Dielectric spectroscopy

[2][3][4][5] It is based on the interaction of an external field with the electric dipole moment of the sample, often expressed by permittivity.

Impedance is the opposition to the flow of alternating current (AC) in a complex system.

Materials or systems exhibiting multiple phases (such as composites or heterogeneous materials) commonly show a universal dielectric response, whereby dielectric spectroscopy reveals a power law relationship between the impedance (or the inverse term, admittance) and the frequency, ω, of the applied AC field.

Almost any physico-chemical system, such as electrochemical cells, mass-beam oscillators, and even biological tissue possesses energy storage and dissipation properties.

This technique has grown tremendously in stature over the past few years and is now being widely employed in a wide variety of scientific fields such as fuel cell testing, biomolecular interaction, and microstructural characterization.

Often, EIS reveals information about the reaction mechanism of an electrochemical process: different reaction steps will dominate at certain frequencies, and the frequency response shown by EIS can help identify the rate limiting step.

There are a number of different dielectric mechanisms, connected to the way a studied medium reacts to the applied field (see the figure illustration).

In general, dielectric mechanisms can be divided into relaxation and resonance processes.

The most common, starting from high frequencies, are: This resonant process occurs in a neutral atom when the electric field displaces the electron density relative to the nucleus it surrounds.

These two facts make dipole relaxation heavily dependent on temperature, pressure,[6] and chemical surrounding.

Ionic conductivity predominates at low frequencies and introduces only losses to the system.

Interfacial relaxation occurs when charge carriers are trapped at interfaces of heterogeneous systems.

A related effect is Maxwell-Wagner-Sillars polarization, where charge carriers blocked at inner dielectric boundary layers (on the mesoscopic scale) or external electrodes (on a macroscopic scale) lead to a separation of charges.

The charges may be separated by a considerable distance and therefore make contributions to the dielectric loss that are orders of magnitude larger than the response due to molecular fluctuations.

Relaxation mechanisms are relatively slow compared to resonant electronic transitions or molecular vibrations, which usually have frequencies above 1012 Hz.

O + e, without mass-transfer limitation, the relationship between the current density and the electrode overpotential is given by the Butler–Volmer equation:[7]

Let us suppose that the Butler-Volmer relationship correctly describes the dynamic behavior of the redox reaction:

Dynamic behavior of the redox reaction is characterized by the so-called charge transfer resistance

Another analog circuit commonly used to model the electrochemical double-layer is called a constant phase element.

appears in series with the electrode impedance of the reaction and the Nyquist diagram is translated to the right.

Under AC conditions with varying frequency ω, heterogeneous systems and composite materials exhibit a universal dielectric response, in which overall admittance exhibits a region of power law scaling with frequency.

[10] Plotting the Nyquist diagram with a potentiostat[11] and an impedance analyzer, most often included in modern potentiostats, allows the user to determine charge transfer resistance, double-layer capacitance and ohmic resistance.

[15][16] It is used in many biosensor systems as a label-free technique to measure bacterial concentration[17] and to detect dangerous pathogens such as Escherichia coli O157:H7[18] and Salmonella,[19] and yeast cells.

[20][21] Electrochemical impedance spectroscopy is also used to analyze and characterize different food products.

Some examples are the assessment of food–package interactions,[22] the analysis of milk composition,[23] the characterization and the determination of the freezing end-point of ice-cream mixes,[24][25] the measure of meat ageing,[26] the investigation of ripeness and quality in fruits[27][28][29] and the determination of free acidity in olive oil.

[30] In the field of human health monitoring is better known as bioelectrical impedance analysis (BIA)[31] and is used to estimate body composition[32] as well as different parameters such as total body water and free fat mass.

[33] Electrochemical impedance spectroscopy can be used to obtain the frequency response of batteries and electrocatalytic systems at relatively high temperatures.

[34][35][36] Biomedical sensors working in the microwave range relies on dielectric spectroscopy to detect changes in the dielectric properties over a frequency range, such as non-invasive continuous blood glucose monitoring.

[37][38] The IFAC database can be used as a resource to get the dielectric properties for human body tissues.

[39] For heterogenous mixtures like suspensions impedance spectroscopy can be used to monitor the particle sedimentation process.