Diffusional deposition of colloidal particles: electrostatic interaction and size polydispersity effects

In the deposition process of charged nanosized particles onto oppositely charged planar substrates two effects are being discussed: (1) The maximum surface coverage sensitively depends on the repulsive electrostatic particle–particle interactions. The ionic strength of the particle suspension defines the magnitude of the electrostatic repulsion between the particles, which in turn modifies the maximum surface coverage. The maximum surface coverage decreases with decreasing ionic strength, a trend that can be well described by an effective hard-sphere model based on random sequential adsorption (RSA), where the effective radius is estimated from the repulsive screened Coulomb potential. Measured radial pair-distribution functions also reveal ideal hard-sphere behavior as compared to RSA simulations for monodisperse disks. The magnitude of the interaction, however, is overestimated with the simple electrostatic model. (2) Particle size polydispersity does also influence strongly the deposition process. Small particles may fill voids left by larger particles such that the maximum surface coverage increases significantly. The size distribution of the deposited particles on the surface changes with time, whereby the small particles are adsorbed preferentially. These trends are observed experimentally and confirmed by computer simulation.