Dynamic Light Scattering (DLS)
DLS measures the diffusion coefficient of particles undergoing Brownian motion in a fluid. From this the size (diameter or radius) of the particles can be determined. To accomplish this, the Zetasizer measures the scattering of the particles as they cross the incident beam of light at a backscattered angle of 175°. A forward scattering angle of 13° is also available for particle size measurement. The Zetasizer can measure particle diameters in the range of 0.6 nm and 6 µm.
Zeta potential is a property of particles in a suspension that relates to the charge on their surface. It is also related to the stability of the particles in the suspension. The experiment is performed by passing an alternating electric field through the suspension and measuring the change in velocity of the particles, called electrophoresis. As a result, the light scattered by the particles exits at a different frequency to the incident beam according to their velocity. Analysis of the superposition of incident and scattered beams, called Laser Doppler Velocimetry, enables the calculation of the zeta potential. A specialized capillary cell is necessary for this measurement in the Zetasizer and can be purchased in the MRL storeroom.
This accessory is available for the Zetasizer and automates the measurement of both particle size and zeta potential as a function of pH, dilution or additive concentration. It is a convenient solution for isoelectric point measurements (pH at which zeta potential = 0).
Surface Zeta Potential
Similar to standard zeta potential, particles in a suspension are subjected to an alternating electrical field and their velocity is measured. However, when a charged surface is introduced (film, membrane, etc.), the particles close to that surface will have a different velocity than those far away and a velocity gradient is formed. This is known as the electroosmotic effect and is related to the charge on the film. Thus by measuring this effect, the zeta potential of the film can be measured. The Zetasizer accessory for surface zeta potential is available for use but must be requested by users as specific training is required.
Static Light Scattering
This technique is used specifically to measure the molecular weight of polymers/molecules in solution. Different sample concentrations are sequentially measured and a Debye plot is created from the results according to the Rayleigh scattering theory. The y-intercept of the plot is equal to the inverse molecular weight of the sample. For accurate results, a reusable glass or quartz cuvette must be used and purchased by the user.