Competitive cation−π interactions between small cations and polycations with phenyl groups in poly(cation−π) hydrogels

Abstract

Although cation−π interactions play a critical role in biological systems, experimental studies in aqueous environments are limited due to a lack of suitable systems and methods. The recent success in synthesizing water-soluble poly(cation−π) bearing adjacent cationic/aromatic residues provides excellent systems for studying cation−π interactions in aqueous environments. In this study, we investigated competitive cation−π interactions between small cations and macrocations with phenyl groups in hydrogels consisting of various poly(cation−π)s. By measuring the Donnan potential of the hydrogels equilibrated in NaCl solutions of different ionic strengths using the microelectrode technique, the binding ratios of small cations (Na+) to the phenyl residues of polymer networks were determined. It was found that when NaCl concentration was lower than the macrocation concentration, the activity coefficient of the counterions (Cl–) in the hydrogels was very high (~2), indicating that a large amount of excess Cl– was trapped in the hydrogels due to Na+ adsorption on the phenyl groups of the polymer chains via cation−π interactions. Further calculations showed that each phenyl group can adsorb a maximum of 1.5 Na+ despite the presence of adjacent macrocations. Additionally, we discuss the mechanism by which small cations compete with adjacent macrocations in poly(cation−π) hydrogels.

Keywords

• Cation−π interaction
• Hydrogel
• Donnan potential
• Activity coefficient
• Adjacent sequence