Electrostatic Interactions and Osmotic Pressure of Counterions Control the pH-Dependent Swelling and Collapse of Polyampholyte Microgels with Random Distribution of Ionizable Groups
R Schroeder and AA Rudov and LA Lyon and W Richtering and A Pich and II Potemkin, MACROMOLECULES, 48, 5914-5927 (2015).
In this work, different systems of colloidally stable, ampholytic microgels (mu Gs) based on poly(N-vinylcaprolactam) and poly(N-isopropylacrylamide), wherein the anionic and cationic groups are randomly distributed, were investigated. Fourier transmission infrared spectroscopy and transmission electron microscopy confirmed the quantitative incorporation and random distribution of ionizable groups in mu Gs, respectively. The control of hydrodynamic radii and mechanical properties of polyampholyte mu Gs at different pH values was studied with dynamic light scattering and in situ atomic force microscopy. We have proposed a model of pH-dependent polyampholyte mu G, which correctly describes the experimental data and explains physical reasons for the swelling and collapse of the mu G at different pHs. In the case of a balanced mu G (equal numbers of cationic and anionic groups), the size as a function of pH has a symmetric, V-like shape. Swelling of purely cationic mu G at low pH or purely anionic mu G at high pH is due to electrostatic repulsion of similarly charged groups, which appears as a result of partial escape of counterions. Also, osmotically active counterions (the counterions that are tiapped within the mu G) contribute to the swelling of the mu G. In contrast, electrostatic interactions are responsible for the collapse of the mu G at intermediate pH when the numbers of anionic and cationic groups are equal (stoichiometric ratio). The multipole attraction of the charged groups is caused by thermodynamic fluctuations, similar to the those observed in Debye Hiickel plasma. We have demonstrated that the higher the fraction of cationic and anionic groups, the more pronounced the swelling and collapse of the mu G at different pHs.
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