Ultra-slow diffusion of hexacyanoferrate anions in poly(diallyldimethyl ammonium chloride)-poly(acrylic acid sodium salt) multilayer films.

Polyelectrolyte multilayer (PEM) films display either a linear or an exponential increase of their thickness with increasing the number of polycation/polyanion deposition cycles. Each of these growth regimes is further associated with a specific mode of internal PEM charge compensation: either intrinsic in the case of linearly growing films, or extrinsic for exponentially growing films with incorporation of small ions from the neighbouring electrolyte solution. In this report, we investigate by electrochemistry the charge compensation mechanism operational for poly(diallyldimethyl ammonium chloride)-poly(acrylic acid sodium salt) (PDAMAC-PAA) films recently found to be out-of-equilibrium immediately after the deposition process despite of their exponential growth mode. It is found here that these films are defined by a negative Donnan potential drop (ca. -160 mV), thus evidencing an extrinsic charge compensation process, and most importantly, by a remarkably slow kinetics for loading of hexacyanoferrate anions, a very unusual property for PEMs with exponential growth. Depending on the film thickness, the diffusion coefficient of the redox probe in the film is found to be of the order of 10 - 10 cm s, i.e. about 8 to 10 orders of magnitude less than that typically measured in aqueous solution. This results in a steady state filling of e.g. a 2.8 µm thick PEM film with the redox probe that is achieved after more than 40 h, a feature in line with the typical 4 to 5 days relaxation timescale of the film structure previously determined by atomic force microscopy and Raman micro-spectroscopy analyses.