Dielectric Properties Of Solvent Research Articles

Effects of environmental exposure on solvent- and water-based epoxy systems

The dielectric properties of latex water-cast and conventional solvent-cast epoxy neat resin systems were investigated under three formation conditions: solvent-based as-made, water-based as-made, and water-based following a boiling fluid extraction procedure to remove residual surfactant. Samples from each of these three groups were cast, cured, and exposed to controlled humidities in an environmental chamber at room temperature and elevated temperature, a steam bath at various temperatures, and soaked in boiling water. Composite laminates, press fabricated using solvent and latex-based epoxy impregnated glass cloth, were subjected to similar environmental exposure. The microwave frequency dielectric properties of these materials, after each environmental exposure, were measured at 9.4 GHz. These dielectric measurements revealed that under identical exposure conditions, water-borne epoxy systems absorbed more water than solvent-based systems and also allowed more free water into the samples. Extraction of water and soluble components in the resin had no effect on the dielectric properties of the water-borne samples. The presence of high quantities of free water at moisture fractions beyond 4.0 wt %, suggests that the differences between the dielectric properties of solvent and water-borne epoxy systems are due to void morphology, which was confirmed by optical and electron microscopy. ©1999Kluwer Academic Publishers

Changes in polypeptide conformer populations induced by the solvent environment

Changes in the solvent dielectric properties are correlated with changes in polypeptide conformer populations. Gramicidin A, a peptide native to membrane environments, forms a variety of well defined dimeric conformations in relatively low dielectric organic solvents. However, changes in this environment lead to changes in the conformer populations with the lowest dielectric solvents favoring the conformers with the lowest net dipole moment. Such changes were characterized by cross-peak intensities in GCOSY solution NMR spectra. In studying membrane-bound polypeptides, it is very important to recognize that the structure is not dictated by just the amino acid sequence, but that the environment plays a very significant role in defining the polypeptide conformation. © 1998 John Wiley & Sons, Ltd.

Picosecond to Microsecond Photodynamics of a Nonplanar Nickel Porphyrin: Solvent Dielectric and Temperature Effects

The lifetime of the (dz2,dx2-y2) metal excited state of nickel(II) 5,10,15,20-tetra-tert-butylporphyrin (NiT(t-Bu)P) exhibits an extraordinary dependence on solvent dielectric properties and temperature. At room temperature, the excited-state deactivation time varies from 2 ps in highly polar solvents to about 50 ns in nonpolar media. The lifetimes increase to several microseconds in both polar and nonpolar solvents near 80 K. In contrast, the (d,d) lifetimes of nominally planar nickel porphyrins such as nickel tetraphenylporphyrin (NiTPP) vary only weakly with solvent dielectric properties and temperature, and typically fall in the range of 100 to 300 ps. All available evidence indicates that NiT(t-Bu)P in solution is highly ruffled (nonplanar) in the ground electronic state. It is proposed that the photoinduced conformational changes that occur in NiT(t-Bu)P in order to accommodate the excited-state electronic distribution are limited by the severe steric constraints imposed by the bulky meso tert-butyl substituents, and result in molecular and electronic asymmetry and thus a polar excited state. Solvent dielectric properties and temperature modulate these conformational excursions and thus the electronic deactivation rates by affecting the excited-state energies, porphyrin/solvent reorganizations, and the populations of low-frequency out-of-plane vibrations of the macrocycle. The novel findings for this nonplanar nickel porphyrin demonstrate the intimate connectivity that exists between the static and dynamic molecular structures of porphyrins and their ground- and excited-state electronic properties. Furthermore, the results obtained provide insights into the interactions between tetrapyrrole chromophores and their host proteins, and suggest the potential use of nonplanar porphyrins as building blocks for molecular photonics applications.

Stokes Shift as a Tool for Probing the Solvent Reorganization Energy

The difference between the maxima of absorption and fluorescence spectra, the Stokes shift, is affected by dynamic properties of the solvent, namely by the solvent reorganization free energy, Es. Stokes shift a dye, proflavine, in polar aprotic solvents and in alcohols is examined. We report that the experimentally observed dependence of Es on the solvent dielectric properties can be explained only when two effects are taken into account. The first effect is the distortion of dielectric properties of a uniform solvent by embedding a bulk solute molecule in the solvent. The second effect is the influence of the solvent polarity on the electronic density redistribution upon the electronic transition. Our model evaluations of Es are based on quantum chemical calculations of solvent effects on charge redistribution of the dye upon excitation and on subsequent calculations of Es in the framework of a model of a cavity in a continuum dielectric. We show that the continuum approach is able in the present case to...

