Zeta Potential – What is it and how can it be characterised?
Zeta potential is the charge that develops at the interface between a solid surface and its liquid medium. Simply put, the surface charge of nanoparticles in solution. It is an important tool for understanding the state of the nanoparticle surface and predicting the long-term stability of a colloidal dispersion.
The unit for Zeta Potential is millivolt (mV).
When a particle is dispersed in a liquid, the functional groups on its surface will react with the surrounding medium. This process results in a surface charge, which attracts the accumulation of oppositely charged ions.
In a colloidal system, dispersed particles have two layers of oppositely charged ions on the surface called the stern (ions strongly bound) and diffuse layer (ions loosely bound). The Zeta potential is defined as the voltage at the edge of the diffuse layer where it meets the surrounding liquid (slipping plane). If the charge of nanoparticles falls below a certain level, the colloid will start to flocculate, conjugate and eventually sediment due to lack of repulsive forces causing instability.
Zeta Potential Measurement
Zeta potential measurement is a technique for determining the surface charge of nanoparticles in a colloidal solution. Its measurement brings detailed insight into the causes of dispersion, aggregation or flocculation, and can be applied to improve the formulation of dispersions, emulsions and suspensions
A zeta potential value on its own without defining the solution conditions is a virtually meaningless number. Measurements are useful only when the measurement conditions are known. These include factors such as pH, buffer concentration, temperature and ionic strength.
Methods to Measure Zeta Potential include electrophoretic mobility and streaming potential
The ZetaView® Nanoparticle Tracking Analyser is primarily an instrument used to sample parameters such as particle size, concentration and fluorescence bio-marker ratios however can also be used to measure Zeta Potential.
The ZetaView® uses micro-electrophoresis in order to calculate zeta potential. An electric field is applied across the sample cell and short video clips are taken of the particles moving through the cell according to their surface charge and polarity. The ZetaView® software then analyses the video clips to determine electrophoretic mobility. The Helmholtz-Smoluchowski equation is then applied in order to calculate zeta potential, as shown in the graphic opposite.
Streaming Potential Measurement using the Stabino or ZETA-check Instruments
The heart of this instrumentation is the cylindrical PTFE measuring beaker and piston. Sample is added to the beaker and the piston oscillates at a constant frequency during measurement. Particles become immobilised on the surface of the beaker and piston, allowing the fluid stream created by the moving piston to shift the mobile ion cloud (double layer) around each particle up and down, creating an oscillating frequency. The potential created is detected and measured by the two electrodes as streaming potential. From this, zeta potential is calculated. Zeta Potential measurements via classical light-scattering methods (e.g. electrophoresis described above) is problematic due to particles agglomerating and then sedimenting out when approaching zero charge. As the Stabino piston is actively mixing the sample, this prevents the process of agglomeration and eventually sedimentation of the particles while the measurement is taking place.
With the Stabino we are able to actively assess the stability of a sample via the added functionality of titrations. +/-30mV is often quoted as a threshold for stability, but this does not give the full picture and is only relevant to the sample as it stands in that exact chemical environment. It is entirely possible for two samples to have -30mV zeta potential but have very different stabilities. The Stabino provides a full insight into the colloid being assessed, primarily via pH and polyelectrolyte titrations.
- pH titrations – allow the assessment of stable pH regions and determination of the iso-electric point (IEP) with extremely high resolution. The Stabino constantly mixes the sample via the oscillating piston and can measure the exact pH at which zero charge is achieved.
- Polyelectrolyte titrations – by titrating a counter-charged polyelectrolyte solution, information on charge density can be estimated. This gives a much greater understanding of how stable a dispersion is. Predictions of shelf-life or stability over time can be built up using this method.
- Salt titrations – allow investigation of the relationship between conductivity and zeta potential. Salt titrations are relevant to applications such as protein aggregation or waste water treatment, where changing the conductivity deliberately causes particles in solution to form aggregates so they can be filtered out of solution.
- Other titrations – ultimately, any titrant can be used on the Stabino and its effect on zeta potential monitored. Some custom applications may include assessing the effect of varying volumes of an additive or stabiliser, different amounts of pigment in an ink solution and so on.
The Stabino affords the user complete control over their experiments, giving extremely high-resolution zeta potential plots thanks to the ability to titrate quickly and frequently, with potential readings taken every 5 seconds. It can also be used in conjunction with NANO-flex Dynamic Light Scattering Instrument to measure particle size vs. zeta potential simultaneously.
The Stabino® II is a second generation instrument for analysing and optimising the stability of colloids and dispersions, based on measurement of streaming potential.
Employing a unique oscillating piston design, the system incorporates rapid pH and polyelectrolyte titrations for investigation of stable pH regions, Iso-Electric Point (IEP), charge density and surface functionalisation. The system is ideally suited to formulation development and has applications in quality control.