Electrostatically Cross-Linked Reversible Gels - Effects of pH and Ionic Strength

Mixing of oppositely charged macromolecules can, under certain conditions, lead to the formation of electrostatically cross-linked coacervate gels. In this simulation study, we determine the conditions under which equimolar mixtures of oppositely charged monodisperse four-armed star copolymers with charged end-blocks are able to form such coacervate gels. The cationic charged blocks consist of quenched charges, whereas the anionic blocks contain pH-responsive weak acid groups. We used the Grand-reaction method to determine the phase stability, equilibrium composition, and structural properties of these systems in equilibrium with a supernatant solution at various pH levels and salt concentrations. Depending on the pH and hence on the charge state of the polyanion blocks, we observed the emergence of three regimes: a solution, a sol of isolated star clusters, and a gel-percolating network of stars. Moreover, we demonstrate that the charge state of the stars in the gel phase can be well described by the ideal Henderson-Hasselbalch (HH) equation, despite the presence of strong interactions violating ideality. We can backtrace this surprising result to two deviations from the ideal titration behavior that almost quantitatively cancel each other. This observation explains why various experiments on coacervate gels can be well described by the HH equation, even though the basic assumptions of ideality are clearly violated.