Electrolyte-Dependent 2-D Aggregation of Colloidal Particles on a Planar a/c Electrode

In an effort to understand the mechanism of 2-D aggregation of colloidal particles observed on a/c electrodes, we have used video microscopy to observe x,y motion of isolated pairs of polystyrene latex particles and Total Internal Reflection Microscopy (TIRM) to monitor the elevation z of a single particle very near one of two parallel horizontal electrodes. In a/c electric fields, the direction of motion depends on which electrolyte composes the aqueous solution: isolated pairs of particles aggregate in solutions of NaHCO₃ or KCl and move apart in solutions of KOH, NaOH, H₂CO₃ or NH₄OH. TIRM measurements of elevation of single particles in a/c fields reveal a phase shift between the oscillating electric field E(t) and the oscillating elevation z(t): in solutions of NaHCO₃ or KCl the phase angle is greater than 90º and in KOH, NaOH, H₂CO₃ or NH₄OH the phase angle is equal to or smaller than 90º. We speculate that the electrolyte dependence is associated with kinetics of electrode reactions.