The effect of electrosteric interactions on the effective charge of thermoresponsive ionic microgels: Theory and experiments

• Published in: Journal of polymer science. Part B, Polymer physics Polymer physics, 2016-10, Vol.54 (20), p.2038-2049
• Main Authors: Adroher-Benítez, Irene, Ahualli, Silvia, Bastos-González, Delfi, Ramos, José, Forcada, Jacqueline, Moncho-Jordá, Arturo
• Format: Article
• Language: English
• Subjects: Addition polymerization; Charge; Charge (electric); effective charge; electrophoresis; electrostatic interaction; Mathematical analysis; Microgels; nanoparticles; Networks; Polyelectrolytes; Solvents; steric interaction; swelling behavior
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.24109

ABSTRACT In this work, the influence of counterion valence and salt concentration on the effective charge of two types of thermoresponsive ionic microgel particles has been studied. The effective charge of the microgel at different swelling states has been experimentally determined from electrophoretic mobility measurements by solving the electrokinetic equations of the solvent for a single polyelectrolyte brush in the presence of an electric field, taking into account the friction of the solvent inside the polymer network. The experimental results have been compared to those obtained by means of the Ornstein‐Zernike integral formalism within the HNC relation. Results show that microgel bare charge is screened by the combined effect of counterion condensation and permeation inside the microgel particle. In addition to the electrostatic interaction, the steric exclusion exerted by the polymer plays an important role on the local ionic concentrations, especially for shrunken configurations. This steric term is responsible for the strong increase of the microgel effective charge experimentally observed when particles shrink for temperatures above the lower critical solution temperature. We also observe that, in the internal region of the microgel, charge electroneutrality is fulfilled, so the effective charge mainly arises from the region close to the microgel surface. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2038–2049. The influence of counterion valence and salt concentration on the effective charge thermoresponsiveness of ionic microgel particles is investigated, comparing the results obtained from electrophoretic mobility measurements with the predictions provided by the Ornstein‐Zernike integral equation theory. The microgel charge is screened by the combined effect of condensed and diffusing counterions. It is observed that the steric exclusion exerted by the polymer network leads to a relevant increase of the effective charge for shrunken states.