Effect of the pH value on compression and array structures of highly charged microgels at the air/water interface

Understanding the interfacial behavior of stimuli-responsive microgels is critical for applications such as foam and emulsion stabilization, as well as for the fabrication of two-dimensional colloidal crystals using the interfaces. In this study, the pH-dependent compression behavior and array structures of micron-sized poly(N-isopropylacrylamide-co-acrylic acid) microgels at the air/water interface was investigated. By combining a Langmuir trough with fluorescence microscopy, microgel arrays under compression and acidic (pH = 3) or basic (pH = 9) conditions were directly visualized. At pH = 9, the carboxyl groups within the microgels are deprotonated, resulting in significant swelling and the formation of ordered hexagonal arrays with high crystallinity (Ψ6 > 0.84) upon compression. In contrast, at pH = 3, the carboxyl groups within the microgels are protonated, leading to a suppression of the electrostatic repulsion between neighboring microgels and a reduction in crystallinity (Ψ6 ∼ 0.70) of the microgel arrays before and after compression. Furthermore, the calculated surface-compression modulus using the compression isotherms indicated higher interfacial elasticity for charged microgels, demonstrating that electrostatic repulsion governs both array ordering and mechanical robustness. These findings provide fundamental insights into the role of charge in controlling the microgel structure and mechanics at interfaces, thus offering further guidelines for the design of stimuli-responsive materials and stabilizers for foams and emulsions.