Ion–Water Cooperative Interactions in Aqueous Solutions of MgCl2 by Dielectric Spectroscopy

The dielectric constant of aqueous MgCl₂ solution has been determined in the frequency range 0.2 MHz to 20 GHz at 298 K using the dielectric relaxation spectroscopy method. The behavior is well described according to four Cole-Cole terms whose evolution with composition is analyzed. The static dielectric constant and relaxation times decreases with the increasing aqueous MgCl₂ solution concentrations. Only one H-bonded water cluster with the aqueous MgCl₂ solution relaxes is reported during the cutoff relaxation time. A distinct ion–water cooperative interaction is observed, and water molecules perturbed by ion contribution on dielectric constant beyond the first hydration shell are obtained.     

Solution Properties of NaHSO4 and Na2SO4: A New Sight from the Water–Ion and Water–Dipole Cooperative Interactions Using Dielectric Relaxation Spectroscopy

The aqueous NaHSO₄ and Na₂SO₄ solutions at concentrations of 0.1–1.0 mol/l in a limited frequency range 0.2–20 GHz are studied by dielectric relaxation spectroscopy with a newly developed fractal concept spectral function. The fractal analysis with α(lnτ) diagrams from dielectric relaxation spectroscopy as functions proposed a new strategy to shed light on the dual nature of ion–water and dipole–water cooperative interactions. A distinct cooperative interaction of ion–water and dipole–water is observed and water molecules perturbed by ions contributing to dielectric constant beyond the first hydration shell is obtained.     

Measurement of the dielectric relaxation property of water-ion loose complex in aqueous solutions of salt at low concentrations

Electrolytes and their dissociated ions are thought to form positive or negative hydration layers around them. In this study, we have developed a method to determine the volume and the dielectric relaxation property (relaxation frequency f c, dispersion intensity delta) of the water hydrating ions in salt solutions. The method consists of four steps: (1) By use of a high-resolution microwave dielectric spectroscopy technique, the dielectric spectra of sample salt solution and bulk water are measured in pair. (2) The dielectric spectrum of solutes (ions) with water layers for a given volume fraction varphi is then calculated from each pair of dielectric spectra of a sample salt solution and reference water according to the Hanai mixture theory. (3) Each spectrum of solutes with water layers at a given varphi is decomposed into a few Debye relaxation functions and the bulk water component. (4) The volume fraction varphi is operationally decreased from 0.5, and steps (2) and (3) are repeated at each varphi until the bulk water component vanished. Then the volume fraction of the hydrated solutes (ions) in solution is determined. The method was applied to NaF and NaCl solutions. As a result the different spectral intensity was nearly proportional to the salt concentration below 0.2 M in the frequency range of 3-26 GHz. The hydration number N h and the dielectric relaxation property of the hydration layer for each salt solution was successfully determined as ( f c1, delta 1, N h)= (18.7, 44.9, 27.9) for NaCl and ( f c1, delta 1, f c2, delta 2, N h) = (26.0, 6.70, 5.64, 19.2) for NaF.     

KCl/NaCl DMSO aqueous solutions dielectric spectra and cooperative interactions

The complex permittivity spectra of KCl/NaCl DMSO aqueous solutions were measured using microwave dielectric spectroscopy. Ion–water and dipole–water cooperative interactions are investigated by using a fractional relaxation process. Only about one cooperative relaxation was found during the cut-off relaxation time interval and the relaxation time increases with concentration increasing. Ions’ concentration dependent on effective number of water molecules in the shell around ions was calculated to reveal the interactions between K⁺, Cl^?, Na⁺ ions and DMSO molecules.     

Temperature changes of dielectric permittivity and relaxation in aqueous lithium iodide solutions

The complex permittivity of aqueous LiI solutions is studied over a wide range of concentrations at temperatures of 288–323 K in the water permittivity dispersion region at seven frequencies in the range of 7.5–25 GHz. One relaxation region describable by the Debye or Cole-Cole equation is observed in these solutions. Dielectric relaxation time τ and static permittivity ?_s are studied as dependent on temperature and concentration. The time and enthalpy of activation of dielectric relaxation decrease in going from water to solutions, which corresponds to the distortion of the initial water structure and the increasing mobility of water molecules in hydration shells of ions. In the initial concentration range, the water activity is a linear function of 1/?_s. The negative temperature dependence of ?_s disappears in going to concentrated solutions. At high concentrations, the static dielectric constant increases in response to increasing temperature. The new trends in ?_s and τ at elevated temperatures of 313–323 K are due to the formation of ion pairs and other ion-water groups having high dipole moments.     

