Role of interfacial interactions in ordering of two-dimensional colloidal self-assemblies on polyelectrolyte multilayer surfaces

Abstract

In this study, we examined the ordering of 2D colloidal particle monolayer structures formed atop polyelectrolyte multilayers (PEM) constructed by layer-by-layer deposition. We focused on the interactions between the particles and the substrate surface by changing the top layer of PEMs, along with the fabrication procedure for the PEMs. For PEMs prepared by rinsing with pure water in the fabrication process, the 2D assembly of negatively charged colloidal spheres exhibited a disordered state for positively charged PEM surfaces, whereas a well-ordered structure was formed on a negatively charged surface. Investigations with a colloid-probe atomic force microscope technique revealed that the friction force between the colloid and substrate at a zero load condition strongly affects the ordering state of the particle monolayers for relatively rigid surfaces. However, swollen PEM surfaces that were prepared by rinsing the layers with a salt solution rendered disordering of the colloidal monolayer for both positively and negatively charged surfaces and shifted to a less ordered state as the number of polyelectrolyte layers increased. For the salt-rinsed PEMs, we found that the range of the steric force of the PEMs rather than the friction force appears to be critical for the ordering of the 2D colloidal assemblies. For such swollen PEMs, we consider that the entanglement of the polymers to the colloidal particles invokes the hindrance of the particle ordering in the 2D colloidal assembly formation even though electrostatic repulsion between the colloids and the polymer layer takes place.