Dielectric Properties Of Aqueous Solutions Research Articles

An electrostatic-variable coarse-grained model for predicting enthalpy of vaporization, surface tension, diffusivity, conductivity, and dielectric constant of aqueous ionic liquid

Electrostatics polarization effects are crucial in predicting various physical properties of pure or aqueous ionic liquids, which have potential applications as green solvents in many fields, such as chemical synthesis and gas separations. Here, we proposed an electrostatic-variable coarse-grained (VaCG) model to predict physical properties of water/[BMIM]+[BF4]- binary mixtures and pure [BMIM]+[SCN]- by introducing a concentration or environment-dependent dielectric constant of ionic liquid, εIL(x), and water, εw(x), in the non-bonded electrostatic interaction term, respectively. The electrostatic polarization induced charge transfer between cation and anion pair of ionic liquid was treated by setting the fragment charges of the three coarse-grained beads, i.e., butyl and methylimidazolium units of [BMIM]+ and anions ([BF4]-or [SCN]-), from the linear scaling generalized energy-based fragmentation electronic structure calculations of clusters with up to 30 cation–anion pairs. This model handles the polarization effect over the range of water mole fraction x = 0 ∼ 100 mol% with the explicit partial charges and implicit treatment of the long-ranged continuum medium without increasing the computational cost. The VaCG model has good performance in predicting thermodynamics (density, enthalpy of vaporization, surface tension), dynamic properties (diffusivity, ionic conductivity), and dielectric properties of aqueous solutions. This model is expected to be applicable to many other complicated systems with significant polarization effects.

Simulation Study on Structure, Transport Proprerties, and Dielectric Properties of Aqueous Solutions of Bio-Molecules Used in Biofuel Cells

Simulation Study on Structure, Transport Proprerties, and Dielectric Properties of Aqueous Solutions of Bio-Molecules Used in Biofuel Cells

Microwave Dielectric Permittivity and the Relaxation of 1,3-Dioxolane and 2,2-Dimethyl-1,3-dioxolane-4-methanol Aqueous Solutions

Results are presented from measuring the microwave dielectric properties of aqueous solutions of 1,3-dioxolane and 2,2-dimethyl-1,3-dioxolane-4-methanol at seven frequencies in the range of 5–25 GHz and temperatures of 288, 298, and 308 K. The static dielectric constants are considered, along with the times and parameters of activation of the dielectric relaxation process. Differences between hydration-induced changes in water and the effects of interaction between different groups of molecules are established for the solutions of the considered compounds. The dynamics of water molecules in solution is slowed, compared to that in pure water in both cases. It is established that this slowing is due to different molecular mechanisms of interaction between the dissolved species and the tetrahedral network of the H-bonds of the initial water.

Radiothermal radiation method for aqueous solutions

For the first time, an ultra-high-sensitivity method for measuring radio-thermal radiation was developed and used in practice in order to establish the difference in the physical properties of aqueous solutions of substances in the millimeter region of the spectrum. The method is used to study the dynamics of the dielectric properties of aqueous solutions depending on the composition of the base substance and its concentration. The dynamics of dielectric properties establishes a one-to-one correspondence between the number and concentration of ions of the dissolved basic substance contained in water and the number of water molecules involved in cooperative interaction, which gives a consistent microscopic picture of ion-water cooperative interactions in the studied aqueous solutions of K2SO4 and Cs2SO4. The density of water molecules perturbed by the ions of the base substance contained in the hydration shell at normal concentrations is proportional to the number of ions, while the transition to weaker solutions leads to the creation of multilayer hydration shells. This means that the number of perturbed water molecules, depending on the number of ions, increases according to a law different from linear. In accordance with the experimental data, the values of the absorption coefficients of aqueous solutions were determined in a wide range of concentrations for alkali metal sulfates. It is noted that alkali metal sulfates have physical properties that generalize the dynamics of dielectric constants depending on the concentration of the base substance. A monotonic increase in the values of the absorption coefficients of solutions with a decrease in the concentration of basic substances in the region of high dilutions was established with individual dynamics for each basic substance, reflecting the total hydration changes in salt solutions. Research has shown that the proposed method for measuring radio-thermal radiation fixes a significant difference in the values of the dielectric constants of aqueous solutions at high dilutions from their values for water.

