Synthesis, physiochemical property and antibacterial activity of gemini quaternary ammonium salts with a rigid spacer

A novel series of adamantane-based gemini quaternary ammonium salts (GQASs) named as m-Ad-m (bromides, alkyl chain length, m = 12, 14, 16) are synthesized from 1,3-adamantanedicarboxylic acid. Their chemical structures are confirmed by H-1 NMR, FT-IR and ESI-MS. The surface activities in aqueous solutions of m-Ad-m are evaluated by surface tension. Unlike conventional GQASs with a flexible spacer, m-Ad-m possessing a rigid adamantane spacer show lower surface tension (gamma(CMC)) values as compared to conventional GQASs bearing the same alkyl chains. The thermodynamic properties of m-Ad-m are examined by electrical conductivity measurements at different temperatures, including molecular interaction parameters (beta), standard Gibbs free energy (Delta G(m)(0)), enthalpy (Delta H-m(0)) and entropy (Delta S-m(0)), which show that the micellization of m-Ad-m is entropy-driven. TEM study exhibits that the aggregate morphologies of m-Ad-m change from micelles and spheroidal vesicles of varying size to network aggregates and then to globular vesicles with the increase in m-Ad-m concentration. Furthermore, the antimicrobial efficacy of m-Ad-m against both Gram-positive bacteria (S. aureus and B. subtilis) and Gram-negative bacteria (E. coli, P. aeruginosa and V. parahaemolyticus) is systematically studied by minimum inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) methods, which indicates the favorable antibacterial activity of 12-Ad-12 against the studied bacteria.