Effect of dielectric properties and structure of aqueous solutions of diallylammonium salts on their reactivity in radical polymerization

The complex dielectric permittivity in the frequency range 7.5–25.0 GHz and the low-frequency specific conductivity of aqueous solutions of diallylammonium salts (diallylammonium and diallylmethylammonium trifluoroacetates and diallyldimethylammonium chloride) were measured at 293–308 K over a wide concentration range. On the basis of the results, the parameters of dielectric relaxation were calculated. The number of water molecules in the solvation shell of the salts was estimated. The concentration behavior of the initial rate of radical polymerization of diallylammonium salts and the rate constant of bimolecular chain termination was correlated with the specific features of the structure of aqueous monomer solutions. The role of “free” water in the initial salt solutions was revealed, a species whose presence in the system determines the character of concentration behavior of the rate constants for the elementary steps of polymerization, such as propagation, chain transfer to the monomer, and bimolecular chain termination.     

The dielectric response of interfacial water—from the ordered structures to the single hydrated shell

Water is the universal solvent in nature. Does this imply, however, that its interaction with its environment is also a universal feature? While this question maybe too fundamental to be answered by one method only, we present evidence that the broadening of the dielectric spectra of water presents universal features of dipolar interactions with different types of matrixes. If in aqueous solutions the starting point of water’s state can be considered as bulk, with only partial interactions with the solute, then the state of water adsorbed in heterogeneous materials is determined by various hydration centers of the inhomogeneous material (the matrix) and it is significantly different from the bulk. In both cases, the dielectric spectrum of water is symmetrical and can be described by the Cole–Cole (CC) function. The phenomenological model that describes a physical mechanism of the dipole–matrix interaction in complex systems underlying the CC behavior has been applied to water adsorbed in porous glasses. It was then extended to analyses of the dynamic and structural behavior of water in nonionic and ionic aqueous solutions. The same model is then used to analyze the CC relaxation processes observed in clays, aqueous solutions of nucleotides, and amino acids.     

A molecular dynamics study of the dielectric properties of aqueous solutions of alanine and alanine dipeptide

Molecular dynamics simulations were used to compute the frequency-dependent dielectric susceptibility of aqueous solutions of alanine and alanine dipeptide. We studied four alanine solutions, ranging in concentration from 0.13-0.55 mol/liter, and two solutions of alanine dipeptide (0.13 and 0.27 mol/liter). In accord with experiment we find a strong dielectric increment for both solutes, whose molecular origin is shown to be the zwitterionic nature of the solutes. The dynamic properties were analyzed based on a dielectric component analysis into solute, a first hydration shell, and all remaining (bulk) waters. The results of this three component decomposition were interpreted directly, as well as by uniting the solute and hydration shell component to a "suprasolute" component. In both approaches three contributions to the frequency-dependent dielectric properties can be discerned. The quantitatively largest and fastest component arises from bulk water [i.e., water not influenced by the solute(s)]. The interaction between waters surrounding the solute(s) (the hydration shell) and bulk water molecules leads to a relaxation process occurring on an intermediate time scale. The slowest relaxation process originates from the solute(s) and the interaction of the solute(s) with the first hydration shell and bulk water. The primary importance of the hydration shell is the exchange of shell and bulk waters; the self-contribution from bound water molecules is comparatively small. While in the alanine solutions the solute-water cross-terms are more important than the solute self-term, the solute contribution is larger in the dipeptide solutions. In the latter systems a much clearer separation of time scales between water and alanine dipeptide related properties is observed. The similarities and differences of the dielectric properties of the amino acid/peptide solutions studied in this work and of solutions of mono- and disaccharides and of the protein ubiquitin are discussed.     

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 of poly(ethylene oxide)s in aqueous solution as studied by dielectric relaxation measurements

The hydration state of poly(ethylene oxide)s (PEOs) in aqueous solutions was investigated using dielectric relaxation measurements at 25 degrees C over a frequency range up to 20 GHz, which is the relaxation frequency of water molecules in a bulk state. The dielectric relaxation spectra obtained indicated decomposition into two major and one minor relaxation modes with relaxation times of 8.3, 22, and 250 ps, respectively. The two major modes were attributed to rotational relaxation of water molecules belonging to the bulk state and water molecules hydrogen bonded to ethylene oxide (EO) monomer units. The number of hydration water molecules per EO unit depended on the molar mass of PEO (M) and reached a constant value of 3.7 at M > 1500, which agrees with the value obtained by other experiments.     

Hydration and dynamic behavior of cyclodextrins in aqueous solution

The hydration state and dynamics of plain and chemically modified cyclodextrins (CDs) in aqueous solution were investigated by using dielectric relaxation measurements at 25 degrees C over a wide frequency range up to 20 GHz, which is the relaxation frequency of pure liquid water molecules. The obtained dielectric relaxation spectra were decomposed into two major and one minor relaxation modes with relaxation times of approximately 8.3, 20-25, and 1000-2500 ps, respectively, depending on the CD species. The two major modes, fast and medium, were attributed to a rotational relaxation process of water molecules belonging to the bulk (free) state and an exchange of water molecules hydrated to CDs owing to hydrogen bond formation. The hydration numbers of the CDs strongly depend on the number of hydroxy (OH) groups controlled by chemical modification such as methylation. Increasing the number of methoxy or 2-hydroxypropoxy groups increases the hydration number of CD molecules, and results in higher solubilities of the chemically modified CDs than those of the plain CDs. The minor, slow mode was assigned to overall rotational relaxation for CDs with finite permanent dipole moments, which also depends on the number of OH groups.     

