Dynamic Covalent Bonding and Bridging: pH-Governed Complexation of Phenylboronic Acid-Modified Alginate with Alkyl...

The dynamic covalent complexation between phenylboronic acid (PBA)-functionalized polymers and diol-containing surfactants presents a powerful strategy for fabricating smart interfacial materials. This work presents a comprehensive investigation into a pH-responsive system composed of Alg-PBA and the diol-bearing surfactant alkyl polyglycoside (APG). The pH-governed formation of boronate ester (B-O) bonds between Alg-PBA and APG dictates their interaction regimes, leading to distinct behaviors in bulk solution, at the oil-water interface, and in macroscopic emulsions. At pH 3, weak hydrogen bonding dominates, while above the pK_a of PBA (pH 9 and 10), strong dynamic covalent complexes form. At intermediate pH (pH 6), the interaction is in a dynamic transitional state. These molecular-level interactions directly govern tunable interfacial activity and adsorption kinetics. Crucially, a pH-dependent "bridging effect" is identified: at low APG concentrations, Alg-PBA chains connect oil droplets into a three-dimensional network, yielding gel-like emulsions stabilized by hydrogen bonds (pH 3) or robust B-O bonds (pH 6, 9 and 10). At high APG concentrations, hydrophobic interactions disrupt this network, fluidizing the emulsion. By decoupling the interplay between pH-tunable binding, polymer conformation, and surfactant concentration, this study establishes the Alg-PBA/APG complex as a versatile, stimuli-responsive emulsifier platform. This provides a design principle for creating emulsions with on-demand rheological properties and stability for applications in drug delivery, food, and cosmetics.