SURFACE ZETA POTENTIAL MEASUREMENTS
In a surface zeta potential measurement, a sample of the surface of interest is placed between the two electrodes of the surface zeta potential cell accessory. The surface zeta potential is determined by the detection of the movement of tracer particles dispersed in the electrolyte in which the sample is immersed. An electric field is applied and the subsequent motion of tracer particles dispersed within the electrolyte is detected. The motion of the tracers will be due to a combination of electrophoresis and electro-osmosis. The movement of the tracer particles is measured as a function of displacement away from the surface. The electro-osmotic flow contribution from the sample surface can be described with a simple model and the surface zeta potential of the sample can therefore be calculated . Therefore, the purpose of the tracer particles is to simply scatter light in order to be able to detect their mobility due to electrophoresis and electro-osmosis.
There are key points which need to be considered with regard to the choice of tracer particles used for surface zeta potential measurements:
Compatibility of the tracer particles with the dispersant
The surface zeta potential value obtained from a measurement will be as dependent upon the chemistry of the dispersant as it is on the surface of the sample of interest. Therefore, consideration has to be made as to what dispersant should be used and whether the tracer particles chosen will be compatible with that dispersant i.e. will the tracer particles aggregate/flocculate in the chosen dispersant.
Compatibility of the tracer particles in the chosen dispersant can be assessed by performing dynamic light scattering measurements to characterize the size.
For some applications, there may be particles/molecules present in the dispersed phase which could act as the tracers during the measurement.
Compatibility of the tracer particles with the sample (surface) being measured
The tracer particles selected should have no affinity for the sample being investigated. Any adsorption of tracer particles onto the sample will influence the value of surface zeta potential obtained. If the surface under investigation is believed to be negatively charged in the chosen dispersant, then positively charged tracer particles would be inappropriate as they would simply absorb onto the surface and fundamentally charge the surface zeta potential value.
Tracer particles with different surface chemistries are commercially available and these include carboxylated, sulphonated, aminated and PEGylated . Different samples and dispersant conditions may require different tracer chemistries to minimise any interaction between the particles and the sample being measured.
If the sign of the zeta potential of a surface is not known in the conditions of the chosen dispersant, the ideal tracer would be one that is neutral which should minimise any interaction with the surface (at least electrostatically). A tracer which is sterically stabilised (i.e. PEGylated) might be an ideal candidate in this case. Method development could involve measuring the zeta potential of various candidate tracers in the dispersant in which the surface zeta potential is going to be measured and see which one has a zeta potential closest to zero. This approach would predict which candidate tracer is least likely to interact with the surface under investigation (at least from an electrostatic standpoint).
Use a minimum concentration of tracer particles
As discussed above, the only purpose of the tracer particles is to scatter light so that the electro-osmotic and electrophoretic contributions can be determined. Therefore, only small tracer concentrations need to be used. The minimum concentration of the chosen tracer can be determined by performing dynamic light scattering (DLS) size measurements at the forward angle (13°), checking the derived count rates (in kilo counts per second) and determining the excess scattering produced (i.e. the intensity of scattered light of the tracer suspension compared to the intensity of scattered light from the dispersant on its own). Working with a low tracer concentration should minimize the interaction with the sample surface under investigation.
Possible interaction of the tracer particles with the sample surface can be deduced by repeating surface zeta potential measurements using increasing concentrations of tracers and seeing whether there is a correlation between the mean surface zeta potential values obtained and the tracer concentration.
In a microrheology measurement, tracer (probe) particles are dispersed in the sample under investigation and a correlation function is obtained. Analysis of the correlation function allows for the calculation of the mean square displacement (MSD) of the tracer particles, from which the rheological properties of the sample can be derived using the Generalised Stokes-Einstein relationship [3,4].
In order to obtain reliable microrheology data, there are key points which need to be considered with regard to the tracer particles used:
Interactions between the tracer particles and the sample matrix should be minimal
The suitability of the tracer particle with the sample under investigation can be determined by measuring the zeta potential of the tracer in the dispersant in which the sample will be prepared and comparing it to the zeta potential value of the tracer dispersed in the sample matrix.
