(31b) Nanoparticles, Chains and Charged Films: What Simulations Can Teach Us about Their Interactions and Self-Assembly

This lecture focuses on the use of simulations to probe structure, interactions and self-assembly of (a) ionic surfactants forming micelles (b) nanoparticles with attached chains and (c) thin films of dissociable polyelectrolytes in solvents of varying quality. The importance of electrostatic interactions on self-assembly of surfactants and biological systems has been widely acknowledged. Most theoretical approaches for these systems are based on mean-field approximations that neglect higher order correlations that are particularly important for multivalent ions and in environments of low dielectric permittivity. We have developed implicit-solvent ionic surfactant models and have used them in Grand Canonical Monte Carlo simulations to investigate the critical micelle concentration, aggregation number and micellar shape in the presence of explicit salt. We illustrate the importance of ion correlation effects by comparing these results with a Yukawa-type surfactant model that incorporates electrostatic screening implicitly. Nanoparticles with attached chains also behave similar to amphiphiles, self-assembling into strings and sheets. There is considerable experimental evidence for the existence of short-range attractions between like-charged surfaces in the presence of multivalent ions, even when excess monovalent salt is present as well. These interactions are also present for charged colloids. We have used simulations to investigate the limits of applicability of Gouy-Chapman theory for ionizable surfaces that respond to the local environment. We have also been studying thin films of grafted chains that contain ionizable groups, with or without added salt. A fine balance exists in these films between electrostatic and solvent interactions that leads to morphological phase transitions and large changes in film structure and height.