Phase stability of phosphatidylcholines in dimethylsulfoxide solutions

The temperature dependence of the phase stability of dispersions of dimyristoyl, dipalmitoyl, and distearoyl derivatives of phosphatidylcholines in excess aqueous dimethylsulfoxide has been examined by differential scanning calorimetry and synchrotron x-ray diffraction methods. There was a close correlation between the enthalpic transitions and the structural changes associated with the pre- and main transitions of the phospholipids in the range of concentrations up to mole fractions of dimethylsulfoxide in water of 0.1333. The temperature of the pre- and main transitions of the three phospholipids were found to increase linearly with increasing mole fraction of dimethylsulfoxide. The difference in phase stability between the lamellar gel and ripple phases induced by increasing dimethylsulfoxide concentration resulted in disappearance of the ripple phase and direct transition between lamellar gel and lamellar liquid-crystal phases. The effect of changing the properties of the solvent by the addition of dimethylsulfoxide on the dimensions of dipalmitoylphosphatidylcholine and solvent layers of the bilayer repeat structure has been determined from electron density distribution calculations. The lamellar repeat spacing recorded at 25 degrees C decreased from 6.36 nm in aqueous dispersion to 6.04 nm in a dispersion containing a mole fraction of 0.1105 dimethylsulfoxide. The results indicate that dipole interactions between solvent and phospholipid and dielectric properties of the solvent are important factors in the determination of the structure of saturated phosphatidylcholines.

Excitation wavelength and solvent dependent emission spectra in weakly polar aprotic solvents at room temperature and 77 K

Excitation wavelength-dependent emission spectra of 4-methyl-2,6-diformylphenol (MFOH) and o -hydroxybenzaldehyde (OHBA) have been examined both in pure weakly polar aprotic solvents and in the presence of a base at room temperature and 77 K. It is shown that fluorescence quantum yield shape, position of the spectra, and number of conformers are dependent upon the excitation energy and also on the proton-accepting ability of the solvents. Fluorescence spectra cannot be correlated with the solvent dielectric properties. At 77 K, deactivation occurs via phosphorescence only at a particular experimental condition in all the solvents studied here. The decay rates are relatively slower in an environment where the probability of hydrogen bonding interaction is stronger.

The relation of solvatochromism and thermochromism to the solvent dielectric constant: The basis of the ET and E'T polarity scales

AbstractWe have measured the thermochromism of pyridinium‐N‐phenoxide betaine and its penta‐tert‐butyl substituted derivative dissolved in MTHF together with the solvent dielectric properties in order to relate polarity scales to the dielectric constants of the solvent. Both thermochromic and solvatochromic shifts of the absorption band of the dyes can be rationalized on the basis of the MSA theory for relating the solvation free energy to the static dielectric constant. Deviations between theory and experiment in the regime of higher viscosities can be attributed to changes in the offset between absorption maximum and electronic transition energy.

Solvent dielectric relaxation properties and the external heavy atom effect in the time-resolved fluorescence quenching of anthracene by potassium lodide and potassium thiocyanate in methanol and ethanol

Steady-state fluorescence quenching and the time-resolved fluorescence decay profiles of anthracene in the presence of iodide and thiocyanate ions in methanol and ethanol have been measured in the temperature range between 253 and 333 K. The Rips–Jortner and Onuchic theories of electron transfer are applied for data analysis. We are able to fit our experimental data with reasonable values of parameters characterizing both chemical reaction and molecular transport. Analysis of the experimental data indicates the importance of solvent dielectric relaxation properties for the fluorescence quenching. In the case of iodide ions as quencher the external heavy atom effect seems also to play an important role in the quenching process.

The influence of wave vector dependent dielectric properties on rotational friction. Rotational diffusion of phenoxazine dyes

The rotational diffusion of three mechanically similar phenoxazine dyes possessing distinct electrical properties was studied in isopropanol. The results, along with previously presented results from other polar solvents, were analyzed in terms of continuum theories for rotational dielectric friction. It was found that a continuum based theory can account for the observed rotational relaxation dynamics, but only with realistic modeling of the solute charge distribution (not a point dipole), and by accounting for both frequency and wave vector dependences of the solvent dielectric properties.

