Dielectric Spectroscopy
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.
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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.
Our group is currently in the process of developing a dielectric
spectrometer.