Cationic gemini surfactants containing both amide and ester groups: Synthesis, surface properties and antibacterial activity

Nine novel cationic gemini surfactants containing both amide and ester groups were synthesized through a three-step reaction. These surfactants have the general formula of C n H 2n+1 OOCCH 2 N + (CH 2 ) 2 -(CH 2 ) 3 -NHOC-(CH 2 ) m -CONH-(CH 2 ) 3 -N + (CH 2 ) 2 CH 2 COOC n H 2n+1 (with n = 8, 10, 12 and m = 2, 3, 4), named BEQA-n, PEQA-n and HEQA-n (B, P, H correspond to surfactants of m = 2, 3, 4 and n = 8, 10, 12, respectively). Chemical structures, surface activity, cleavable properties, electrolytes tolerance, foam properties and antibacterial activity of these surfactants were successively investigated. The results show that the surfactants have lower critical micelle concentration (CMC) than traditional monomer surfactants. The values of CMC and the degree of counter ion dissociation (α) depend on the lengths of hydrophobic chain and spacer. The values of Krafft temperatures (K t  < 0 °C) and the standard Gibbs free energy (∆G 0 mic  < 0) indicate that all surfactants have good water solubility and micelle-forming ability. The simultaneous presence of the amide and ester groups makes these surfactants have good hydrolytic ability in both basic and the acidic solutions. In addition, the surfactants exhibit greater electrolyte tolerance to NaCl than CaCl 2 . It is worth mentioning that EQA-8 and PEQA-12 have 200 g/L NaCl tolerance. Moreover, all of the surfactants have certain antibacterial activity against Gram-positive bacteria (S. aureus ATCC 25923) and Gram-negative bacteria (E. coli ATCC 25922), and the surfactants with hydrophobic chain length n = 10 have the strongest antibacterial activity.