Polymer-mediated electrostatic interactions between charged lipid assemblies and electrolyte solutions: A tentative model of the polyethylene glycol-induced cell fusion

We developed a theoretical model to investigate the interaction between charged lipid aggregates and a water solution containing ions and uncharged polymers. The local concentration of ions and polymer chains around the lipid aggregate have been treated as variational parameters which can be found by minimizing the total energy of the system. We divided the energy into the following main contributions: (a) Solvation energy of the ions. This depends on the local polymer concentration through the variation of the solvent dielectric properties. (b) Ions-lipid aggregate interactions. These depend on the local concentrations both of the ion cloud and polymer chains. (c) Conformational energy of the polymer. This term is related to the inhomogeneous spatial density of the polymer segments. Any direct interaction between the charged lipid surface and the polymer coils has been intentionally neglected. The minimization procedure leads to a non-linear Poisson-Boltzmann equation coupled with a non-linear algebraic equation describing the polymer distribution. The solution of the above system allows one to calculate the ions and polymer spatial distribution around the lipid aggregate. The knowledge of such parameters is useful to predict the effect of non-ionic polymers on the structure and properties of lipid assemblies such as the mean area per lipid molecule, the aggregation number, the critical micellar concentration and the formation of immiscibility gaps in mixed lipid systems. A possible involvement of these parameters into the fusion process between lipid vesicles is discussed.

Solute thermodynamic properties at high temperature and pressure modeled on solvent dielectric properties

Solute thermodynamic properties at high temperature and pressure modeled on solvent dielectric properties

Solvent reorganization in optical and thermal electron-transfer processes: solvatochromism and intramolecular electron-transfer barriers in spheroidal molecules

Expressions for the shift of the absorption and emission band maxima of a solute with changes of the solvent's dielectric properties are presented for solute molecules with shapes and polarizabilities that can be approximated by those of spherical dielectric-continuum cavity. The derivations use a nonequilibrium thermodynamic approach developed by Marcus. An series of accurate approximations for the solvent shifts are presented. The new expressions are shown to reduce to equations previously derived by McRae, Ooshika, Mataga, and others when point-dipole approximations are made and particular values of the solute polarizability are assigned. The polarizability of the solute can be related to the internal dielectric constant of the cavity, and general expressions for the band shift in the point-dipole limit containing the dielectric constant of the cavity as a parameter are derived.

High-pressure studies of rotational reorientation dynamics: The role of dielectric friction

Picosecond, time-resolved, fluorescence depolarization spectroscopy is used to measure the rotational reorientation times of rhodamine 6G (R6G) and p-terphenyl (PTP) as a function of solvent viscosity. The viscosity is varied either by changing the solvent or by changing the pressure in a single solvent. The differences between the two molecules, PTP and R6G, provide a means of evaluating the role of solute structure and solute–solvent interactions on the dynamics of rotational reorientation. The rotational behavior of PTP is well described by simple hydrodynamic models as embodied by the Stokes–Einstein–Debye equation. In contrast, the rotational reorientation dynamics of the charged molecule R6G are not well described by these models. It is demonstrated that dielectric friction plays an important role in governing the rotational motion of charged molecules in polar solvents. When the solvent dielectric properties are varied, the dielectric friction model accurately predicts the observed experimental trends under a wide variety of experimental conditions. This model is also shown to explain anomalous effects previously attributed to the presence of solute–solvent hydrogen bonded complexes.

The temperature dependence of the dielectric properties of unassociated organic solvents at polarizable interfaces

As it was shown previously [1, 2], the three state model for solvent dipoles can be used successfully to describe the differential capacity at Hg/nonaqueous interface in the case of unassociated or weakly associated organic solvents. The model may be tested further by examining the temperature dependence of the surface potential and the adsorption entropy as was done previously for Hg/aqueous interface [3, 4]. The temperature coefficient of the surface potential for a number of organic solvents with the exception for methanol is positive [5]. However the interpretation of this quantity in terms of solvent dipole orientation and the sign of the surface potential depends on the molecular model used to describe the dielectric properties of the solvent monolayer. Another quantity which is accessible experimentally is the solvent adsorption entropy. The only data available for nonaqueous solvents are those Hg/propylene carbonate interface [6]. It is evident that the accepted interpretation based on a two state model is not valid in this case. The molar adsorption entropy has a maximum at a negative charge density which would suggest a preferred orientation at the uncharged mercury with the negative end of the propylene carbonate dipole pointing towards the metal and consequently, a negative surface potential. However the occurrence of the solvent capacity ‘hump’ at a high positive charge density would imply just the opposite. The purpose of this paper is to discuss the temperature dependence of the dielectric properties of the Hg/solution interface in the case of N,N-dimethylformamide, methanol and acetone including also the literature data for propylene carbonate [5]. The adsorption entropy has been calculated on the basis of the temperature dependence of the double layer differential capacity following the thermodynamic arguments of Harrison et al. [7]. The results have been compared with the prediction of three state model.