Influence of the oxidation rate on the dielectric properties of water confined in graphene bilayers

In recent years, numerous studies have focused on enhancing water permeability and ion retention by oxidizing pristine graphene membranes. Despite advanced modification and successful applications, the physical properties involved in the separation processes by graphene oxide nanochannels, such as the dielectric constant are not fully understood. A series of molecular dynamics simulations are carried out in this work to investigate the dielectric properties of aqueous solution in both pristine and oxidized bilayer graphene membranes. Results show that the dielectric constant decreases with the increase of the oxidation ratio as the oxidation ratio of the channel is less than 20%. However, the dielectric properties violate the previous trend as the oxidation rate continues to increase up to 40%. We show that the former is due to the formation of hydrogen bonds between water and graphene oxide substrate, leading to the decrease of water dipole fluctuation and the dielectric constant and the latter is caused by the formation of hydrogen bonds between adjacent OH groups on the graphene sheets at high oxidation rate, thus reducing the ability to attract water and formation of water-substrate hydrogen bonds.

Hydration and dielectric properties of aqueous solutions of protonated diallylmethylammonium polymer: transition from monomer to polymer

The high-frequency dielectric relaxation of aqueous solutions of protonated diallylammonium polyelectrolyte, namely poly(diallylmethylammonium trifluoroacetate) has been studied at the maximum water dispersion frequencies, 7.5–25 GHz, and temperatures of 288, 298, and 308 K. Dielectric relaxation parameters have been calculated and compared with similar characteristics of aqueous solutions of monomer, diallylmethylammonium trifluoroacetate, and pyrrolidinium trifluoroacetate salt simulated structure of pyrrolidinium polymer link. It has been concluded that although the monomer features hydrophobic hydration, its polymer exhibits hydrophilic hydration properties. This change is related to conformation of hydrophilic-hydrophobic polycations in aqueous solutions and the change in the structure of polymer links.

Super-High-Frequency Permittivity and Relaxation of Aqueous Solutions of Aluminum Chloride

The super-high-frequency dielectric properties of aqueous solutions of aluminum chloride were measured at five frequencies (13–25 GHz) at 298 K. The static permittivities and the times and other parameters of dielectric relaxation of the solutions were calculated. The concentration dependences of the hydration properties of the solutions within various concentration ranges were studied.

Hydration effects accompanying the formation of DNA complexes with some ligands

In the range of millimeter wavelengths the dielectric properties of aqueous solutions of some biologically active ligands (potential anticarcinogen chlorophyllin; pharmacological drug caffeine; polyamine putrescine; mutagens proflavine and ethidium bromide; actinocin derivative, an analogue of antitumor antibiotic actinomycin D) and DNA complexes with these substances were studied. It was shown that complex formation is accompanied by the change in dielectric properties of the solution. These changes during interaction of DNA with the first three compounds correspond to a decrease in hydration (compared with the total hydration of free components), and in other cases they cause an increase in hydration. The number of water molecules bound with both the ligand and DNA nucleotide in the complex was estimated. The results were compared with existing models of DNA interaction with the studied substances.

Surveying colloid sedimentation by coplanar waveguides

By using coplanar waveguides, direct access to the dielectric properties of aqueous solutions of polystyrene beads with different diameters from 330 nm to 10 μm is provided. The relative variation of the transmission parameter with respect to water is monitored, ranging from obtained for a 9.5% solution with 330 nm diameter beads to ∼22% for 10 μm diameter particles at the same concentration. To highlight its applicability in biosensing, the technique was further employed to survey the clustering between biotin and streptavidin-coated beads. The transmission parameter displays a ∼50% increase for mixtures containing nine volumes of biotin and one volume of streptavidin-modified beads (4.5 ng μl–1 of streptavidin) and reaches ∼400% higher values when equal volumes of biotin and streptavidin-coated beads (22.5 ng μl–1 of streptavidin) were mixed.

