RESUMEN
Polyelectrolyte "complexes" have been studied for almost a century and find more and more applications in cosmetics and DNA transfection. Most of the available studies focused on the thermodynamic aspects of the "complex" formation, mainly to determine phase diagrams and the influence of diverse physicochemical aspects on the formation of "complexes", but conversely less effort has been given to the kinetics of such processes. We describe herein the "complexation" kinetics of a short linear sodium polyphosphate (PSP) with poly(allylamine hydrochloride) (PAH) in the presence of 10 mM, 0.15 M and 1 M NaCl. We find, by using a combination of physicochemical techniques, that mixtures containing a 1 to 1 molar ratio of phosphate and amino groups allow the formation of "complexes" having a few 100 nm in diameter which progressively grow to particles up to 1.5 microns in hydrodynamic diameter, the growth process being accompanied by some progressive sedimentation. During this slow aggregation kinetics, the polyelectrolytes undergo a release of counterions and the zeta potential changes from a positive value to a negative one of -20 mV which is close to the zeta potential of (PSP-PAH)(n) films deposited under identical physicochemical conditions. Even though the complexes have a negative electrophoretic mobility, they contain an equimolar amount of amino and phosphate groups. This allows us to make some assumption about the structure of such "complexes" and to compare them with other published structures. We will also compare them with the aggregates found during the "layer-by-layer" deposition of the same species under the same conditions.
RESUMEN
Because of its versatility, the layer-by-layer (LBL) assembly method has become a popular tool for preparing multimaterial films, yet astonishingly little is known about the fundamental rules governing their deposition. Here we show an unusual case of self-patterning LBL films made from poly(allylamine hydrochloride) and poly(sodium phosphate). In such films, both the film thickness and the film roughness increase linearly with the number of deposition steps up to a thickness of approximately 60 nm. Even more surprising is the fact that the adsorption of individual "layers" of polyanions and polycations proceeds without a regular inversion of the zeta potential and with the occurrence of a growth instability at approximately 75 layers. These findings underline the need to reconsider the fundamentals of polyelectrolyte multilayer film deposition.