Dielectric Constant

Ebatco’s NAT Lab is equipped with a Brookhaven BI-870 Dielectric Constant Meter. The BI-870 utilizes two concentric stainless-steel cylinders as a probe. The instrument applies a low-distortion, 10 kHz sine wave of electrical signal between the outer cylinder and inner cylinders while the resulting current is measured. Using the current and precise knowledge of the probe’s geometry and capacitance, the dielectric constant of a liquid is determined.

A material’s dielectric constant, ε, is a measure of a material’s ability to store electrical energy when placed in an electrical field. It is often normalized to ε_0, the permittivity of vacuum.  This normalized value is called relative permittivity, ε_r, and is defined as:

Relative permittivity is the factor by which the electrical field between two point charges is reduced by the material relative to vacuum. Materials with large relative permittivity’s often make good capacitors.

While the dielectric constants for most pure liquids are well documented, those of mixtures and the effects of impurities are still being studied. The complex molecular interactions in the liquid state mean that there is no simple way to accurately calculate the dielectric constant of a liquid mixture, so the best way to determine the dielectric constant is to measure it directly. This is especially useful for Zeta potential measurements where the liquid used to keep particles in suspension may be comprised of several different liquids.

Typical Experimental Results

Dielectric Constants of Commonly Used Liquids

Applications

Instrument: Brookhaven Instruments 780 Liquid Dielectric Constant Meter

Instrument Key Specifications

For more information, please read our application notes:
Dielectric Constant Determination for Aqueous Solutions of Alcohol, PDF
All application notes can be found here

Dielectric Constant Determination for Aqueous Solutions of Alcohol

Dielectric constant is a measure of a material’s ability to store electrical energy when placed in an electrical field. It is also referred to as relative permittivity; it is typically denoted as ε_r; and it is defined as:

where ε is the permittivity of the material, and ε₀ is the permittivity of vacuum. Relative permittivity is the factor by which the electrical field between two point charges is reduced by the material relative to vacuum. It can also be expressed as the ratio of the capacitance of a capacitor using the material as a dielectric, C_x, compared to the capacitance of a similar capacitor using vacuum as a dielectric, C₀:

While the dielectric constants for most pure liquids are easy to find, they may vary depending on the impurities present in the liquids or if the liquids are mixed with other liquids. Since there is no universal way to calculate the dielectric constant of a mixture, the best way to determine the dielectric constant is to measure the dielectric constant of the liquid mixture directly. This is especially useful for Zeta potential measurements where the liquid used to keep particles in suspension may be comprised of several different liquids.

The Brookhaven BI-870 Dielectric Constant Meter, equipped in Ebatco’s Nano Analytical and Testing (NAT) Lab, is capable of measuring the dielectric constants of liquids. The BI-870 utilizes two concentric stainless-steel cylinders as a probe. The instrument generates a 10 kHz very low distortion sine wave of electrical signal at the outer cylinder and measures the conductive and capacitive currents between the two cylinders at the inner cylinder. By measuring the currents generated by this stable voltage source and knowing precisely the probe parameters, the dielectric constant of a liquid can be readily measured.

Figure 1 and Table 1 show the measured dielectric constants of aqueous solutions with various concentrations of denatured alcohol at room temperature. The results show a relatively linear relationship for the dielectric constants of water and denatured alcohol solutions with their alcohol concentrations.

In this instance, the dielectric constants for the solutions containing other alcohol concentrations can be approximated using the linear relationship. However, for many other mixtures, especially for more complex mixtures, this may not be the case. As the instrument manufacturer has stated “Although it is very tempting to try to apply simple mixing rules to approximate a new dielectric constant, there is no simple equation that will adequately characterize the interaction of fluids to calculate dielectric constants of mixed liquids. Thus direct measurement is the only recourse.” When the dielectric constants of liquids are needed for their intended uses, measurements should and can be made in order to obtain precise and accurate values.

ASTM Standards