The swelling of poly(acrylamide) (PAAm) gels and the osmotic pressure of linear PAAm in aqueous solutions were predominantly affected by anion type and increased according to the lyotropic series ranking of sodium halide anions: F− < (H2O) < Cl− < Br− < I−. The osmotic pressure of PAAm in all examined salt solutions followed the scaling theory, with an exponent of 2.3 ± 0.1. In solutions of a sodium halide series, the value of the pre‐exponential factor seemed to depend on salt concentration, anion radius, and the apparent “anionic‐portion radius” of the water molecule. This radius, extracted from the literature data, marks a transition point of the anion radius effect. Larger anions increase the osmotic pressure of PAAm more significantly as their concentration increases and vice versa. The effects of the anions on the osmotic pressure of PAAm are related to their preferential interactions with the polymer. Iodide, which increased the osmotic pressure of PAAm with respect to its value in pure water, seemed to preferentially adsorb onto the polymer with a binding constant of Kb = 9.7 ± 2.0 M−1 determined by isothermal titration microcalorimetry. However, fluoride, which decreased the osmotic pressure, was preferentially repulsed. The mechanisms of attraction and repulsion were attributed to ion‐water‐polymer interactions and the solvent quality of the hydrated ions. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 508–519, 2003
The swelling of polyacrylamide (PAAm) gels increased with rising glucose concentrations, and so did the osmotic pressure of the soluble polymer and its intrinsic viscosity. A Flory-Huggins-based model for the osmotic pressure of a nonionic hydrophilic polymer in a ternary solution consisting of a main solvent, a polymer, and a nondissociating low-molecular-weight cosolute was developed and examined. The model-calculated values were in reasonably good agreement with experimental results for the water-PAAm-glucose system studied when PAAm-water and glucose-water interaction coefficients from the binary systems were used, and only the PAAm-glucose interaction coefficient was adjusted. Its negative value suggested a favorable interaction of glucose and PAAm, supporting the notion of glucose being a good cosolvent for PAAm. Isothermal titration microcalorimetry results showed no evidence for the binding of glucose to PAAm, but an exothermic interaction was indicated between glucose and PAAm. Microcalorimetrically determined enthalpic contributions to the Flory-Huggins interaction coefficients showed enthalpically favorable binary interactions, particularly the enthalpic component of the PAAm-glucose interaction coefficient ( H23 ), which was slightly negative. The enthalpically favorable interaction between glucose and PAAm may explain the increased osmotic pressure of PAAm in glucose solutions.
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