Dielectric relaxation time-domain spectroscopy

The observation of slow, confined water motion in AOT reverse micelles is also supported by measured dielectric relaxation of the water pool. Using terahertz time-domain spectroscopy, the dielectric properties of water in the reverse micelles have been investigated by Mittleman et al. [36]. They found that both the time scale and amplitude of the relaxation was smaller than those of bulk water. They attributed these results to the reduction of long-range collective motion due to the confinement of the water in the nanometer-sized micelles. These results suggested that free water motion in the reverse micelles are not equivalent to bulk solvation dynamics. 

Figure 5 Ranges of some dielectric relaxation spectroscopy techniques (a) conventional transient methods (b) newer transient methods (c) low frequency impedance bridge (d) conventional impedance bridges and analyzers (e) high frequency impedance analyzers and time domain spectroscopy (f) microwave cavities, transmission lines ...

In most cases, the measurements are carried out isothermally in the frequency domain and the terms dielectric spectroscopy (DS) and dielectric relaxation spectroscopy (DRS) are then used. Other terms frequently used for DRS are impedance spectroscopy and admittance spectroscopy. Impedance spectroscopy is usually used in connection with electrolytes and electrochemical studies, whereas admittance spectroscopy often refers to semiconductors and devices. Isothermal measurements in the time domain are often used, either as a convenient tool for extending the range of measurements to low frequencies (slow time-domain spectroscopy, dc transient current method, isothermal charging-discharging current measurements) or for fast measurements corresponding to the frequency range of about 10 MHz - 10 GHz (time-domain spectroscopy or time-domain reflectometry). Finally, TSDC is a special dielectric technique in the temperature domain, which will be discussed in Section 2.2. 

Usually, the depolarization current is measured to avoid the dc conductivity contribution. The dielectric relaxation spectrum is then obtained by Fourier transform or approximate formulas, e.g., the Hamon approximation [14]. By carefully controlling the sample temperature and accurately measuring the depolarization current, precision measurements of the dielectric permittivity down to 10" Hz are possible [18]. In fast time domain spectroscopy or reflectometry, a step-like pulse propagates through a coaxial line and is reflected from the sample section placed at the end of the line. The difference between... 

Dielectric relaxation spectroscopy can probe the very broad time domain between 10 and 10 s. In principle, therefore, this method can be used... 

The FT technique has been applied in a multitude of different areas. Starting at low frequencies, FT methods have been used for dielectric response spectroscopy of solids (sometimes called time domain reflectometry). A short picosecond voltage pulse is applied to a dielectric and the current response is measured. Fourier transformation of the current gives the dielectric response function, s v), which is typically interpreted as the Debye relaxation of dipoles. 

Recently, the same behavior was demonstrated for the system water (NaCl 8%) + decane 1 1-butanol-A-octylribonamide (CsNg), by using H chemical shift and relaxation time data. At saturation, the molar ratio of bound water to OH groups is again about 1 [135]. This is somewhat surprising, as usually the water solubilization behavior revealed by spectroscopic techniques is entirely different. Thus, NMR [14] (Fig. 17), time domain dielectric spectroscopy (TDS) [136], ESR [137], and Fourier transform infrared (FTIR) [15] measurements indicate that upon the addition of even a small amount of water, an equilibrium between free and bound water is established. This apparent discrepancy is readily understood because the spectroscopic techniques sense the water molecules most near the surfactant. 

Dielectric spectroscopy also allows monitoring the structural relaxation time and, eventually, its slowdown as expected in the case of higher fractions of MROs. Furthermore, investigation of the form of the relaxation peaks in the frequency domain permitted us to characterize the dynamic heterogeneity of our films. For this purpose, a quantitative analysis was achieved via fits of the experimental data to the Havrialiak-Negami equation... 

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