Dielectric dispersions

Anotlier teclmique used for stmctural inference is dielectric dispersion in tlie frequency [25] or time [26] domains. The biopolymer under investigation must have a pennanent dipole moment p. It is first dissolved in a dielectrically inert solvent, e.g. octanol, which may be considered to bear some resemblance to a biological lipid membrane, and tlien tlie complex impedance i +j( is measured over a range of frequencies / typically from a... 

Blanton S A ef a/1997 Dielectric dispersion measurements of CdSe nanocrystals colloids observations of a permanent dipole moment Phys. Rev. Lett. 79 865... 

Changes of s and z" with frequency are shown in Fig. 1.4. The frequency is displayed on a logarithmic scale. The dielectric dispersion covers a wide range of frequencies. 

Figure 12.5. Photograph of microdispensing system depositing an inorganic dielectric dispersion onto a patterned, metallized polyester film. The pattern is of a transistor gate electrode array. The microdispensing head atomizes the dispersion, generating a liquid spray much like a dual orifice atomizer found on an airbrush.

Polysulfone Cheiin DvncUtiics. Carbon-13 nmr has been used recently to resolve a puzzling problem in the dynamics of polysulfone chains (9,10,11). Some years ago. Bates, Ivin and Williams (12) reported that measurements of the dielectric dispersion in solutions of alternating 1 1 copolymers of sulfur dioxide with hexene-1 and 2-methylpentene-1 show no loss in the high frequency region, Le. 

At high temperatures above Tb 617 K PMN behaves Hke all other simple perovskites. The dynamics of the system is determined by the soft transverse optical (TO) phonon which exhibits a normal dispersion and is imderdamped at all wave vectors. Below Tb, in addition to the soft mode—which becomes overdamped—a new dielectric dispersion mechanism appears at lower frequencies which can be described by a correlation time distribution function /(t). 

Fig. 10 Relaxation time distribution function/(r) describing the dielectric dispersion in relaxor PMN. The short timescale maximnm describes the glassy-type dynamics, whereas the long timescale part refers to the polar clnster dynamics. The same featnres are obtained in PMN, PLZT, and SEN relaxors...

Raicu, V. 1999. Dielectric dispersion of biological matter Model combining Debye-type and universal responses. Phys. Rev. E 60 4667-80. 

Before leaving SD applications of the LRA, it is worth stressing that a different approach has often been taken in relating the electrostatic C i) to pine solvent dynamics. In this approach, the connection is made to solvent dielectric relaxation. Early theories made the connection through the longitudinal dielectric relaxation time," while more recent ones use as input the dielectric dispersion s or its generalization to finite wavevectors, e(k, co). 23.24.29,68,89... 

Axe, J. D., and G. D. Pettit, 1966. Infrared dielectric dispersion and lattice dynamics of uranium dioxide and thorium dioxide, Phys. Rev., 151, 676-680. 

Lane, J. A., and J. A. Saxton, 1952. Dielectric dispersion in pure polar liquids at very high radio frequencies I. Measurements on water, methyl and ethyl alcohols, Proc. R. Soc. Lond., A2I3, 400-408. 

The rotational diffusion coefficient Dr of a rodlike polymer in isotropic solutions can be measured by electric, flow, and magnetic birefringence, dynamic light scattering, and dielectric dispersion. However, if the polymer has some flexibility, its internal motion makes it difficult to extract Dr for the end-over-end rotation of the chain from data of these measurements. In other words, Dr can be measured only for nearly rodlike polymers. 

Some time ago we measured the dielectric dispersion of a plant virus particle and found a critical frequency in the kHz region, therefore in a region that may well correspond to the eventual maximum of your ultrasonic absorption. We were able to describe the mechanism leading to this dispersion in terms of the rotation of the complete virus particle and of the motion of associated (or bound) counterions on the elongated surface of the particle. 

Chapter E is devoted to the mean-square dipole moment and mean rotational relaxation time derived from dielectric dispersion measurements. Typical data, both in helieogenic solvents and in the helix-coil transition region, are presented and interpreted in terms of existing theories. At thermodynamic equilibrium, helical and randomly coiled sequences in a polypeptide chain are fluctuating from moment to moment about certain averages. These fluctuations involve local interconversions of helix and random-coil residues. Recently, it has been shown that certain mean relaxation times of such local processes can be estimated by dielectric dispersion experiment. Chapter E also discusses the underlying theory of this possibility. 

The most familiar method of evaluating is by dielectric dispersion experiments, in which the real and imaginary parts of the complex dielectric constant over those of the solvent are determined as functions of frequency. It is the value of referring to the state of vacuum that can be correlated with the molecular structure of the solute. Polymers cannot be dispersed in the gaseous state. Furthermore, solvents effective for polypeptides are usually polar, and only approximate theories are presently available for the estimate of vacuum < 2> from dielectric measurements with polar solvents. Therefore the dipolar information about polypeptides is always beset with ambiguity in absolute magnitude as well as in interpretation. 

