to determine the above features. According to circular dichroism, lysozyme adsorbed onto nanotubes could essentially retain its native conformation, but the significant amount of free protein does not allow drawing definitive conclusions on this regard. The state of charge and charge distribution around nanotubes was inferred by combining electrophoretic mobility and dielectric relaxation methods. The former gives information on changes in the surface charge density of the complexes, the latter on modifications in the electrical double layer thickness around them. Such results are complementary each other and univocally indicate that some LYS molecules take part to binding. Above a critical protein/nanotube mass ratio, depletion phenomena were observed. They counteract the stabilization mechanism, with subsequent nanotube/nanotube aggregation and phase separation. Protein-based depletion phenomena are similar to formerly reported effects, observed in aqueous surfactant systems containing carbon nanotubes.

Polarizability of spherical biological cells in the presence of localized surface charge distributions at the membrane interfaces

The electrical polarizability α(ω) of a biological cell in the presence of a layer of localized, partially bounded, charges at the two cell membrane interfaces has been calculated within the dipolar approximation. The cell is modeled in the light of the single-shell spherical model, but the results can be easily extended to shelled particles of more complex shape. Under the influence of an external electric field, the presence of these charge distributions, which added to the ones originated by the mismatch of the complex dielectric constants of the different media, produces a further dielectric relaxation, besides the one due to the usual Maxwell-Wagner effect. We explicitly find the contribution that must be added to the electrical polarizability α(ω) in order to take into account the surface electrical currents originated by the localized charges free to move on the membrane surfaces. Our results, maintaining their validity whatsoever the values of the surface charge distributions and, moreover, whatsoever the values of the membrane conductivity are, extend the applicability of the model recently proposed by Prodan [Biophys. J. 95, 4174 (2008)], who developed an analytical solution which offers reliable results only in the case of weak surface charge distributions and, moreover, for negligible small values of the membrane conductivity. Our approach, removing these constrains, represents a valuable improvement toward more realistic biological cell models and widens the use of dielectric relaxation methods to a larger class of biological systems.

Electrostatic Interactions between a Protein and Oppositely Charged Micelles

Micellar solutions made of a fully fluorinated surfactant, LiPFN, form water-soluble complexes with lysozyme in a wide concentration range. Such complexes are stabilized by electrostatic and, very presumably, double-layer interactions. The mixtures were investigated by combining electrophoretic mobility, DLS, and dielectric relaxation methods. The former gives information on the surface charge density of protein-micelle complexes and indicates that the resulting adducts retain a negative charge (i.e., charge neutralization is incomplete). The double-layer thickness of proteins, micelles, and protein-micelle complexes is also connected to the dielectric relaxation frequency. Changes in particle size (inferred by DLS), charge density, and double-layer thickness are closely interrelated to each other. A model was developed to quantify such properties.

Dielectric and X-ray Studies of Substances with the Smectic B phase

Several substances of different chemical structures exhibiting the smectic B polymorphism in a wide temperature range [accompanied by the nematic (N) phase in few cases] were studied with the aid of the dielectric relaxation and X-ray methods. The dielectric relaxation time τ∥ connected with the end-over-end rotations of molecules was determined. It elongates considerably at the N–B transition whereas the activation barrier hindering the flip--flop motion decreases. The parameters of the hexagonal unit cell were determined as functions of temperature. The packing parameter, p = ZV mol /V u.c. , was calculated which exceeds value of c. 0.6, thus is typical for the plastic crystals. The dynamical properties of molecules are discussed in relation to their packing in the smectic layers. The applicability of the Arrhenius equation to calculation of the activation barrier from the temperature dependence of τ∥ in the B phase seems to be doubtful in view of the experimental data collected. It was concluded that all examined substances have rather the hexatic than the crystalline B phase.

Some Properties of Lysozyme−Lithium Perfluorononanoate Complexes

Mixtures containing lysozyme, LYSO, and a fully fluorinated surfactant, lithium perfluorononanoate, LiPFN, were investigated in a wide range of concentrations and mole ratios. To ensure consistency to the data, a comparison was made, when possible, with the more conventional SDS as surfactant. Molecular solutions, precipitates, and micellar phases have been observed. The region of existence for each phase depends on the LiPFN/LYSO mole ratios, r, and was determined by different experimental methods. Optical absorbance, CD, 19F NMR, viscosity, electrical conductivity, and dielectric relaxation methods were used. Some methods give information on the protein conformation, others on the state of the surfactant or on the collective system properties, respectively. Addition of LiPFN gives rise to a solution, a poly phase dispersion (at low surfactant to protein ratios) and to a micelle-mediated redissolution of the precipitates. Concomitant to the above macroscopic properties, peculiar effects in the state of LYSO are observed. Low amounts of surfactant reduce significantly the amount of alpha-helix in favor of the beta-sheet conformation of the protein. The former is almost completely regained once micelle-assisted redissolution of the complex occurs. The tertiary structure of the protein, conversely, is lost at low surfactant content and never recovered. Such evidence suggests the occurrence of a molten globule conformation for LYSO in micellar media.

