Biobased polymeric surfactant: Natural glycyrrhizic acid-appended homopolymer with multiple pH-responsiveness

Even though amphiphilic natural products have been widely employed in cosmetics, food, and pesticide formulations, the development of a compound with stimuli-responsiveness is still highly significant. Inspired by the low cost, biocompatibility, pH resistance and amphiphilicity of natural glycyrrhizic acid (Gly), we designed and synthesized an amphiphilic homopolymer, namely, poly(glycyrrhizic acid) (PGly), via reversible addition-fragmentation chain transfer (RAFT) polymerization. The two carboxylic groups on the side chain of PGly promoted it as a multiple pH-responsive polymeric surfactant. As the pH was decreased from 5.0 to 2.0, the PGly transformed from the extension to the coil state and further aggregated into nano/micro particles. During this process, the surface charge, surfactivity and diffusion rate of PGly decreased along with the decreasing pH. Moreover, with ultrasonic treatment (UT), the aggregates that formed at lower pH values decreased (pH 3.0 and 2.0) or almost disappeared (pH 4.0), indicating that these insoluble aggregates at lower pH were broken by UT and were then reassembled into the compacted PGly nanoparticles. Based on the above results, the emulsion (20 wt% xylene) stabilized by 0.1 wt% PGly was fabricated using ultrasonic emulsification for which the diameter distribution and the dispersion state can be reversibly regulated by pH (2.0-5.0). This natural polymeric surfactant exhibited a favorable surfactivity and multiple pH responsiveness in the preparation of emulsions, showing its potential for application in the controlled release of pesticide formulations and in the recovery of organic pollution.