Zetapotential measurement with electrophoretic light scattering

The zeta potential is the electrical potential at the shear plane of a moving particle in a liquid medium. It is characteristic of the electrochemical properties of an emulsion droplet or particle in a liquid. The zeta potential can be used to draw conclusions about dispersion stability and particle mobility in external fields. A characteristic value is the so-called isoelectric point (IEP), where the zeta potential is zero, the particles agglomerate and the system flocculates.

In principle, there are two direct measurement methods for determining the zeta potential of particles dispersed in a dispersion: electrophoretic light scattering (ELS) and electroacoustics. While electroacoustics focuses on the investigation of hard material suspensions such as ceramic slurries, cement or battery suspensions, ELS is particularly suitable for characterizing liquid systems with soft, non-sedimenting particles such as proteins, macromolecules, polymers or oil droplets (emulsions). A special version of ELS, the so-called phase analysis light scattering technology (ELS-PALS), is implemented in the devices of the BeNano series, which also allows zeta potential measurements on non-polar or highly conductive dispersions.

Instruments

BeNano series Particle size measurement 0,3 nm – 15 µm (ISO 22412)

Measurement principle/technology

ELS-PALS-measurement principle

The principle of ELS-PALS – realized in the measuring devices of the BeNano series – is shown in the following illustration:

Classic ELS: The laser beam is split into a main beam and a reference beam using a beam splitter. The main beam hits the particles moving through the applied electric field. The resulting scattered wave undergoes a phase shift due to the Doppler effect

Δf = nvsin(θ)/λ

Where v is the particle velocity, λ is the laser wavelength and θ is the scattering angle. The wave from the scattered beam and reference beam interfere behind the electrophoresis cell has a certain beat frequency (wave troughs), which depends on the particle velocity. This results in the absolute value of mobility µ of the particles. The sign of the mobility µ is determined using the phase modulator by changing the reference wave frequency; the zeta potential then results from the mobility.

PALS technology: The main problem with the classic ELS technique occurs when measuring samples with organic solvents or highly conductive systems, as it is difficult to measure the frequency shift properly here. With PALS technology – a further development of the classic ELS – a phase shift between the reference and scattered wave is measured instead of direct measurement of the frequency shift due to the Doppler effect: This is realized by applying an alternating voltage of 20 Hz (FFR – Fast Field Reversal) or 2Hz (SFR – Slow field Reversal) to the electrophoresis cell during the measurement. The phase difference between the scattering wave and the reference wave is recorded as a function of time (phase plot). The increase of the individual segments in the phase plot is proportional to the frequency shift Δf. The frequency shift Δf determined in this way then results in mobility µ and zeta potential ζ as follows

M = λ/nsin(θ)·1/E·Δf          µ = 2ε_rε₀ζ/3η·f(κa)

where ε is the dielectric constant, f(κa) is the Henry function and η is the viscosity of the dispersing medium.

Literature and Norms

/1/ ISO 13099:2 – Colloidal systems — Methods for zetapotential determination — Part 2: Optical methods
/2/ CIT (Chemie Ingenieur Technik) 01-02/2023; „Partikelgrößen- und Zetapotentialmessung an verdünnten und original konzentrierten Nano-Oxidsuspensionen“
/3/ Particle World 24/2023: “BeNano series for particle analysis: New Autotitrator and DLS” Microrheology Option”
/4/ Particle World 23/2022: “Comprehensive nanoparticle characterization with optical and acoustic methods: BeNano 180 Zeta Pro vs. DT-1202”