"Dielectric spectroscopy can provide information about the segmental mobility of a polymer by probing its dielectric properties. The complex dielectric properties, the loss factor(e") and the relative permittivity(e'), are determined by performing several isothermal scans as a function of frequency. An alternating current(Vrms=0.005-1.1 volts) external electric field is applied across the DUT(Device Under Test) in a capacitor plate configuration. The applied alternating electric field interacts with the electric dipole moments of the DUT."
"Each dielectric mechanism effect has a characteristic relaxation frequency. As the frequency becomes larger, the slower mechanisms drop off. This in turn leaves only the faster mechanisms to contribute to the dielectric storage."
"Dielectric relaxation is the result of a movement of dipoles or electric charges due to a changing electric field in the frequency range of 10^2-10^10 Hz. This mechanism is a relatively slow process when compared with electronic transitions or molecular vibrations which have frequencies above 10^12 Hz. Only when sufficient time is allowed after the application of an electric field for the orientation to attain equilibrium will the maximum polarization, corresponding to the highest observable dielectric constant, be realized in a material."
Dielectric spectroscopymeter
In dielectric spectroscopy the current flowing through a sample cell containing a colloidal suspension and the voltage across this cell are measured as a function of frequency. From this data one can obtain the impedance of the solution as a function of frequency. The impedance can then be separated into the frequency dependent conductivity and relative permittivity of the solution. A schematic of a dielectric spectrometer is given below.
An oscillatory field applied to a colloidal suspension changes the distribution of ions in the electrostatic double layer, as well as the neutral region just outside of the double layer. The applied field polarizes the double layer when time scales of ionic transport processes are fast compared with the period of the oscillatory field. High polarization is manifested as a relative dielectric permittivity that may be much greater than that of the suspending medium. If we increase the frequency of the applied field, the polarization and relative dielectric permittivity decrease and the latter eventually approaches that of the suspending medium. This process, dielectric relaxation, can therefore indicate the time scales of ionic transport processes near particle surfaces.Dielectric spectroscopy characterizes the dynamics of double layer relaxation and yields more information per measurement than static methods such as electrophoresis. Full interpretation of dielectric models requires the use of colloidal electrodynamics. These models usually rely upon electrostatic parameters that are obtained through electrokinetic methods. Thus the availability of both electrokinetic and dielectric techniques offer an advantage for reconciling and interpreting measurements of particle surface structure and electrochemistry.
Other application:
The range of potential applications of dielectric spectroscopy is quite broad. Virtually any physical process change leads to changes in dielectric properties of samples. Process variability is a primary concern for the pharmaceutical industry (1). Exposure to mechanical and thermal stress can cause a change in the physical properties of pharmaceuticals. Such variations are important to control because physical properties generally determine the efficacy of the drug...There are two major properties of dielectric spectroscopy that are typically varied to suit the desired application: the spatial distribution of the interrogation field and the interrogation frequency range....Direct sensing or preconcentrators? There are many cases in which gas or liquid analytes must be sensed in small concentrations close to, or below, the detection threshold of a dielectric spectroscopy measurement device. Direct sensing is simpler than preconcentration; however, direct sensing is not always possible. Preconcentration should be performed in the following cases:
When the analyte is at a concentration at or below the measurement threshold
To select an analyte of interest from a mixture of gases or liquids....
Single frequency or spectroscopy? Relaxation processes in dielectric spectroscopy are very similar to relaxation processes in the optical regime. However, the interrogation frequencies used in dielectric spectroscopy are lower than optical frequencies, so this technique studies molecular interactions such as polymer reconfiguration within a matrix, percolation processes, and moisture diffusion. A major advantage of dielectric spectroscopy is that it can be performed over a wide band of measurement frequencies. The lowest boundary for frequency in existing dielectric spectroscopy is around 1 μHz, and the highest is in THz range. It is rarely practical to go to such extremes; most practical industrial measurements are accomplished in the range from 1 Hz to 100 MHz...
Further reading
A significant amount of literature is available that describes the theoretical aspects of dielectric behavior, algorithms, and signal processing methods used for the processsing of dielectric data and sensor design for dielectric measurements. One of the earliest models describing the frequency dependence of dielectric behavior was proposed by Debye (43). Jonscher describes dielectric relaxation emphasizing solids materials (44). Dielectric spectroscopy also has been explored in detail for polymeric materials (36). An understanding of dielectric behavior for engineers is presented by Coelho (45). Specifically for pharmaceuticals, a comprehensive review of applications is provided by Craig (14). MacDonald provides a review of methods for measurement of dielectric properties (46). A number of algorithms have been proposed for calibration and correlating data to physical property distributions (47–49). Rapid advancements in microtechnology have resulted in an increase in the number and complexity of electrode structures available for dielectric measurements. Notable references for sensor design are also available (50–56). A comprehensive overview of interdigital dielectric sensors is provided by Sundara-Rajan (57). A detailed review of currently available dielectric spectroscopic systems can be found at www.ee.washington.edu/research/seal/pharmatech/ .
Get more from pharmtech.
Reference.
1. author Beau Lambert (Partially From His Master's Research)http://www.psrc.usm.edu/mauritz/dilect.html
2. wikipedia. http://en.wikipedia.org/wiki/Dielectric_spectroscopy
3. Division of Information Technology, Engineering and the EnvironmentLaser Light Scattering and Materials Science Group.
http://www.unisa.edu.au/laser/Research/Dielec.asp
4. Dielectric Spectroscopy: Choosing the Right Approach, Sep 2, 2008By: A. Mathur, K. Sundara-Rajan, G. Rowe, A. V. MamishevPharmaceutical TechnologyVolume 9, Issue 32, pp. 8293
Posts
No comments:
Post a Comment