Influence of polyelectrolyte multilayer films on calcium phosphate nucleation

The nucleation of calcium phosphate crystals from a weakly supersaturated calcium phosphate solution on the surface of polyelectrolyte multilayers was investigated in dependence on the chemical nature of the outermost layer. Scanning angle reflectometry was used to follow in situ the initial stages of the nucleation kinetics. The multilayers were constructed by alternate adsorption of poly(styrene sulfonate) (PSS) and poly(allylamine) (PAH), leading to oppositely charged surfaces. It was verified that films terminating with either PSS or PAH exhibited a negative or positive 5-potential, respectively. Surprisingly, both types. of surface layers induced a nucleation process for supersaturations smaller than the one observed on the bare silica surface. According to the literature, such an effect should only be expected on a negatively charged surface. Infrared spectroscopy showed that the nucleated crystals are hydroxyapatite (OHAP) or octacalcium phosphate (OCP) but not dicalcium phosphate dihydrate (DCDP) which is found on bare silica surfaces. On both PSS and PAH surfaces, the nucleation processes started only after a given induction time. The evolution of the induction times with different supersaturations was analyzed within the framework of the classical nucleation theory. The effective surface free energies of the formed crystals were estimated to be on the order of 32 mJ.m(-2) on a multilayer terminating with PSS and to be about 37 mJ.m(-2) on a multilayer terminating with PAH. A mechanism for the enhanced nucleating effect of such polyelectrolyte surfaces is proposed.