Coarse-Grained Modeling of Colloid-Nanoparticle Mixtures

Colloid-nanoparticle mixtures have attracted much recent attention for their rich phase behavior. The potential to independently vary size and charge ratios greatly expands the possibilities for tuning interparticle interactions and stabilizing unusual phases. Experiments have begun to explore the self-assembly and stability of colloid-nanoparticle mixtures, which are characterized by extreme size and charge asymmetries. In modeling such complex soft materials, coarse-grained methods often prove essential to surmount computational challenges posed by multiple length and time scales. We describe a hierarchical approach to modeling effective interactions in ultra-polydisperse mixtures. Using a sequential coarse-graining procedure, we show that a mixture of charged colloids and nanoparticles can be mapped onto a one-component model of pseudo-colloids interacting via a Yukawa effective pair potential and a one-body volume energy, which contributes to the free energy of the system. Nanoparticles are found to enhance electrostatic screening and to modify the volume energy. Taking the effective interactions as input to simulations and perturbation theory, we calculate structural properties and explore phase stability of highly asymmetric charged colloid-nanoparticle mixtures.