Phase behavior of pH-responsive polyelectrolytes and hydrogels

Polyelectrolytes and hydrogels in solution acquire charge through the dissociation or association of ionic 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. Using a Monte Carlo/molecular dynamics scheme that dynamically adjusts the charges of individual ionizable groups we calculate pH–temperature phase diagrams of linear polyelectrolytes as well as hydrogels. Using histogram reweighting techniques we uncover that charge regulation effects lead to, respectively, discontinuous coil–globule and collapse–swelling transitions. The extent of the discontinuous region and the existence of two critical points at low and high pH values depends sensitively on the size of the counterions. Computational results are supported by an analytical mean-field theory. These insights are used to design efficient hydrogel-based nano-actuators.