A 1H NMR relaxation study of hydration water in polyelectrolyte mono and multilayers adsorbed to colloidal particles

The adsorption of polyelectrolytes to charged surfaces can be applied to build up multilayers of polyelectrolytes of alternating charge by layer-by-layer assembly. Here, poly-(allylamine hydrochloride) (PAH) is adsorbed to colloidal latex particles in aqueous solution, and to particles precoated with four layers of polyelectrolytes. The method of H-1 NMR transverse relaxation is applied to monitor the hydration water in such layers, where relaxation rates R-2 serve as a parameter describing the average immobilisation of water molecules. Electrophoretic and relaxation measurements are performed as a function of polymer coverage, surface precoating and salt content in the solution. While the zeta-potential increases monotonously with initial polymer concentration due to a charge reversal from a negatively to a positively charged surface, pronounced structures occur in the R-2 curve for monolayers, which are attributed to the contributions arising from different water populations. For adsorption to multilayers, the electrophoretic curves are shifted to higher concentrations, and relaxation rates show an unexpectedly high water immobilisation. The electrophoretic and relaxation data are consistent with a molecular picture of rodlike chains in the absence of salt, and a coil shaped structure for adsorption from salt solution. In both cases, the properties of the fifth layer differ significantly from the first one. this is attributed to the entanglement of adsorbing chains in the precoated layers, leading to larger adsorbed amounts. (C) 2002 Elsevier Science B.V. All rights reserved.