Multilayer films made from poly(allylamine) and phosphorous containing polyoxometalates: focus on the zeta potential

The physicochemical characterization of coatings deposited in a step-by-step (SBS) manner relies most often on thickness and homogeneity measurements by means of atomic force microscopy. In the case of coatings produced from oppositely charged species, their surface potential, estimated through their zeta potential, was for a long time expected to change alternatively as a prerequisite for the film deposition. However, some counterexamples appeared in the literature where the growth of the coating was observed when the number of deposition steps was increased but without a regular change in the sign of the surface potential. These data showed that the interpretation of the zeta potential of SBS deposited films should be subjected to more attention. In this article, we show the occurrence of ionic strength dependent instability of the surface potential of films made from the alternated deposition of poly(allylamine hydrochloride) and polyoxoanions. This instability may be due to a structural change in the coating, but we cannot exclude some shear induced desorption during the zeta potential measurement. In addition, a change in ionic strength between the deposition of the coating and the measurement of its streaming potential can have a huge importance on the result.