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The counteranion specific effects on interpolyelectrolyte neutralization in aqueous solutions and at surfaces

The interpolylectrolyte neutralization of poly(diallyldimethylammonium)(PDADMAC) and (poly(N-ethyl-4-vinylpyridinium) (P4VPEt) cations with poly(styrene sulfonate) (PSS) anion in aqueous solutions of binary sodium salts was studied by means of dynamic light scattering (DLS), microcalorimetry, spectrophotometry and quartz crystal microbalance with dissipation monitoring (QCM- D). The reactions upon stepwise titrant to titrand addition in solution proceeded as described by Fuoss and Sadek. At first, charged primary (the excess of titrand monomers at surface) and then secondary complexes (the excess of titrant monomers at surface) were being formed, whose coalescence resulted with precipitate formation. In contrast, sequential deposition of polycation-polyanion pairs was realized by exposing the SiO2 surface to reactant solutions. At lower salt concentrations, no influence of the counterion type could be noticed. The insoluble, metastable polyelectrolyte complexes formed in solution, contained almost equal amounts of oppositely charged monomers and the multilayer thickness was only weakly influenced by the type of the salt present. The anion- specific aggregation and more pronounced overcharging of positive polyelectrolyte nanocomplexes was observed at higher electrolyte concentrations. This led to asymmetric interpolyelectrolyte neutralization (i.e. direction dependent monomer ratio in metastable precipitates). The polycation affinity towards explored counteranions decreased in the order: ClO4– ≈ I– > NO3– ≈ Br– > ClO3– > Cl– for both PDADMAC and P4VPEt. The type of the anion introduced also effected the composition of the negative nano-complexes, obtained during titration of the polyanion with investigated polycation solutions, though the effect was much less pronounced than in the opposite direction. The reactions of investigated polyelectrolytes were predominantly (or entirely) entropically driven at ionic conditions explored. The thickness of P4VPEt-PSS and PDADMAC-PSS multilayers was in complete accord with the results of experiments in solution. The equilibrium establishment in solutions containing equal amounts of oppositely charged PDADMAC-PSS and P4VPEt-PSS polyelectrolytes proceeded slowly and resulted with formation of virtually insoluble polysalts. Though being slow, the transformation of investigated metastable products into equilibrium ones, proceeded much faster than in the case of previously investigated poly(allylammonium) (PAH) - PSS pair.

interpolyelectrolyte neutralization, counteranion specific effects, primary complexes, secondary complexes, multilayers

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