Gemini surfactant-engineered di-Schiff bases: Tailoring hydrophobicity to enhance catalytic and biological activities

This study explores the enhancement of catalytic performance of silver nanoparticles (AgNPs) using a series of gemini surfactants based on di-azomethine with varying hydrophobic tail lengths: DSGO, DSGD, and DSGH. By fine-tuning the molecular structure of these surfactants, we aim to develop more effective catalysts for purifying polluted water sources. The high surface energy of AgNPs often results in their aggregation, reducing their catalytic efficacy. However, increasing the surfactant tail length from 8 (DSGO) to 16 (DSGH) carbon atoms improves the stability and dispersibility of AgNPs, as confirmed by DLS, TEM, and UV analyses. DSGH, with its longer carbon tail, facilitates AgNPs synthesis with superior stability and smaller particle size, indicated by a higher zeta potential of +55.6 mV and a reduced size of 15.1 nm. This is attributed to the increased surface activity and hydrophobicity of the surfactants. As the tail length increases from 8 to 16, the investigated surfactant's tendency for micellization and adsorption rises. Notably, DSGH exhibits the lowest critical micelle concentration (CMC) of 406 μM and the highest adsorption Gibbs free energy (ΔG°ads) of −59.08 kJ·mol⁻¹ at 20 °C, enabling strong adsorption onto AgNPs. By modulating surfactant hydrophobicity, we effectively regulated AgNP catalytic activity. The DSGH/AgNPs composite shows enhanced catalytic activity, efficiently converting p-nitrophenol (p-NP) and methylene blue (MB) into less toxic species with apparent rate constants (k_app) of 0.368 and 0.537 min⁻¹, respectively. Additionally, the AgNP-incorporated gemini surfactants exhibit promising antimicrobial activity against tested bacteria and fungi. Remarkably, the DSGD/AgNPs hybrid system surpasses commonly used commercial antimicrobial agents in efficacy. This study underscores the potential of manipulating surfactant structure to control the catalytic and antimicrobial activity of AgNPs.