Ion-specific and charge effects in counterion binding to poly(styrenesulfonate) anions.
ABSTRACT In order to obtain a deeper insight into effects occurring when an electrolyte solution is added to a solution of a strong polyelectrolyte, the microcalorimetric and potentiometric titrations of poly(sodium 4-styrenesulfonate) (Na(+)PSS(-)) solution with different alkali, earth-alkali and tetraalkylammonium nitrate, perchlorate and chloride solutions were performed. From the calorimetric titrations the differences in sign and magnitude of enthalpy change upon addition of various electrolytes were observed depending on the salt used. Potentiometric titrations using a sodium ion selective electrode have revealed that addition of an electrolyte is accompanied by the increase in sodium activity until a certain critical value is reached, which seems to be the consequence of counterion substitution on the polyelectrolyte chain. In the case of addition of lithium and sodium salts the experimental results for ΔH of mixing can be qualitatively correctly explained by the Poisson-Boltzmann and Monte Carlo calculations based on the continuum solvent models. This is not the case for the mixtures with KNO(3), RbNO(3) and CsNO(3) salts. The results suggest that the ion-specific effects, associated with the changes in the water structure, have to be taken into account when thermodynamic properties of polyelectrolytes in solution are concerned. The calorimetric results imply that the enthalpically observed cation specificity for binding to a poly(styrenesulfonate) group could be correlated with corresponding cation hydration enthalpies. The counterion substitution of sodium with divalent cations was found to be endothermic, which is in qualitative agreement with the electrostatic theory.
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ABSTRACT: Aliphatic x,y-ionenes are polyelectrolytes in which x and y denote the numbers of methylene groups separating quaternary ammonium ions. They represent useful model substances for studying hydrophobic and charge effects in aqueous solutions. We used isothermal titration calorimetry to measure the enthalpies of mixing, ΔH(mix), of 3,3- and 6,6-ionene fluorides and bromides with low molecular weight salts (NaF, NaCl, NaBr, and NaI) at 298 K in water. The signs and magnitudes of the measured enthalpies depend on the hydrophobicity of the ionene and on the nature of the added salt. For example, addition of sodium fluoride to solutions of 3,3- and 6,6-ionene fluorides produced endothermic effects, while addition of sodium bromide to 3,3-ionene bromide resulted in a strong exothermic effect. Interestingly, mixing of 6,6-ionene bromide and NaBr solutions in water gave a small exothermic heat effect. Polyelectrolyte theories, based on continuum-solvent models, predict enthalpies of mixing to be positive (endothermic) for all the solutions examined in this work. The ion-specific effect is more strongly expressed in ionene solutions with higher charge density (3,3-ionene). The most important result of this work is the finding that the enthalpy of mixing of 3,3- (and of 6,6-ionene) fluorides with sodium halides can be expressed as a linear function of the enthalpy of hydration of the halide counterions. The experimental results were complemented with an explicit water molecular dynamics simulation of solutions of oligoions modelling 3,3- and 6,6-ionenes. The computer simulation results for various nitrogen-counterion pair distribution functions were in most cases consistent with the enthalpy measurements.Physical Chemistry Chemical Physics 02/2012; 14(6):2024-31. · 4.20 Impact Factor
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ABSTRACT: The (de)protonation equilibrium of the poly(allylammonium) cation (PAH) in an aqueous solution of various binary 1 : 1 electrolytes of different concentrations (0.1 ≤ c(NaX)/mol dm(-3) ≤ 1.0; X = Cl(-), Br(-), I(-), NO3(-)) was investigated potentiometrically at 25 °C. The mixed and concentration apparent equilibrium deprotonation constants (Kap) were calculated from the experimentally collected data and concentration profiles of dissociated and undissociated functional groups were obtained. The standard pK value of monomers was estimated by extrapolating the pKap values determined at various concentrations of added electrolyte to the degree of dissociation α = 1. The dependence of pKap on the degree of dissociation could be well described by the two parameter model according to Mandel. The variation of with monomer dissociation degree was found to be in satisfactory agreement with the cylinder Stern model, based on the Poisson-Boltzmann (PB) equation, and a constant Stern capacitance. Generally, the derived apparent constants showed a pronounced dependence on the concentration of binary electrolytes and a weak dependence on the type of anion counterbalancing the polyion charge. The influence of the PAH chain length (polymers containing on average 150 and 700 monomers were examined) on the protonation equilibrium of PAH could not be observed.Physical Chemistry Chemical Physics 04/2013; · 4.20 Impact Factor
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ABSTRACT: Complexation between polyallylammonium cations and polystyrenesulfonate anions was investigated in aqueous solutions of binary 1 : 1 sodium electrolytes (NaX, X = F, Cl, Br, I, NO3, ClO4) by means of microcalorimetry, dynamic light scattering, electrokinetics and spectrophotometry. At lower molar ratios of monomer units charged polyelectrolyte complexes were formed. At molar ratios close to equivalence and at lower salt concentrations (c(NAX)/mol dm(-3) ≤ 0.1) flocculation occurred. The obtained precipitates contained approximately equimolar amounts of oppositely charged monomer units. At c(NAX)/mol dm(-3) ≥ 0.5 (X = NO3, ClO4) and in the case when the polycation was present in excess, the amount of positively charged monomer units in the precipitate was higher than that of negatively charged monomers (asymmetric neutralisation). In addition, the aggregation of positively charged complexes in concentrated solutions of all investigated electrolytes was noticed. The onset of aggregation was strongly anion specific. However, the aggregation of negatively charged complexes did not occur even at c(NaX) = 3 mol dm(-3). The composition of the insoluble products at equimolar ratio of monomer units and higher concentrations of NaNO3 and NaClO4 was dependent on the order of addition, indicating non-equilibrium interpolyelectrolyte neutralisation under all ionic conditions. At 25 °C and c(NaClO4) = 1 mol dm(-3) equilibrium was not reached after two months. In contrast, the supernatants showed no traces of free polyanion chains after being heated for a week at 60 °C. The pairing of monomer units was predominantly entropically driven, irrespective of the type of reaction products formed (polyelectrolyte complexes, precipitates) and the electrolyte type. The results obtained indicate that the overcharging is not an enthalpically demanding process. The calorimetric measurements also suggest that the strong influence of counteranions on the composition of the reaction product must be related to differences in ion distribution around polycations. However, despite rather similar energetics for complex and precipitate formation in the presence of various sodium salts a clear correlation of formation enthalpies with corresponding anion hydration enthalpies (Hofmeister series) was observed. Somewhat surprisingly, the titration calorimetry experiments have also revealed that the increase in electrolyte concentration affects the enthalpy of interpolyelectrolyte neutralisation negligibly.Soft Matter 07/2014; · 4.15 Impact Factor