Sequential addition of anionic and cationic polyelectrolytes may lead to the formation of multilayers at a solid surface. The buildup of such multilayers is characterized by a stepwise increase of the adsorbed amount and layer thickness and by alternating highly positive and highly negative values for the ζ-potential. The prime variables which determine the stability of these structures are the polymer charge and the ionic strength. Very stable multilayers are formed when both polymers are highly charged and when the ionic strength is low. For weakly stable multilayers complexation at the surface may first occur, followed by desorption of the complexes. For strongly charged polyelectrolytes the charge stoichiometry, which is not always 1:1, seems to be unique for each pair of polyelectrolytes; no influence of the substrate, of the pH, or of the ionic strength could be observed.

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Clustering of latex particles 4−10 μm in diameter during and after electrophoretic deposition of the particles onto flat electrodes has been reported by Böhmer (Langmuir 1996, 12, 5747). The particles interacted over length scales comparable to their size in the formative stages of the clusters. Combinations of two or more clusters already deposited approached each other to form larger agglomerates. A model based on electroosmotic flow about charged particles near surfaces is developed here to explain these observations. A charged, nonconducting particle near or on a flat conducting surface creates flow in the adjacent fluid due to electroosmosis about the particle's surface. Fluid is drawn laterally toward the particle near the electrode and pushed outward from the particle farther away from the electrode above the particle. Another particle near the electrode will be drawn toward the central particle by this convection. We first solve for the flow field about a single particle and then compute the rearrangement of neighboring particles in response to the flows. The clustering times for different initial configurations of sets of particles (e.g., regular versus irregular spacing) are calculated. The average clustering times for irregular configurations are greater than those for regular arrays. The qualitative and quantitative features of the experimental observations are captured by this model if the hindrance effect of the solid wall is taken into account. For example, the model correctly predicts the observed declustering (separation) of particles when the polarity of the electric field is reversed as well as the observed cluster-to-cluster motion.

Electrophoretic deposition of micron-sized polystyrene latex particles on an indium tin oxide (ITO) electrode was studied in situ using optical microscopy. Strong 2-dimensional clustering of the particles on the electrode surface was observed upon application of a potential. The clustering decreases somewhat with increasing salt concentration. Upon reversal of the direction of the field, the clusters broke up. The aggregation on the electrode cannot be explained by DLVO or dipole interactions. A possible interpretation may be formulated in terms of electro-osmotic flow.