Salt softening of polyelectrolyte multilayer capsules.

The changes in the morphology and the mechanical properties of hollow polyelectrolyte multilayer capsules made from poly(styrenesulfonate)/poly(allylamine hydrochloride) in response to added salt were investigated. We found that capsules shrink in response to salt exposure. The effect depends strongly on the nature of the salt added and follows trends of the Hoffmeister series, with weakly hydrated cations inducing the strongest shrinking. For NaCl, we have investigated additional effects on capsule mechanical properties that are occurring above a 3 M salt concentration and we found that the morphological changes are accompanied by a pronounced softening of the capsule wall material, which we can quantify by analyzing the force response of capsules in the prebuckling regime. This shows that salts can act as plasticizers in the multilayers and induce annealing effects.