If the concentration of the sample is too high, the laser beam will become attenuated by the particles reducing the scattered light that is being detected. This optical configuration imposes restrictions on the maximum turbidity of the sample which can be measured. In a typical optical configuration used for electrophoretic mobility (and hence zeta potential) measurements, the laser beam has to penetrate the sample for scattered light at a forward angle of 13 В° to be detected ( figureВ 2).

Hence the data are given in the form of mobility, and the more complex formulations needed to deal with the derivation of zeta potential, where the particle is not isolated and the zeta potential not small, are addressed in a later section.

This limit is relevant to some but not all of the sample measurements presented in this paper. 100) and the double layer is thin in comparison with the particle radius. A simpler form in which F( Оєa)=1.5 is known as the Smoluchowski equation ( Smoluchowski 1921), and applies where Оєa is large (approx. This equation strictly only applies for isolated particles of zeta potential less than around 25вЂ‰mV. Where Оµ is the dielectric constant of the dispersant, F( Оєa) is the Henry function and О· is the viscosity ( Hunter 1988 Delgado et al.

The potential at this boundary (surface of hydrodynamic shear or slipping plane) is the zeta potential ( figureВ 1 Hunter 1988 Lyklema 2000 Delgado et al. Those ions beyond the boundary stay with the bulk dispersant. owing to electrophoresis), ions within the boundary move with it. Within the diffuse layer there is a notional boundary inside which the ions and particles form a stable entity. The liquid layer surrounding the particle (the electrical double layer) exists as two parts-an inner region (Stern layer) where the ions are strongly bound and an outer (diffuse) region where they are less firmly associated. An increase in the concentration of counter ions close to the surface results in the formation of an electrical double layer. Most colloidal dispersions in aqueous media carry an electric charge and the development of this charge at the particle surface affects the distribution of ions in the surrounding interfacial region. It can be used to optimize the formulations of suspensions and emulsions and aid in predicting long-term stability ( Shaw 1992 Hunter 1993 Everett 1994). Zeta potential is the key parameter that controls electrostatic interactions in particle dispersions and, as such, it is important in understanding the stability of colloidal dispersions. Conclusions and recommendations are made from the zeta potential values obtained at high concentrations. Further, a comparison of the data against theoretical models is presented and discussed. For both of the sample types studied, the electrophoretic mobility results show a gradual decrease as the sample concentration increases and the possible reasons for these observations are discussed. Results are presented on two different sample types, titanium dioxide and a polyurethane dispersion, as a function of sample concentration. This paper discusses electrophoretic mobility measurements made on turbid samples at high concentration using a novel cell with reduced path length. However, the ability to measure turbid samples using light-scattering techniques presents a number of challenges. The ability to measure samples at or close to their neat concentration would be desirable as it would minimize any changes in the zeta potential of the sample owing to dilution. Therefore, depending upon the size and optical properties of the particles, many samples will be too concentrated and will require dilution. Traditionally, samples measured by this technique have to be optically transparent. Laser Doppler electrophoresis is an accepted method for the measurement of particle electrophoretic mobility and hence zeta potential of dispersions of colloidal size materials. Zeta potential is the key parameter that controls electrostatic interactions in particle dispersions.

Malvern zetasizer open measurement file