Dielectric dispersion measurements also provide a means of determining rotational diffusion coefficients or mean rotational relaxation times of solute molecules. In principle, data for these hydrodynamic quantities can be used for a... 

This chapter.surveys applications of the dielectric dispersion method to polypeptides in dilute solution. 

Since the mathematical expression for < u2) is equivalent to that for , measurements of should provide information which can be utilized to check the theory of , e.g. Eq. (C-3), for polypeptides in the helix-coil transition region. This idea, however, cannot be developed in straightforward fashion because there is no available theory to estimate of interrupted helical polypeptides from dielectric dispersion curves. Therefore, we are forced to proceed on some yet unproven assumptions, or even drastic approximations. 

Fig. 38. Helix-coil transition of a PBLG sample (Mw = 59000) in a DCA-CHa3 mixture (70 30) detected by ORD ( ) and by dielectric dispersion (O). (+) (US). Here d and e represent the quantities defined by Eq. (E-6) and Eq. (E-7), respectively, and tj f. denotes the critical frequency of the dispersion corrected for solvent viscosity...

Thus the time constant t may also be estimated from (e )J h, the value of (e )ch extrapolated to zero frequency, and (co< ")SU the value of (coc")ch extrapolated to infinite frequency. For a chemically induced dielectric dispersion to be observable experimentally, the time constant t ought to be much shorter than the mean rotational relaxation time of the solute molecule, yet still in the range accessible to available experimental techniques. 

Wada et al. (120) reported the observation of a secondary dielectric dispersion with a high-molecular-weight sample of PBLA in wi-cresol and showed that the resulting values of t as a function of / were interpretable in terms of their own theory essentially identical to Schwarz s one. 

Fig. 1.1 Dielectric dispersion spectra for a polar solvent with a single Debye relaxation process in the micro-wave region and two resonant transmissions in the IR and UV ranges [5 b].

A dynamic rotational isomeric state description is presented of a,high frequency dielectric dispersion. 

In the case of electron transfer reactions, besides data on the dynamic Stokes shift and ultrafast laser spectroscopy, data on the dielectric dispersion (w) of the solvent can provide invaluable supplementary information. In the case of other reactions, such as isomerizations, it appears that the analogous data, for example, on a solvent viscosity frequency dependence 17 ( ), or on a dynamic Stokes fluorescence shift may presently be absent. Its absence probably provides one main source of the differences in opinion [5, 40-43] on solvent dynamics treatments of isomerization. 

Saito,K., Kaneko,M., Furuichi,J. Dielectric dispersion of poly-y-benzyl L-glutamate. J. Phys. Soc. Japan 19,577 (1964). 

Baur and Stockmayer (13) have recently observed a low frequency dielectric dispersion zone in liquid poly(propylene) oxide which is dependent upon molecular weight in the manner of Eqs. (2.8) and (2.9). Due to the method of synthesis of their samples, there are only infrequent reversals of dipolar sense along the chain, and the model discussed above... 

Table 6. Dielectric dispersion behavior in the continuum limit...

The Onsager cavity description of solvation treats the solvent as a dielectric continuum. The dielectric dynamics of the solvent is typically characterized by the frequency-dependent complex dielectric constant s(co). The measurement of (co) for a neat solvent is conventionally called a dielectric dispersion measurement. 

The dielectric dispersion for some solvents is poorly modeled by a multiple Debye form. Alternative, e(cu) distributions such as the Davidson-Cole equation or the Cole-Cole equation are often more appropriate. 

Besides the obvious practical importance of solvation dynamics in water, from a fundamental standpoint, water offers a unique opportunity to evaluate theoretical models for solvation. Only for water have extensive molecular dynamics simulations been accomplished (see below). Also semi-empirical models for solvation dynamics, such as MSA, can be carefully examined for water because the necessary information on the dielectric dispersion 2(high level of accuracy. The results of the solvation... 

Thcse equations require that the dielectric constant decrease from the static to the optical dielectric constant with increasing frequency, while the dielectric loss changes from zero to a maximum value f" and back to zero. These changes are the phenomenon of anomalous dielectric dispersion. From the above equations, it follows that... 

Sengwa, R.J. and Kaur, K., Dielectric dispersion studies of poly(vinyl alcohol) in aqueous solutions, Polym. Int., 2000, 49, 1314. 

Buckley, F. and Maryott, A. 1958. Tables o Dielectric Dispersion Data for Pure Liquids and Dilute Solutions. National Bureau of Standards Circular 589. Washington, DC. (Available through National Technical Information Service, Springfield, VA. 

Dielectric dispersion 10 3-10 12 Power loss, capacitance change... 

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