(528g) Simulation Studies for the Adsorption of Colloidal Brushes on Flat Surfaces: Steric Repulsion Vs. Bridge Attraction

The development of nanotechnology probably requires novel strategies for the directed assembly of nanoparticles into well-defined patterns, sometimes supported on a surface. Polymer molecules grafted to one flat surface are known to alter the interactions between the surface and its surroundings (other surfaces and/or colloidal particles). This is often employed to stabilize colloidal dispersions and to reduce non-specific protein adsorption. However, our understanding of the molecular phenomena that take place on flat substrates may not be applicable to systems in which nanoparticles are characterized by large curvature. Stimulated by recent AFM experiments reported by Goodman et al. [Langmuir 20 (2004) 2333], we decided to employ a simple model to investigate, via molecular simulations, the interactions between a colloidal brush and a flat surface. The experimental data showed that at low brush density an attractive force arises between the colloidal brush and a flat cantilever tip. This was unexpected because the solvent was ?good' for the polymer chains grafted on the colloidal particles. However, as the grafting density increased the brush-surface interaction became repulsive, as expected from our understanding in flat geometries. In our model the colloidal particle is treated as a Lennard Jones sphere, the grafted brushes as freely-jointed-tangent-Lennard-Jones-sphere chains, and the surface is within the Steele's 10-4-3 potential. Thus our model only accounts for dispersive interactions (colloid-surface, segment-surface, and colloid-segment) and entropic effects. Our results qualitatively reproduce the experimental observations, and in addition help us understand the molecular mechanisms that can be used to promote or prevent the adsorption of colloidal brushes on flat surfaces.