Charge regulation effects in nanoparticle self-assembly and polyelectrolyte complexation

Nanoparticles, polyelectrolytes and biomolecules in solution acquire charge through the dissociation or association of surface groups. Thus, a proper description of their electrostatic interactions requires the use of charge-regulating boundary conditions rather than the commonly employed constant-charge approximation. Although this phenomenon has been theoretically studied since the seminal work by Kirkwood in the 1950s, the assumption that all objects carry a constant charge is still widely employed. We present a hybrid Monte Carlo/molecular dynamics scheme that dynamically adjusts the charges of individual ionizable groups of objects while evolving the trajectories of these objects. Charge regulation effects are shown to result in global charge redistribution and thereby qualitatively alter important structural properties, such as the nature of self-assembled aggregates and the polyelectrolyte coil–globule transition. These insights are used to design efficient hydrogel-based nano-actuators.