When multilayer films are built up with polycations and polyanions, their thickness can grow either linearly or exponentially with the number of deposited layer pairs, depending for example on the nature of the polyelectrolytes used. We investigate in the present work the construction of a film using a binary mixture of polyanions as polyanion solution. The two anionic components are chosen such that one of them causes the film to grow linearly while the other causes the film to grow exponentially. It is observed that a mixture of both components leads to a hybrid growth law, depending on its composition. At the beginning of the construction, the thickness of the film increases exponentially with the number of deposited bilayers. Once a given thickness is reached, one observes the crossover to a linear growth regime. This finding is discussed on the basis of the diffusion coefficients of the polyanions that are assumed to diffuse "in" and "out" of the film.

We show, in this paper that multivalent ferrocyanide anions can penetrate into exponentially growing (PGA/PAH)n multilayer films whatever the nature of the last deposited layer. These ions are not able to diffuse out of the film when it is brought in contact with a pure buffer solution. However, the contact of this film with a poly(allylamine) (PAH) or a poly(L-glutamic acid) (PGA) solution leads to the release of ferrocyanide ions from the multilayer. It is shown that the release of ferrocyanide anions, when the film is in contact with a PGA solution, is due to the diffusion of the PGA chains into the film so that an exchange between ferrocyanide ions and PGA chains takes place inside the film. On the other hand, PAH chains do not diffuse into PGA/PAH multilayers. When the film is then brought in contact with a PAH solution, the PAH chains from the solution are expected to strongly interact with the ferrocyanide ions and thus induce a diffusion mechanism of the multivalent anions out of the film, the film/solution interface playing the role of a sink for these ions. This work thus shows that interactions between multivalent ions and exponentially growing films are much more complex than expected at first sight and that polyelectrolyte multilayers must be seen as dynamic entities in which diffusion and exchange processes can take place.