Hydrodynamic size and electrophoretic mobility of latex nanospheres in monovalent and divalent electrolytes

The hydrodynamic size and electrophoretic mobility of sulfate latex nanospheres are given as a function of ionic strength for KCl and MgCl2 and at different volume fractions. The results were obtained from Doppler velocimetry and Dynamic Light Scattering using a Malvern ZetaSizer Nano (ZetaNano) at 173° scattering angle and a Malvern ZetaSizer 3000HSA (Zeta3000) at 90° scattering angle. Although the instrumentation is similar, we observed differences in the hydrodynamic radii. The reason for these discrepancies are most probably due to the conversion of the detected light intensity into particle radius since the particles have a diameter of about 530nm (measured by TEM) quite close to the lasers’ wavelength (633nm). From the study, we found in particular that the hydrodynamic radius decreases upon addition of monovalent or divalent salt. This behavior is generally attributed to the presence of a hairy layer on the particle's surface. We also found that the electrophoretic mobility decreases logarithmically with increasing particle concentration, as predicted for nearly salt-free conditions. The change in viscosity at higher ionic strength and the shift in the shear plane position are shown to play a role in the aggregation behavior.