Effect of Counterion Binding to Swelling of Polyelectrolyte Brushes

Chunda Ji; Chao Zhou; Bintao Zhao; Jingfa Yang; Jiang Zhao

doi:10.1021/acs.langmuir.1c00309

Effect of Counterion Binding to Swelling of Polyelectrolyte Brushes

Langmuir. 2021 May 11;37(18):5554-5562. doi: 10.1021/acs.langmuir.1c00309. Epub 2021 May 3.

Authors

Chunda Ji^{1

2}, Chao Zhou^{1

2}, Bintao Zhao^{1

2}, Jingfa Yang^{1

2}, Jiang Zhao^{1

2}

Affiliations

¹ Beijing National Research Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
² University of Chinese Academy of Sciences, Beijing 100049, China.

PMID: 33934597
DOI: 10.1021/acs.langmuir.1c00309

Abstract

The effect of binding strength of counterions with the polyelectrolyte chain to the swelling of polyelectrolyte brushes is studied, by investigating the swelling of both the polycation and polyanion in response to the variation of the salt concentration under the change of counterion's identity. Two polyelectrolyte brushes grafted on solid substrates are adopted: the cationic poly [2-(methacryloyloxy)ethyltrimethyl ammonium] (PMETA-X, X = F, Cl, Br, and I) and the anionic polystyrene sulfonate (M-PSS, M = Li, Na, K, and Cs). The swelling change with the salt concentration is investigated by ellipsometry, quartz crystal microbalance with dissipation, and dielectric spectroscopy. It is discovered that although the thickness of PMETA-X brushes is larger than that of M-PSS brushes of similar grafting density, the former has much less solvent incorporated than the latter. Such a difference is attributed to the weaker interaction between the PMETA⁺ chain and its halide counterions than that between the PSS^- chain and its alkali counterions, discovered by dielectric spectroscopy. This makes the original charges on the PMETA-X chain less neutralized and therefore have a higher charge density, compared with the M-PSS chain. The results demonstrate that the stronger binding of the counterions to the polyelectrolytes makes the main chains less charged, resulting in the weaker inter-chain electrostatic repulsion and less swelling of the brushes. Investigations into the effect of ion identity show the following order of binding strength: for the cationic PMETA⁺ chain, F^- < Cl^- < Br^- < I^- and for the anionic PSS^- chain, Li⁺ < Na⁺ < K⁺ < Cs⁺.