pH-Controlled swelling of polyelectrolyte multilayer microcapsules

We explore quantitatively the effect of pH on the osmotically induced swelling of polyelectrolyte, multilayer microcapsules filled with solutions of a strong polyanion. The shells of the microcapsules are composed of strong polyanions and weak polycations via a layer-by-layer electrostatic self-assembly. The equilibrium radius of the swollen capsules was found to be the same at low and neutral pH but increases at high pH, indicating the softening of the capsule shell. Such a softening probably reflects a decrease in the charge density of a polycation, which leads to a reduction in the number of ionic cross-links. The dramatic increase in size and softening of the capsules at high pH is accompanied by polyanion release from the capsule interior. No release was observed at low and neutral pH.