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

摘要

Colloidal particle diffusivities are usually measured by light scattering and zeta-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, zeta-potentials are often calculated from a radius determined by transmission electron microscopy or light scattering. In either case, a false picture emerges since the zeta-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 mm as the salt concentration increased from (approximately) 10 mu M 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 zeta-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. (C) 1998 Elsevier Science B.V. [References: 8]