Micelle Fibrillization, Gelation, and Mechanical Property Tuning of a Green, Stimuli-Responsive, and Na ⁺ Ion-Specific Low-Molecular-Weight Amino Acid-Based Cationic Surfactant─Hofmeister and Ionic Strength Effects

A novel low-molecular-weight hydrogel (LMWH) was designed through the intricate process of molecular self-assembly of a biocompatible and biodegradable Nα-lauroyl-l-arginine methyl ester, a cationic arginine-based surfactant, abbreviated as LAM. Transitioning from spherical to smart worm-like micelles (SWLM), confirmed through DLS and SAXS, this surfactant displayed an extraordinary capability to induce gelation in aqueous solutions above pH 8.5, even at a mere 0.5% weight ratio. The hydrogel exhibited reversible responsiveness to changes in temperature (long-lasting thermal tolerance), with Tgel observed at approximately 38.5 °C. Additionally, gel–sol transition was observed upon pH change and mechanical agitation. The mechanical characteristics of the LAM hydrogel studied through rheology were found to be exquisitely adaptable, enabling its fine-tuning through adjustments in the (i) concentration of LAM, (ii) Hofmeister chaotropic anions, and (iii) ionic strength. The critical micellar concentration (CMC) and critical gelation concentration (CGC) of LAM decreased, whereas the gelation temperature, Tgel, increased from 38.5 to 53.5 °C, upon addition of 0.6 M NaI. At this ionic strength, the mechanical strength of the gel increased by 66.9 times. The hierarchical microstructure properties were analyzed through an inverted microscope, FESEM, and a THUNDER microscope. Besides the structural versatility of the LAM hydrogel, it exhibited exceptional biocompatibility, biodegradability, and injectability. Moreover, it displayed notable antimicrobial properties, effectively countering both bacterial and fungal pathogens. The hydrogel also finds better applications in environmental remediation by sequestering heavy metal ions (HMIs).