Introduction：Zeta Potential

When an electric field is applied to a system in which charged particles are dispersed, the particles move toward the electrode, since the velocity is proportional to the charge of particles, zeta potential is obtained by measuring the migration speed of particles.

The electrophoretic light scattering method is also known as the laser Doppler method, which states that “when light or sound waves hit a moving object and are reflected or scattered, the frequency of light or sound waves changes in proportion to the speed of object.” The “effect” is used to determine the migration rate of particles. When the electrophoresed particles are irradiated with laser light, the frequency of scattered light from the particles shifts due to the Doppler effect. Since the shift amount is proportional to the migration speed of particles, the migration rate of particles can be known by measuring this shift amount.

In fact, the relationship between the migration speed (V) and the Doppler shift amount (Δν) when a sample dispersed in a solvent having a refractive index (n) is irradiated with laser light of wavelength (λ) and detected at the scattering angle (θ) is expressed by the following equation.

［n: Refractive index of solvent, θ: detection angle］

The electrical mobility (U) can be obtained from the migration speed (V) and electric field (E) obtained here.

　　　Ｕ＝Ｖ／Ｅ ・・・ (2)

Most of the electrical mobility (U) to zeta potential (ζ) can be obtained using the Smoluchowski equation.

［η: Viscosity of solvent, εr: Specific dielectric constant of solvent, ε0: Dielectric constant in vacuum  ］

In this way, zeta potential is obtained by observing scattered light from moving particles.