If the difference in zeta potential values is less than 5mV, the tracer chemistry is suitable for the sample being studied. However, if the zeta potential of the tracer in the presence of the sample is significantly different from the value obtained in the dispersant, there may be strong particle-matrix interactions present . Such interactions will inevitably affect the rheological data obtained and an alternative tracer needs to be found where the surface chemistry of the tracer is more compatible with the sample i.e. tracers where their zeta potential values in the presence of the sample is very similar to the value obtained in the dispersant alone.
For people with access to a rotational rheometer, tracer-sample interactions can be evaluated by measuring the rheological behaviour of the sample with and without tracer present on a rotational rheometer. If no interaction is occurring then the rheological response (specifically G') should be similar between the two samples, while a significant difference would suggest interaction.
Tracers with different surface chemistries are commercially available and include carboxylated, sulphonated, aminated and PEGylated . Different samples may require different tracer chemistries to minimise any interaction between the particles and the sample matrix.
The concentration of tracer particles should ensure that the scattering intensity is dominated by the tracer particle scattering compared to the scattering from the sample matrix
It is important that the intensity of scattered light from the tracer particles dominates the scattering intensity from the sample matrix. An ideal tracer therefore is one which has a large relative refractive index and can be obtained at high concentration so that the addition of small aliquots to the sample under investigation will not result in any significant change in the rheology of the sample. Normally, 5µL of tracer particles would be added to 1mL of sample. The exact concentration used can be entered into the tracer properties window in the Zetasizer software (Measure – Optimisation – Tracer Concentration) . A size measurement is taken and the intensity particle size distribution (PSD) obtained will hopefully consist of only one peak i.e. the tracer. If more than one peak is present in the intensity PSD, the intensity of the tracer particles at the inputted concentration relative to that of the sample is used to calculate the tracer concentration required to give a relative intensity ratio of 95% (19:1), which is the minimum recommended to ensure the tracer scattering will dominate the measurement. This calculation is made on the basis that for single scattering there should be a linear correlation between concentration and intensity.
The tracer particles should be well dispersed in the sample
It is important that proper dispersion of the tracer particles within the sample is achieved prior to a microrheology measurement. To achieve this, the use of a roller mixer for up to 24 hours might be required to ensure proper dispersal of the tracer within the sample matrix.
The tracer particle size should be larger than the relevant microstructural length scale e.g. mesh size in a polymer network, such that it probes the bulk material response i.e. the assumption of continuum viscoelasticity holds.
The particle tracer size must be significantly larger than the measured size of the sample. Under these conditions, the tracer particles probe the bulk material response i.e. the assumption of continuum viscoelasticity holds. If a tracer particle size is small on the microstructural length scale of the sample matrix, then bulk rheological properties will not be recovered from the microrheology measurement [4,5].
If the tracer particle size is too small, the microstructure of the sample may not be probed during a DLS microrheology measurement and the bulk rheological properties will not be correctly recovered. Typical suitable particle diameters of tracers for microrheology measurements would be 500nm and larger.
Another important point is that the tracer particle size should be measured to make the microrheology result as accurate as possible. The size of the tracer particle may change slightly from the nominal size, depending on the pH of the dispersant or presence of electrolyte. This is because the electrical double layer (Debye layer) will change depending on the ionic strength of the dispersant, thus affecting the measured hydrodynamic size of the particle [6,7].
The choice of the most appropriate tracer particles for either surface zeta potential or microrheology measurements with the Zetasizer Nano will determine the success of the results obtained.
There is no universal tracer particle available which would work with every sample. However, it is hoped that the guidelines given in this technical note should help in determining the most appropriate tracer to select.
 J.C.W. Corbett, F. McNeil-Watson, R.O. Jack and M. Howarth (2012) Colloids and Surfaces A: Physicochem. Eng. Aspects 396 (2012) 169– 176.
 Functionalised Micro- and Nanoparticles available from MicroMod Partikletechnologie GmbH (http://www.micromod.de/en/functionalized-micro-and-nanoparticles--1.html).
 S. Amin, S. Blake, R.C. Kennel and E.N. Lewis (2005) Materials 8, 3754-3766.
 An Introduction to DLS Microrheology Whitepaper
 Microrheology: Running Measurements on the Zetasizer Nano ZSP/ZS Technical Note available from malvernpanalytical.com
 R.J. Hunter (2000) Foundations of Colloid Science, OUP Oxford UK.
 R. Pecora (1985) Dynamic Light Scattering: Applications of Photon Correlation Spectroscopy, Plenum Press, New York, USA.