Characterization of DNA degradation using direct current conductivity and dynamic dielectric relaxation techniques

The purpose of this study was to evaluate DNA degradation upon thermal heating using dielectric relaxation and direct current (DC) conductivity methods. Herring sperm DNA, human growth hormone (HgH) plasmid DNA, and secreted alkaline phosphatase (SEAP) plasmid DNA were used as the examples. DNA was heated at 80 degrees C for 1 hour. The dielectric relaxation spectra as a function of the applied field frequency were measured for HgH DNA at 0.5 hours and at 1 hour. The frequency range covered was from 10 kHz to 100 kHz. The DC conductivity measurements were made for all 3 kinds of DNA at 4 time points: 0 hours, 0.5 hours, 0.75 hours, and 1 hour. At each time point the DC conductivity was measured for each sample as a function of concentration via water dilution. The results show that the dielectric relaxation method is less sensitive in characterizing heat-driven DNA degradation. Conversely, DC conductivity is very sensitive. The semiquantitative dependence of the conductivity upon heating suggests that DNA degradation involves more than plasmid DNA nicking. Double strand and single strand breaks may also occur. In addition, herring sperm DNA, HgH DNA, and SEAP DNA, though similar in their DC conductivity functional forms upon dilution, exhibit significant differences in their responses to sustained heating.

Chain Reorientation in β-PVDF Films Upon Transverse Mechanical Deformation Studied by SEM and Dielectric Relaxation

Chain reorientation may be induced in polyvinylidene fluoride (PVDF) in its β-phase by applying a deformation perpendicular to the pre-oriented polymeric chains. This reorientation begins right after the yielding point and seems to be completed when the stress-strain curve stabilizes. We have studied the chain reorientation mechanism by scanning electron microscopy (SEM) and dielectric relaxation methods and we have analyzed its influence in the α- and β-relaxations of the polymer. SEM images confirm the existence of a reorientation upon transversal deformation. The decrease of crystallinity with increasing reorientation and the dependence of the dielectric permittivity upon deformation indicate that a model of stress-induced melting followed by an incomplete recrystallization properly describes the reorientation mechanism.

Characterization of fumed silicas and their interaction with water and dissolved proteins

Fumed silicas synthesised with varied amounts of reactants (SiCl 4, O 2, H 2) at the flame temperature of 1100–1400°C were studied using adsorption, infrared (IR), 1H NMR, dielectric relaxation (DRS) and photon correlation spectroscopy methods. These samples possess different specific surface area (100–410 m 2 g −1), concentration of silanols (2.4–4.8 μmol m −2), weakly (0.6–3.0.wt.%) and strongly (0.5–3.8 wt.%) bound water (in air). Elevating synthesis temperature and decrease in the amounts of hydrogen/oxygen in the flame reduce the size of primary particles and the hydration level of fumed silica surfaces. Changes in synthetic conditions impact the characteristics not only of silica powders (reflected in their IR, DRS and NMR spectra in air) but also of their aqueous suspensions and adsorption of dissolved polymers such as albumin and gelatin. Increase in the concentration of surface silanols with growing size of primary particles enhances protein adsorption in mg m −2 of the oxide surface.

The structure and dipole moment of globular proteins in solution and crystalline states: use of NMR and X-ray databases for the numerical calculation of dipole moment.

The large dipole moment of globular proteins has been well known because of the detailed studies using dielectric relaxation and electro-optical methods. The search for the origin of these dipolemoments, however, must be based on the detailed knowledge on protein structure with atomic resolutions. At present, we have two sources of information on the structure of protein molecules: (1) x-ray databases obtained in crystalline state; (2) NMR databases obtained in solution state. While x-ray databases consist of only one model, NMR databases, because of the fluctuation of the protein folding in solution, consist of a number of models, thus enabling the computation of dipole moment repeated for all these models. The aim of this work, using these databases, is the detailed investigation on the interdependence between the structure and dipole moment of protein molecules. The dipole moment of protein molecules has roughly two components: one dipole moment is due to surface charges and the other, core dipole moment, is due to polar groups such as N--H and C==O bonds. The computation of surface charge dipole moment consists of two steps: (A) calculation of the pK shifts of charged groups for electrostatic interactions and (B) calculation of the dipole moment using the pK corrected for electrostatic shifts. The dipole moments of several proteins were computed using both NMR and x-ray databases. The dipole moments of these two sets of calculations are, with a few exceptions, in good agreement with one another and also with measured dipole moments.