Narrow-Band Water-Based Absorber With High Return Loss for Terahertz Spectroscopy

A simple, compact, high-performance absorbing load (absorber) for narrow-band THz spectroscopy is described. Current commercially available THz absorbers offering more than 20 dB return loss are bulky, made of proprietary materials, and intrinsically broadband. This 240 GHz absorber consists of a precisely machined thin layer of PMMA (plexiglas) placed over a small volume of water. By diluting the water with glycerol, the effective refractive index of the back medium may be adjusted finely to maximize return loss. The resonant frequency of the absorber is tuned with the PMMA thickness. Reflectometry measurements of the absorber with a vector network analyzer (VNA)-based terahertz spectrometer show a peak return loss of over 47 dB at 240 GHz and an overall peak return loss between 230 and 250 GHz of over 60 dB. Preliminary calculations suggest that such absorbers can be designed to be effective over much of the sub-THz band. In addition, the absorber is highly sensitive to the refractive index of the back medium, suggesting this type of absorber can be used to track minute changes in the dielectric properties of aqueous solutions.

Functional group dependent dielectric properties of sulfated hydrocolloids extracted from green macroalgal biomass

Dielectric properties of aqueous solutions of sulfated hydrocolloids (ulvan and rhamnan sulfate) extracted from green macroalgal biomass were studied in a frequency range of 100MHz–10GHz. Counterion exchange of native hydrocolloids (mixture of Na+, Mg2+ and Ca2+) to H+-form showed significant increase in loss factor due to ionic conduction. On the other hand, desulfations decreased their loss factors. The results suggested that ionic conduction of H+ has significant contribution to loss factors. Additionally, H+-form hydrocolloids showed significant improvement in hydration, which might also affect the dielectric property of the solution by reducing the amount of free water. The viscosity, however, did not show apparent relevance with the dielectric property.

Variation of dielectric properties of aqueous solutions of ethanol and acids at various temperatures with low acid concentration levels

Microwave-assisted extraction is gaining in popularity for the production of plant extracts and the dielectric properties determine the behaviour of different materials in a microwave environment; hence the measurement of the dielectric properties of different solvents used for analytical purposes and the interpretation of their heating rate can help in minimising the time and solvent consumption. In this study, the dielectric constant and dielectric loss of different combinations (generated using central composite design (CCD)) of ethanol concentrations and acid (HCl and citric acid) at different temperature levels were measured at frequencies varying from 0.5 GHz to 6 GHz, and the dissipation factor and depth of penetration were calculated for the respective cases, which were analysed for the interpretation of heating rates of the different solvent combinations. Overall, for all the responses (observed and calculated), ethanol concentration levels and temperature levels were found to be the most significant factors; it was interpreted and observed that the heating rate increases with an increase in ethanol concentration and decreases with increasing temperature. In general, the dissipation factor was interpreted to increase with increase in acid concentration (observed from screening analysis); however, citric acid concentration was found to have an insignificant effect on the dissipation factor (as interpreted from CCD with response surface methodology (CCD-RSM)), and hence was found to have insignificant contribution to the heating rate at lower concentrations. In case of CCD-RSM analysis of the values of dissipation factor for the combinations of ethanol with HCl, the effect of acid concentration on the dissipation factor was found to be significant; however, it was still found to be negligible when the solvent combinations were heated in the microwave reactor.

Assemblies of Thermoresponsive Diblock Copolymers: Micelle and Vesicle Formation Investigated by Means of Dielectric Relaxation Spectroscopy

