Influence of different salts on micro-sized polyelectrolyte hollow capsules

Polyelectrolyte capsules of poly( styrene sulfonate, sodium salt) and poly( allylamine hydrochloride) prepared on weakly polymerized melamine formaldehyde colloidal particles have been incubated in solutions of different salts ( carbonates, phosphates, chlorides). The salt concentration was varied from 0.05 to a maximum of 1.7 M. The effect of the salts on capsule topology was studied by confocal laser scanning microscopy and atomic force microscopy. The salt mediated change of permeability of the capsules was studied by fluorescence recovery after photo-bleaching. Significant size reduction with a simultaneous increase in wall thickness was found for all salt incubated samples to different extents. The changes were completed during a few minutes and were irreversible. Clear pH dependency could not be confirmed. The most remarkable effect on size, wall thickness and permeability of the capsules was found after incubation in solutions of carbonate salts. The diameter of the capsules was reduced by 20 to 25%, the thickness of the capsule walls increased by 100 to 400% and the capsules became almost impermeable for FITC labeled dextran of low molecular weight and fluorescein. Additionally, a reverse of the surface charge from positive to negative was measured for all samples treated with carbonates. A partial dissolution of the uppermost layer of poly( allylamine hydrochloride) was detected by fluorescent labeling. Layers placed deeper in the polymer film were not desorbed by the carbonate treatment. Isothermal titration calorimetric measurements showed a strong interaction of the carbonate ions with poly( allylamine hydrochloride). Carboxylation of amino groups of poly( allylamine hydrochloride) can not be excluded. The stability of the capsules at high pH values was discussed with respect to the melamine content in the multilayer. A mechanism of shrinkage due to dehydration of poly( allylamine hydrochloride) and disruption of the ion hydration shell, electrostatic screening, coiling and reorganisation of the polyelectrolytes in the multilayer was concluded.