Microwave dielectric permittivity and relaxation of aqueous potassium trimethylacetate solutions

The MW dielectric properties of aqueous potassium trimethylacetate (pivalate) solutions have been measured at six frequencies (10–25 GHz) at 288, 298, and 308 K. The static dielectric constants and dielectric relaxation times and activation parameters have been calculated. Trimethylacetate ion leads to the decrease in the mobility of water molecules and strengthening of their hydrogen bonds. These changes of water are similar to those in solutions of other carboxylates with a large number of nonpolar groups. The hydrophobic hydration of the trimethylacetate ion is maximal in this series.     

The complete morphological,electro-optical and dielectric study of dichroic dye-doped polymer-dispersed liquid crystal

Polymerisation-induced phase separation method was used to prepare dye-doped polymer-dispersed liquid crystal (DPDLC) films consisting of nematic liquid crystal, photo-curable pre-polymer and a dichroic dye. The change occurring on droplets morphology with the change in dye concentration and voltage was investigated by means of polarising optical microscope and scanning electron microscope. Electro-optic behaviour of these composite films in the presence of an externally applied AC electric field (0–100 V, 200 Hz) and temperature range 25–50°C was evaluated using He–Ne laser. The maximum absorbance of dye and effect of electric field on the absorbance of dye was studied using UV-Vis spectrophotometer. Dielectric relaxation spectroscopy was carried out in the frequency range 20 Hz–20 MHz and over the temperature range 25°C–90°C. Dielectric parameters such as distribution parameter, relaxation frequency, relaxation strength and relaxation time of these DPDLC films were calculated, and the modelling of experimental data was done using Debye and Cole–Cole method.     

Hydration and dielectrical properties of aqueous pyrrolidinium trifluoroacetate solutions

Results from microwave measurements of the dielectrical properties of aqueous pyrrolidinium trifluoroacetate solutions at maximum water dispersion frequencies (13–25 GHz) and temperatures of 288, 298, and 308 K are given. The static dielectrical constants, times, and activation parameters of the dielectrical relaxation of solutions are calculated. The enthalpy and time of dielectrical relaxation activation are increased by deceleration of the motion of water molecules in the hydrate shells of ions. The changes in dielectrical parameters are in this case minimal in a series of aqueous solutions of diallylammonium salts with cations of different structures and degrees of substitution. It is shown that pyrrolidinium ions are characterized by weak hydrophobic hydration.     

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

Abstract

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.     

Dielectric relaxation study of glycine–water mixtures using time domain reflectometry technique

The complex permittivity of glycine in water mixture for various temperatures and concentrations have been measured as a function of frequency between 10?MHz and 30?GHz using time domain reflectometry technique. Dielectric parameters, i.e. static dielectric constant and relaxation time were obtained from the complex permittivity spectra using nonlinear least square fit method. The dielectric relaxation parameter increases with an increase in molar concentration of glycine due to the formation of hydrogen bond groups by glycine molecule in an aqueous solution medium. The activation entropy, activation enthalpy and Kirkwood correlation factor have also been determined for glycine–water mixtures.     

Hydration Dynamics of Water near an Amphiphilic Model Peptide at Low Hydration Levels: A Dielectric Relaxation Study

A dielectric relaxation study of aqueous solutions of the amphiphilic model peptide N‐acetyl‐leucine amide (NALA) at 298 K over a wide range of hydration levels is presented. The experiments range from states where water builds up several hydration layers to states where single water molecules or small water clusters are shared by several NALA molecules. The dielectric spectra reveal two modes on the 10 and 100 ps timescales. These are largely broadened with regard to the Lorentzian shape caused by simple Debye‐type relaxation, and are well described by the Kohlrausch–Williams–Watts stretched exponential function. The fast mode is assigned to water reorientation comprising bulk water as well as hydration water. Even when all water molecules are in contact with the solute, this fast component is dominant, and its mean relaxation time is retarded by less than a factor of two relative to neat water. The amplitude of the slow process is far higher than expected for the dipolar reorientation of the solute. The observations are consistent with results from molecular dynamics simulations for a similar model peptide reported in the literature. They suggest that the slow relaxation mode is mainly founded in peptide–water dipolar couplings, with some additional contribution from slowly reorienting hydration water molecules. The results are discussed with regard to the hydration dynamics of proteins and the interpretation of dielectric spectra of protein solutions.