Electrostatic Interactions and Osmotic Pressure of Counterions Control the pH-Dependent Swelling and Collapse of Polyampholyte Microgels with Random Distribution of Ionizable Groups
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- 作者：Ricarda Schroeder ; Andrey A. Rudov ; L. Andrew Lyon ; Walter Richtering ; Andrij Pich ; Igor I. Potemkin
- 刊名：Macromolecules
- 出版年：2015
- 出版时间：August 25, 2015
- 年：2015
- 卷：48
- 期：16
- 页码：5914-5927
- 全文大小：919K
- ISSN：1520-5835
文摘
In this work, different systems of colloidally stable, ampholytic microgels (渭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 渭Gs, respectively. The control of hydrodynamic radii and mechanical properties of polyampholyte 渭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 渭G, which correctly describes the experimental data and explains physical reasons for the swelling and collapse of the 渭G at different pHs. In the case of a balanced 渭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 渭G at low pH or purely anionic 渭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 trapped within the 渭G) contribute to the swelling of the 渭G. In contrast, electrostatic interactions are responsible for the collapse of the 渭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鈥揌眉ckel plasma. We have demonstrated that the higher the fraction of cationic and anionic groups, the more pronounced the swelling and collapse of the 渭G at different pHs.