engleski

Complexation between polyallyammonium cation and polystyrene anion ; The effect of ionic strength and electrolyte type

Complexation between polyallylammonium cation (PAH) and polystyrenesulfonate (PSS) anion was investigated in aqueous solutions of binary sodium (NaX, X = F, Cl, Br, I, NO3, ClO4)[1] and chloride (MCl, M = Li, Na, K, Rb, Cs and Et4NCl)[2] salts by means of microcalorimetry, dynamic light scattering, electrokinetics and spectrophotometry. At lower molar ratios of monomers charged polyelectrolyte complexes were formed irrespectively of the electrolyte type. At csalt / mol dm3 ≤ 0.1 and close to equivalence their flocculation occurred. The obtained precipitates contained approximately equal amounts of PAH and PSS monomers. At c(NAX) / mol dm3 ≥ 0.5 and in the case when the polycation was present in excess strongly anion specific aggregation of positively charged complexes was noticed. Further more, the amount of charged monomer units, incorporated in the precipitates obtained in NaNO3 and NaClO4 solutions (equimolar ratio), was found to be dependent on the reactant addition order. The neutralisation was more asymmetric at higher salt concentrations, leading to considerable surplus of positive monomers in precipitates obtained. Even at extremely high perchlorate concentrations (2 mol dm3) and at 60 ºC the final reaction product was virtually insoluble polysalt, indicating its extremely high stability. However, the equilibrium establishment proceeded extremely slowly. On the other hand, the composition of the precipitates obtained in chloride salt solutions (equimolar ratio) did not significantly depend on the addition order up to 1.0 mol dm3, suggesting the formation of metastable precipitates with high degrees of monomers paired. The negatively charged complexes were not prone to aggregation even at electrolyte concentrations as high as 3.0 mol dm3. The interpolyelectrolyte neutralisation of investigated polyelectrolytes was predominantly entropically driven, irrespectively of the type of reaction products formed (polyelectrolyte complexes, precipitates), and the electrolyte present. Somewhat surprisingly, the increase in electrolyte concentration affects the enthalpy of interpolyelectrolyte neutralisation negligibly. The obtained results hence indicate that the strong influence of counteranions on the composition of reaction product must be related with differences in anion distribution around polycation. On the other hand, the large differences in PAH and PSS counterion binding abilities are most likely due to differences in the distances of the charged groups from the backbones of investigated polymers, though the functional group type could play an important role as well.

Polyelectrolytes, Polyelectrolyte complexation, Ion-specific effects, Ion hydration

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