Adsorption of Charge-Bidisperse Mixtures of Colloidal Particles

Careful control of the microstructure of an adsorbed monolayer of colloidal particles is important for creating nanostructured devices through self-assembly processes, and the structural and functional complexity of self-assembled particulate monolayers increases with the number of components in the system. Here, we perform simulations of the adsorption of binary mixtures of Brownian, colloidal particles to explore and identify combinations of parameters that produce technologically interesting surface structures. The system contains two types of particles of identical radii but differing surface potentials. In one scheme, Brownian dynamics simulations begin with an evenly distributed mixture above a charged planar surface, and the particles adsorb to the surface until the system achieves a steady state. In the second scheme, two different single-component suspensions are exposed to the substrate sequentially. Volume fractions in the bulk control relative surface coverages, and the observed structures include isolated, high-potential particles and chains and clusters of low-potential particles. Substitutionally disordered lattices form for ratios of particle potential ranging from about 1.5 to 4, depending on parameters:  ordered lattices are more stable to bidispersity at higher wall potentials and higher particle potentials. Terminal fractional bidispersities based on equivalent hard disk (EHD) radii vary from 3.6 to 10%. In sequential adsorption, small amounts of the second component adsorb only for parameter combinations with minimal repulsions from preadsorbed particles and sufficient attraction to the surface, since colloidal adsorption is a kinetically frustrated process. High-potential particles added to a monolayer of low-potential particles create isolated dots, and in reverse, low-potential particles dope lattices of high-potential particles. The results of the simulations are discussed in the light of lattice models and EHD models.