Simulations of ionization equilibria in weak polyelectrolyte solutions and gels (original) (raw)
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The influence of ion size and surface charge model in titrations of ionizable polyelectrolytes is studied by means of the Semi Grand Canonical Monte Carlo simulation method in the context of the primitive model. Three models describing a discrete distribution of charged functional groups on the polyelectrolyte and different values for the radius of the background electrolyte spanning from ionic to hydrated radii values were analyzed. The polyelectrolyte titrations were simulated by calculating the degree of ionization versus pH curves at two ionic strengths. The results allow us to quantify the impact of the sizes of the background salt ions and surface functional groups of the polyelectrolyte on the dissociation degree. This influence is explained in terms of the effectiveness of the screening of the charged surface sites. Finally, by comparison with the Non-Linear Poisson-Boltzmann model, the influence of ionic correlations and finite size of the solution ions is assessed.
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A molecular model for linear and cross-linked polyelectrolytes is described. The model emphasizes the effect of interactions between neighboring charged groups upon both configurational and thermodynamic properties of the polymeric systems. Ion binding is introduced in a ,phenomenological manner, and it is shown that the model predicts far larger amounts of binding to polymers than to small molecules containing similar functional groups. It is found that ion binding is necessary to explain the configurational properties and titration curves of linear polyelectrolytes. Moreover, equilibria among ion pairs and unbound ions are shown to provide a means for understanding of the variation of ion-exchange resin selectivity with cross-linking, exchange capacity and the composition of the solution in contact with the resin.
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The static and dynamic structure factors of weakly charged polyelectrolytes in concentrated solutions are derived in the random phase approximation. The effects of polymer stiffness, excluded volume, charge fraction, and salt concentration are obtained and compared with the small angle neutron scattering experiments on poly(acrylic acid) gels. An accurate description of the short range interactions in the unperturbed polymer chain together with consideration of a moderate excluded volume effect is required to approximate the complicated experimental behavior.
Short-range interactions from simple ions to polyelectrolyte solutions
Although many properties of electrolyte solutions can be successfully described by theories at the McMillan Mayer level of approximation, there are other phenomena that cannot be explained without taking into account the explicit nature of solvent molecules. One of these that have received much attention is the Hofmeister effect that describes the influence of different types of ions on the solubility of hydrophobic molecules in water. In this work we use two simple water models, the 'fused-spheres' and the two-dimensional 'Mercedes-Benz' models to study ion solvation in water, and test suppositions about their effect on hydrophobicity. Both models give good qualitative agreement with experiment, such as Samoilov ion hydration activation energies, and Setchenow coefficients, which describe the salt concentration dependence of the solubilities of hydrophobic solutes. The results suggest that the interactions of ions with water are governed mostly by the ionic charge densities. Water structure is determined by the balance of electrostatic forces and the tendency for hydrogen bond formation. Ions with a high charge density bind water molecules very tightly and therefore exclude the hydrophobe from their first shell, leading to salting-out. The effect decreases with decreasing charge density of the ion. ᮊ
Polymer Science, series A, 2003, Vol. 45(5) p. 485, 2003
The structural and thermodynamic characteristics of highly charged rigid-chain polyelectrolytes with explicit incorporation of univalent counterions were studied using the Monte Carlo method and integral equation theory (in the PRISM approximation). Computations were performed for dilute and semidilute solutions over a wide temperature range. It was found that, at temperatures below the Manning threshold, the integral equation theory does not ensure exact quantitative agreement with the computer simulation results. On the basis of Monte Carlo calculations, it was shown that the reason for the lower accuracy of the PRISM approximation in the low-temperature region is the formation of locally anisotropic structures in the polymer subsystem. In a semidilute solution in a thermodynamically good solvent, aggregation processes due to the effective attraction of polyions can lead to LC ordering of short rodlike macromolecules.