The conductance of tetraethylammonium salts in water-methanol mixtures

Conductance measurements are reported for tetraethylammonium halide and perchlorate salts in water-methanol mixtures over the complete composition range at 25° C. Limiting conductances and association constants are evaluated together with the distance parameters. The maxima in the limiting ionic Walden products are considered in some detail. Ionic association is generally weak, and the lack of reliable correlation betweenK A and the solvent dielectric properties supports the assumption of specific ion-solvent interaction.

Tissue ultrastructure and alterations as a result of applied hydrostatic pressure in two marine teleosts

1.1. The cell ultrastructure of gill, liver and skeletal muscle has been investigated in two abyssal teleosts, Antimora rostrata and Synaphobranchus sp. Tissues removed immediately after capture were used to identify any alteration due to ascent decompression, and also, as controls during in vitro pressurization and decompression experiments.2.2. Major ultrastructural changes are observed only during rapid, in vitro, decompression. These changes, particularly noticeable in gill and muscle tissues, consist of disrupted membranes and the loss of structural protein integrity. Minor disruption occurred in some membrane systems of the eel, Synaphobranchus sp., even with fixation at pressure, but none were evident in Antimora tissues.3.3. It is postulated that the ultrastructural disruption during rapid decompression may result from volume changes due to ionization and macromolecular conformation, submicroscopic gas formation, variations in solvent dielectric properties, or other unknown forces. These factors are examined and testable predictions established which may suggest the importance of each. Also, these benthic fish tissues appear to be unusual in not showing the extensive cellular disorganization following pressurization seen in many unicellular and multicellular organisms.

Conductances, densities, and viscosities of concentrated solutions of lithium chlorate and of sodium chlorate in water–dioxane mixtures, at 25°

The conductances, densities, and viscosities of sodium chlorate and of lithium chlorate in the solvents 44.5% dioxane–55.5% water and 64.5% dioxane–35.5% water were determined, at 25 °C, almost up to saturation. The apparent molal volume of the electrolytes, calculated from the density data, was found to be greater in the dioxane–water solvents than it is in water, indicating a decrease in solvation in the mixed solvents. The relative viscosity increased as the dioxane content of the solvent increased, showing a larger ordering effect of the electrolytes as the solvent became less hydrogen bonded and less ordered. The equivalent conductance of both electrolytes was greatly reduced as the dioxane content of the solvent was increased, in accord with predictions based on solvent viscosity and dielectric properties. Also, sodium chlorate was found to be more associated than lithium chlorate in the mixed solvents.

Effect of substituents on the chemical shift of benzylic protons

The chemical shift of the benzylic protons in fifteen series of para-substituted toluenes bearing different groups in the alpha position are reported. In each series a plot of the chemical shift in tau units of the benzylic protons against the Hammett σ value of the para substituent is linear. The slopes of these plots vary from −0.20 to +0.01 p.p.m./σ. The variation of slopes appears to be conformationally dependent, an observation at odds with current theory. In two rigid systems having the same geometry the slope has been found to vary from −0.12 to −0.18 indicating a dependence on other factors as well. The effect of solvent on the slopes has been determined, and shows no correlation with solvent dielectric properties.

Vibrational spectra and intermolecular association

The nature of the intermolecular association of furfural and of some nitroso aromatic compounds has been examined by measuring the infrared absorption spectra in different states of aggregation and under a variety of physical conditions. In the case of furfural, the behaviour of several bands suggests that dipolar aggregation may occur. Variations of intensity or the shifts in position of some of these bands have been correlated with the dielectric properties of solvents in which they arise. From the spectra of nitrosobenzene, nitroso-mesitylene and p-iodo-nitrosobenzene, vibration frequencies have been suggested for the NO group in the monomeric and dimeric forms.

Influence of Ion on the Dielectric Properties of Polar Solvents

Influence of Ion on the Dielectric Properties of Polar Solvents