The determination of hydrodynamic size and zeta potential from electrophoretic mobility and light scattering measurements

Abstract

Colloidal particle diffusivities are usually measured by light scattering and ζ-potentials determined from electrophoretic mobilities. A hydrodynamic size can be calculated from the diffusivity by use of the Stokes–Einstein equation, although this ignores the influence of the surface charge and the ion cloud surrounding each particle. Similarly, ζ-potentials are often calculated from a radius determined by transmission electron microscopy or light scattering. In either case, a false picture emerges since the ζ-potential is defined as the potential at the electrokinetic shear surface. Here we show that a self-consistent picture emerges upon combining diffusivity and electrophoretic mobility measurements so as to include the effects of the diffuse layer in the hydrodynamic/electrokinetic particle size determination. The diffusivity and electrophoretic mobility of an anionic polystyrene latex were measured over a range of salt strengths. When diffuse layer effects were ignored, the particle diameter decreased monotonically from 180 nm to 158 nm as the salt concentration increased from (approximately) 10 μM to 10 mM. When diffuse layer effects were incorporated into the analysis by simultaneously calculating the particle size and charge, the ionic strength dependence weakened. Above an ionic strength of 10 −4 M, the size varied by less than 1% and exceeded the dry size (from transmission electron microscopy) by about 4%. Although making use of the two experimental parameters has a significant impact on the calculated size, the effect on ζ-potentials is small. Some evidence of a fuzzy layer was present at ionic strengths below 10 −4 M since the particle size increased with diminishing ionic strength.