Electrolyte Dependence of Particle Motion Near An Electrode During AC Polarization

The directed motion of colloidal particles on a planar electrode has been studied for more than two decades because of its importance to the fabrication of colloidal crystals, the separation of colloidal dispersions, and the high throughput evaluation of electrocatalysts.  The phase angle between the imposed ac electric field and the oscillations in particle height is the key parameter governing the sign of inter-particle force and, consequently, the equilibrium microstructure of the colloidal crystal. The phase angle depends on a number of experimental parameters, including the strength and frequency of the imposed electric field. Surprisingly, the phase angle also depends on the dispersing electrolyte. Negatively charged polystyrene spheres 5.7 μm in diameter dispersed in 0.15 mM KCl or NaHCO₃ have a phase angle > 90°, but particles dispersed in 0.15 mM NH₄OH or KOH have a phase angle < 90° despite all other experimental parameters being identical to the previous case. Electrolyte dependence arises in both the polarization of the particle’s diffuse layer and the conductivity of the dispersing electrolyte. The first a priori model for electrolyte dependent particle motion normal to an electrode during ac polarization will be presented.