Restructuring of exponentially growing polyelectrolyte multilayer films induced by salt concentration variations after film deposition

Abstract

It is known that changes of the ionic strength of the solution in contact with a polyelectrolyte multilayer film leads to the swelling or the deswelling of this film. Recently, we found that increasing the ionic strength of the solution in contact with an exponentially growing polyelectrolyte multilayer changes vigorously its internal structure with the formation of holes before the film finally dissolves (Mjahed et al., Soft Matter 2009, 5, 2269). Here we generalize these observations and show that any change of ionic strength of the solution (increase or decrease) in contact with the film induces a film restructuring with the formation of holes. Two types of holes are found: spherical holes formed by a direct increase or decrease of the ionic strength and non-spherical holes, resembling cracks which appear after an increase followed by a decrease sequence of the ionic strength. Later on, these holes heal. The film restructuring thus depends upon the path of ionic changes imposed to the film and takes place through complex processes which are far from being fully clear.