Diffusion dielectric spectroscopy

When a chain has lost the memory of its initial state, rubbery flow sets in. The associated characteristic relaxation time is displayed in Fig. 1.3 in terms of the normal mode (polyisoprene displays an electric dipole moment in the direction of the chain) and thus dielectric spectroscopy is able to measure the relaxation of the end-to-end vector of a given chain. The rubbery flow passes over to liquid flow, which is characterized by the translational diffusion coefficient of the chain. Depending on the molecular weight, the characteristic length scales from the motion of a single bond to the overall chain diffusion may cover about three orders of magnitude, while the associated time scales easily may be stretched over ten or more orders. 

KEYWORDS interfaces, impedance, dielectric spectroscopy, non-destructive testing, durability, diffusion, coatings, water, neutron reflectivity... 

The dielectric spectroscopy is an essential probe for nondestructive studies required for biomolecule analysis. The study of internal motion/dynamics related to the dielectric relaxation in biomolcules require the coverage of periodic vibration along with other mechanisms, such as diffusion, molecular orientations, and relaxation processes. 

Dielectric spectroscopy has proved to be an important tool to describe emulsion and microemulsion systems. Professor Yuri Feldman and his Norwegian colleagues review in Chapter 6 the fundamentals of this technique and its plausible applications. The span of applications is very wide and includes flocculation processes in emulsions, diffusion processes and porosity measurements in solid materials, characterization of particulate biosuspensions, and percolation phenomena in microemulsions. 

Dielectric relaxation thus resembles self-diffusion. Both processes observe the motion of single macromolecules through a uniform albeit fluctuating background. In a two-component polymer-solvent system, dielectric spectroscopy reveals the effect of intermacromolecular interactions on single-molecule size and reorientation. Dielectric measurements on a three-component polymer-polymer-solvent mixture, in which a tracer polymer has a nonzero type-A dipole and a potentially nondilute matrix polymer has none, can be used for example to separate the effects of probe and matrix molecular weights on dielectric relaxation. This motif in the comparative study of binary and ternary solutions appears repeatedly below. Finally, dielectric measurements on block copolymers in which some copolymer subchains have been inverted end-to-end or have no dipole moment allow one to observe internal motions and dynamic cross-correlations of subchains. 

Three types of methods are used to study solvation in molecular solvents. These are primarily the methods commonly used in studying the structures of molecules. However, optical spectroscopy (IR and Raman) yields results that are difficult to interpret from the point of view of solvation and are thus not often used to measure solvation numbers. NMR is more successful, as the chemical shifts are chiefly affected by solvation. Measurement of solvation-dependent kinetic quantities is often used (<electrolytic mobility, diffusion coefficients, etc). These methods supply data on the region in the immediate vicinity of the ion, i.e. the primary solvation sphere, closely connected to the ion and moving together with it. By means of the third type of methods some static quantities entropy and compressibility as well as some non-thermodynamic quantities such as the dielectric constant) are measured. These methods also pertain to the secondary solvation-sphere, in which the solvent structure is affected by the presence of ions, but the... 

A variety of techniques has been employed to investigate aliovalent impurity-cation vacancy pairs and other point defects in ionic solids. Dielectric relaxation, optical absorption and emission spectroscopy, and ionic thermocurrent measurements have been most valuable ESR studies of Mn " in NaCl have shown the presence of impurity-vacancy pairs of at least five different symmetries. The techniques that have provided a wealth of information on the energies of migration, formation and other defect energies in ionic solids are diffusion and electrical conductivity measurements. Electrical conductivity in ionic solids occurs by the motion of ions through vacancies or of interstitial ions. In the case of motion through vacancies, the conductivity, a, is given by... 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

However, in an attempt to integrate the SFA and spectroscopic techniques, the use of silver for optical interferometry has been seen as a drawback due to the fact that it precluded sufficient excitation source intensity to illuminate the buried interface. In order to circumvent this problem Mukhopadhyay and co-workers in an experimental set-up where the SFA was combined with fluorescence correlation spectroscopy (FCS) used, instead of silver, multilayer dielectric coatings that allowed simultaneous interferometry and fluorescence measurements in different regions of the optical spectrum [75]. Using this set-up they succeeded in measuring diffusion in molecularly thin films with singlemolecule sensitivity. 

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