Solute-induced wettability control at naphthalenediimide polymer brushes via anion–π interactions

Abstract

Wettability at solid/liquid interfaces is a critical property that influences material functions such as sensing, mass transport, interfacial reactions, and adhesion. While selective wettability toward specific substances is desirable, established methods to achieve this remain limited. Noncovalent interactions, especially those driving molecular recognition and self-assembly, are essential for biological functions and functional molecular systems. However, their role in controlling the interfacial wettability has not been fully explored. In this study, we aimed to control the water wettability using anion–π interactions between anions and electron-deficient aromatic rings. To enhance these interactions, polymer brushes bearing naphthalenediimide (NDI) units serving as anion receptors have been fabricated on solid substrates. Quartz crystal microbalance with dissipation (QCM-D) measurements revealed strong and selective adsorption of nitrate ions (NO₃⁻). Force curve analysis was used to further quantify the anion–π interactions at the interface. When aqueous solutions containing various anions were applied to the NDI-modified surfaces, a decrease in the contact angle was observed compared to that of pure water, indicating enhanced wettability. Notably, the NO₃⁻ solutions led to a significant reduction in the contact angle, which is consistent with the adsorption data. These findings demonstrate successful modulation of wettability through molecular recognition, specifically via anion–π interactions. This approach offers a promising strategy for developing selective wetting interfaces. Future work may involve integrating electrowetting mechanisms to expand their applications in sensors, controlled substance transport, and separation technologies.