Structure-sensitive liquid flowable compositions based on polyvinyl alcohol and silicotungstic acid

Conditions of the formation of structure-sensitive liquid flowable media based on 1.5 and 10% aqueous solutions of polyvinyl alcohol containing silicotungstic acid and water-soluble carboxy-methylcellulose in the form of its potassium salt (Na-CMC) are examined. Rheological, optical, and dielectric relaxation methods revealed the formation of several types of interpolymer complexes in the examined liquid flowable media. This leads to the formation of associates and an increase in the molecular mobility of the macromolecules and their fragments. The structure and properties of the complexes depend on the composition of the media as well as the method of introducing polyacids. It was found that it was possible to control the structure of such solutions by applying a mechanical of magnetic field. The compositions obtained can be used for producing anisotropic light-, electric-, and heat-sensitive film materials, as well as sensors of different types.

The dielectric modulus: relaxation versus retardation

Experiments which access the quantity ε*( ω) are usually termed dielectric relaxation methods, although ε*( ω) and ε( t) actually refer to dielectric retardation. The true dielectric relaxation, the modulus M( t), corresponds to the decay of the electric field under the conditions of a constant dielectric displacement. We have measured the polarization in terms of a real dielectric relaxation technique by studying the decay of the electric field E( t) ∝ M( t) for times 10 −3 s≤ t≤10 6 s under constant charge conditions. This conceptually straightforward method bears the advantage of the ability to measure extremely large retardation times. Using the equivalent thermally stimulated technique we also investigate the equilibrium and non-equilibrium dynamic response of amorphous condensed matter well below its caloric glass-transition.

Single‐Phase blends of polycarbonate and poly(phenyl methacrylate)

AbstractMiscibility studies on blends of polycarbonate (PC) and poly(phenyl methacrylate) (PPMA) were undertaken by means of differential scanning calorimetry (DSC), dynamic mechanical, and dielectric relaxation methods. PC and PPMA were mixed by dissolving in tetrahydrofuran (THE) and subsequently coprecipitated in methanol. DSC studies showed a single glass transition (Tg) that shifts systematically with composition. These Tgs are reproducible in repeated DSC heating cycles, suggesting true miscibility of the pair. The dry PC and PPMA pellets were melt mixed in a Mini‐Mixer/Molder. The extrudates were compression molded. These melt‐mixed PC/PPMA blends exhibited glass like transparency and also showed a single Tg in the DSC scans. The true miscibility of PC and PPMA was further confirmed by dynamic mechnaical and dielectric relaxation methods. The net birefringence has been reduced substantially because of the opposite sign of the itrinsic birefringence of PC and PPMA molecules. At the 12/88 PC/PPMA, the birefringence remains zero at all draw ratios, indicating the achievement of birefringence free polymer alloys.

Role of the hinge in the molecular motion in polydiaminophosphazenes

Nuclear magnetic and dielectric relaxation methods have been used to investigate the relaxational transitions in two polydiaminophosphazenes—polydimethylaminophosphazene and polydibutylaminophosphazene. Their characteristics are compared with those previously obtained for polydialicoxyphosphazenes similar side radicals. The effect of replacing the hinge on the thermal motion of the polymers and also on their structure is demonstrated.

ESR study of the rotational mobility of spin labels and spin probes in divinyl rubber (in the two millimetre and three centimetre ranges) together with investigations based on the electron spin echo and dielectric relaxation methods

The mobility of a spin label stably bound to SKD macromolecules has been investigated witthin the correlation time intervals 5 × 10 −11−3 × 10 −4 sec. In addition, the mobility of a spin probe introduced into the rubber has been studied. At high temperatures the mobility of the probe exceeds that of the label by a factor of ∼2, whereas in the region of the glass transition temperature T g the difference in their mobility is insignificant. With regard to α relaxation processes the dielectric relaxation times for the spin probe and for the divinyl rubber are in good agreement with characteristic times of rotational mobility determined at high temperatures by ESR for the probe and the label. In the area of slow motions stationary methods of ESR indicate the high-frequency branch of the characteristic times distribution for the β relaxation process, while the electron spin echo method indicates the low-frequency branch.

Segmental mobility of spin-labelled polystyrene in solution

To determine the characteristic times of segmental mobility for spin-labeled polystyrene (PS) in dilute solutions a technique based on a model of anistropic rotation of a spin label on the polymer chain was used. The results obtained by ESR are in good agreement with those based on the polarized luminescence and dielectric relaxation methods. A conclusion was reached that approximately the same number of monomer units participate in relaxation processes of PS in solution when recorded by ESR, by polarized luminescence and by dielectric relaxation techniques.

Molecular motion in supercooled liquids and glasses

Recently obtained data on dielectric and NMR relaxation, pulsed EPR, and other methods indicate the presence in glassy liquids of two types of molecular rotational motion. One type corresponds to macroscopic motion of the molecules and is apparent in measurements of mechanical relaxation, macroviscosity, and dielectric ..cap alpha.. relaxation. A small number of molecules display the second type of motion, which is distinguished from the first type by much higher frequency of motion and is apparent in dielectric ..beta.. relaxation, NMR relaxation, and pulsed EPR of spin probes. It is especially characteristic for impurity molecules. We discuss the connection between molecular motion and the structural features of glasses. We indicate examples of chemical reactions whose kinetics are determined by the second type of motion.