The dielectric properties of aqueous solutions of two different thermoresponsive mixed copolymers, (3-acrylamidopropyl)trimethylammonium chloride and N-isopropylacrylamide, PAMPTMA-b-NIPAAM, and sodium 2-acrylamido-2-methylpropanesulfonate and N-isopropylacrylamide, PAMPS-b-PNIPAAM, have been investigated in the frequency range where marked interfacial polarization mechanisms occur, both below and above the lower critical solution temperature. In the presence of poly(ethylene oxide)-PNIPAAM block polymers, PEO-b-PNIPAAM, these classes of copolymers give rise to different types of aggregates with different compositions and different architectures. By the combined results from dielectric relaxation spectroscopy, dynamic light scattering, and ζ-potential measurements, we give evidence for assembling into two different composite structures, a core-shell-type micellar structure built up by a hydrophobic core surrounded by a hydrophilic charged layer, in the case of the PEO-b-PNIPAAM + PAMPS-b-PNIPAAM system, and a vesicular structure encompassing an aqueous core in the case of the PEO-b-PNIPAAM + PAMPTMA-b-PNIPAAM system. These different structures, governed by a delicate interplay between electrostatic and hydrophobic interactions, are characterized by dielectric parameters (dielectric increments and relaxation frequencies) that can be properly deduced from suitable dielectric models, in the framework of heterogeneous system theory. These structures and in particular hallow particles with a large internal aqueous core, where large hydrophilic compounds can be encapsulated, offer novel and interesting properties in biomedical technologies and in particular in drug delivery as drug mesoscopic carriers.

Bound water: Evidence from and implications for the dielectric properties of aqueous solutions

Broadband dielectric spectra of a variety of aqueous solutions are evaluated as to indications of water that may be considered bound. Static permittivity decrements due to depolarizing internal electric fields, from kinetic depolarization, as well as from dielectric saturation are discussed. The latter effect reflects the preferential orientation of water permanent dipole orientations within strong Coulombic field of small ions, especially multivalent cations. Such water may be considered bound even though rapid rotations around the orientation of the electric dipole moment are definitely possible and also a fast exchange of water molecules between the hydration region and the bulk may take place. Water exhibiting large dielectric relaxation times, as typical for regions with large local concentration of foreign matter, may also be named bound. However, no clear evidence for interaction energies exceeding the hydrogen bond energy of pure water has been found. Rather enhanced relaxation times at low water content reflect the small concentration of hydrogen bonding sites and thus low probability density for the formation of a new hydrogen bond. Potential interferences of the water relaxation with relaxations from other molecules or from ionic structures are mentioned briefly.

Dielectric Properties of Micellar Aggregates Due to the Self-Assembly of Thermoresponsive Diblock Copolymers

The radiowave dielectric properties of aqueous solutions of thermosensitive copolymers, consisting of poly(2-acrylamido-2-methylpropanesulfonate) [PAMPS] and poly(N-isopropylacrylamide) [PNIPAAM] with different block lengths, have been investigated over a broad temperature and frequency range. These copolymers PAMPS(n)-b-PNIPAAM(m) form temperature responsive aggregates (micelles) that represent a class of self-assembled structures in water of great interest because of their potential use as drug delivery formulations and in diverse biotechnological applications. Copolymers formed by hydrophilic segments covalently attached to a hydrophobic segments are capable of forming a micellar structure as soon as the temperature is raised above their lower critical solution temperature. We have investigated the dielectric properties of PAMPS(n)-b-PNIPAAM(m) diblock copolymers with different lengths of the hydrophilic and hydrophobic segments during the whole aggregation process driven by the progressive increase of temperature. The process has been followed by the changes resulting in the dielectric parameters (the dielectric increment Δε and the relaxation frequency ν(0)) of the whole aqueous solution. The dielectric response of the micelles has been described within the framework of the standard electrokinetic model for charged colloidal particles, and the main characteristic parameters have been evaluated. Subsequent cross-linking of these diblock copolymers by a cationic PEO(x)-b-PAMPTMA(y) polyelectrolyte yields hybrid core-shell-corona systems, with the PNIPAAM hydrophobic blocks collapsed in the core, an interpolyelectrolyte chain complex forming the shell, and the hydrophilic PEO chains as an external corona. In this case too, the dielectric spectra can be appropriately accounted for within the same theoretical framework.

