Microwave dielectric spectroscopy

J. Grigas, Microwave Dielectrics spectroscopy of Ferroelectrics and Related Materials, Gordon and Beach Publishers, 1996. 

Also the question How polar are ionic liquids has been addressed by many methods that previously have been used to characterize the polarity of common molecular solvents. The macroscopic constant, generally used by the chemists to evaluate the solvent power of a molecular liquid, the dielectric constant, has been evaluated in the case of ILs initially using indirect metliods. and more recently by microwave dielectric spectroscopy. Generally, the values found for the investigated ILs are moderate and, at least those obtained by microwave dielectric spectroscopy, insignificantly affected by the IL structure. 

Wakai C, Oleinikova A, Ott M (2005) How polar are ionic liquids Determination of the static dielectric constant of an imidazolium-based Ionic liquid by microwave dielectric spectroscopy. J Phys Chem B Lett 109 17028-17030... 

Microwave dielectric spectroscopy has been used to estimate values for the static dielectric constant of a small number of [RMIM]" ionic liquids (Table 3.5-1) [5]. Values ranging from 9 to 15 were found, depending on the ionic liquid. This is of the order found for molecular solvents of modest polarity. The dielectric constant was found to decrease as the length of the alkyl chain increased. This is similar to the behavior of homologous series in molecular solvents. The dielectric constant also decreased in the order [OTf] > [Bp4] > [PFe] -... 

The dielectric constant (Cj.) of the pure hquid is by far the most commonly used polarity scale. Estimates of static dielectric constants of ionic liquids have been derived from microwave dielectric spectroscopy [93]. Most values found ranged from 9 to 15 depending upon the ionic hquid, characterizing these as nonpolar hquids. However, values in excess of 30 were found for some protic ionic liquids. 

From a hydration study with microwave dielectric spectroscopy the protein surface changes its surface... 

Arnold, C.B. Sutto, T.E. Kim, H.K. Pique, A. (2004). Direct-wiite laser processing for miniature electrochemical systems. Laser Focus World, 40., 5., 9-12., 1043-8092 Benenson, W., Harris, J. W., Stocker, H, Lutz, H. (2002). Handbook of Physics, Springer-Verlag New York, Inc., ISBN 0-387-95269-1, New York, USA Bright, F., Baker, G. A (2006). Comment on "How Polar Are Ionic Liquids Determination of the Static Dielectric Constant of an Imidazohum-based Ionic Liquid by Microwave Dielectric Spectroscopy". The Journal of Physical Chemistry B, 110., 11., 5822-5823., 1520-6106... 

Wakai and coworkers ( Wakai et al., 2005) determined the static dielectric constants of 1-alkyl-3-methylimidazolium ionic liquids by microwave dielectric spectroscopy in the megahertz/gigahertz regime. The obtained results classify the ILs as moderately polar solvents. The observed e-values at 298.15 K fall between 15.2 and 8.8 and decrease with increasing chain length of the alkyl residue of the cation. The anion sequence is trifluoromethylsulfonate > tetrafluoroborate tetrafluorophosphate. The results indicate markedly lower polarities than the ones found by spectroscopy with polarity-sensitive solvatochromic dyes ( Wakai et al., 2005). 

Marand, E., Baker, K.R. and Graybeal, J.D., Comparison of reaction-mechanisms ofepoxy-resins undergoing thermal and microwave cure from in situ measurements of microwave dielectric-properties and infrared-spectroscopy, Macromolecules, 1992, 25, 2243. 

To date, a number of chemically selective near-field imaging methods have been demonstrated. Near-field contrast mechanisms that rely on electronic spectroscopy (UV-visible absorption and fluorescence),204 vibrational spectroscopy (IR absorption and Raman spectroscopies), dielectric spectroscopy (microwave dispersion), and nonlinear spectroscopy (second harmonic generation) have all been demonstrated at length scales well below the diffraction limit of light. 

There is a clear difference between microwave spectroscopy and microwave dielectric heating effects. Thus, in microwave spectroscopy, molecules are examined in the gas phase and the microwave spectrum for a molecule exhibits many sharp bands [15] over the frequency range 3-60 GHz. Such sharp bands arise from transitions between quantized rotational states of the molecules. Microwave spectroscopy provides an excellent fingerprinting method for identifying molecules in a gas phase and has been used, for example, to confirm the presence of a wide range of molecules in outer space. 

From microwave dielectric relaxation , from impedance spectroscopy , from NMR , from a.c. conductivity reference 17,... 

Progress in the understanding of superionic conduction is due to the use of various advanced techniques (X-ray (neutron) diffuse scattering, Raman spectroscopy and a.c.-impedance spectroscopy) and-in the particular case of protons - neutron scattering, nuclear magnetic resonance, infrared spectroscopy and microwave dielectric relaxation appear to be the most powerful methods. A number of books about solid electrolytes published since 1976 hardly mention proton conductors and relatively few review papers, limited in scope, have appeared on this subject. Proton transfer across biological membranes has received considerable attention but is not considered here (see references for more details). 

Dielectric spectroscopy permittivity, static permittivity Dielelectric polarization, microwave radiation Frequency-dependent, complex... 

Apart from the role that sub-THz radiations play in polymer chemistry, they are also very usefully applied in polymer physics. As an example, this might involve dielectric spectroscopy, which is used widely to characterize the molecular dynamics of polymers. Moreover, microwaves are employed for the nondestructive testing of polymer materials, an example being the detection of heterogeneities in plastic articles. 

The dielectric permittivity as a function of frequency may show resonance behavior in the case of gas molecules as studied in microwave spectroscopy (25) or more likely relaxation phenomena in soUds associated with the dissipative processes of polarization of molecules, be they nonpolar, dipolar, etc. There are exceptional circumstances of ferromagnetic resonance, electron magnetic resonance, or nmr. In most microwave treatments, the power dissipation or absorption process is described phenomenologically by equation 5, whatever the detailed molecular processes. 

The dipole moment of a molecule can be obtained from a measurement of the variation with temperature of the dielectric constant of a pure liquid or gaseous substance. In an electric field, as between the electrostatically charged plates of a capacitor, polar molecules tend to orient themselves, each one pointing its positive end toward the negative plate and its negative end toward the positive plate. This orientation of the molecules partially neutralizes the applied field and thus increases the capacity of the capacitor, an effect described by saying that the substance has a dielectric constant greater than unity (80 for liquid water at 20°C). The dipole moments of some simple molecules can also be determined very accurately by microwave spectroscopy. 

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