Radiowave Dielectric Properties of Sodium Maleate Copolymers in Aqueous Solutions in Light of a Scaling Approach

The concentration dependence of the dielectric properties of aqueous solutions of sodium maleate copolymers with comonomers of different hydrophobicities have been investigated by means of frequency domain dielectric spectroscopy in the frequency range from 1 kHz to 2 GHz. The dielectric relaxation falling between the process at low frequencies (polarization involving the whole polymer chain) and the one at high frequencies (polarization of the bulk aqueous phase) has been analyzed in light of the scaling properties of polyelectrolyte solutions. Within this framework, the dielectric properties are governed by two characteristic lengths, the distance R(cm) between chains in the dilute regime and the correlation length xi in the semidilute regime. We find that, for both these regimes, the exponents of the scaling laws, which describe the dielectric increment Deltaepsilon and the relaxation time tau(ion) are in a reasonably good agreement with the ones experimentally observed. This analysis gives further support to the scaling approach of the dynamic behavior of polyelectrolytes, appearing very suitable to modeling our dielectric results.

Dielectric Properties of Thermo-Reversible Hydrogels: The Case of a Dextran Copolymer Grafted with Poly(N-isopropylacrylamide)

We investigated the dielectric properties of aqueous solutions of a grafted copolymer, consisting of a polysaccharide, Dextran, grafted with a thermo-sensitive polymer, poly(N-isopropylacrylamide), [pNIPAAM], over broad temperature and frequency ranges. The graft copolymers, prepared by atom-transfer radical polymerization [ATRP], form temperature-responsive materials that represent a class of self-assembled structures in water of great interest because of their potential use as drug delivery formulations and in diverse biotechnological applications. In these systems, in the dilute regime and below the lower critical solution temperature, relaxation modes corresponding to two different length-scales have been observed and analyzed in terms of ion fluctuation dielectric models specifically developed to describe the dielectric relaxation in highly charged polyion aqueous solutions. Regardless of whether the ions were produced by the ionization of the polymer chain, as in polyelectrolyte solutions, or not, as in the present case, they represent a probe at a microscopic level that is expected to reveal the structural characteristics of the system at different scales. We have identified a characteristic length associated with the size of the polymer coil in the dilute regime and a length due to fixed cross-links, where ions are partially localized by the local profile of the Coulombic field. These lengths are in reasonable agreement with analogous lengths derived from structural information and from the hydrodynamic radius of the polymer coils, measured by means of a dynamic light-scattering technique.

Temperature dependent dielectric relaxation study of triol-water mixtures using picosecond time domain technique

Picosecond time domain reflectometry method has been used in the frequency range of 10 MHz to 20 MHz to determine dielectric properties of aqueous solutions of triol in temperature range of 15 to 35 °C. The excess permittivity, Kirkwood correlation factor and Bruggeman factor of triol-water system have also been determined. A hydrogen bonded theory is applied to compute correlation terms for the mixtures. A hydrogen bonded model gives good qualitative account of the dielectric constant for triol-water mixture.

Microwave dielectric properties of aqueous solutions of potassium and cesium fluorides

The microwave dielectric properties of aqueous solutions of potassium and cesium fluorides are studied over a wide concentration range at frequencies of 13 to 25 GHz and temperatures of 288, 298, and 308 K. The static dielectric constant ɛs and the time τ and enthalpy of activation ΔH ɛ ++ of dielectric relaxation for the solutions are calculated. It is demonstrated that, for fluorides, the disruptive effect of ions on the hydrogen bond network of water is weaker than that for the other halogenides.

Complex dielectric constant and relaxation parameters of aqueous solutions of barium nitrate

Microwave dielectric properties of aqueous solutions of barium nitrate are studied as a function of concentration at frequencies from 3.4 to 23.5 GHz and temperatures from 283 to 313 K. The following dielectric parameters of the solutions are determined: static dielectric constant es, dielectric relaxation time τ, and enthalpy of activation of dielectric relaxation ΔH e ++ . In transfer from water to the solutions, the static dielectric constant decreases for all test solutions. Decreases in τ and ΔH e ++ are induced by the increasing mobility of water molecules in ion hydration shells. The ionic effect disappears as temperature increases.