Synthesis, physiochemical properties, and antimicrobial activities of a novel gemini surfactants with biphenyl and multiple amide groups

The structure of aqueous solution surfactants, particularly the spacer group used to link the polar head groups, can significantly affect its surface activities and application properties. Herein, we designed and synthesized cationic gemini surfactants (i.e., CnDBH, n = 12, 14, 16, 18) with biphenyl and multiple amide groups as rigid spacer and characterized their structures using H-1 NMR, C-13 NMR, FT-IR and ESI-MS. Surface-active properties and micellization thermodynamics of CnDBH in aqueous solution were studied by surface tension and isothermal titration calorimetry at 298.2 K. Compared to the traditional surfactant of dodecyl dimethyl benzyl ammonium chloride (1227), the synthesized cationic gemini surfactants were found to have lower critical micelle concentrations (0.494-0.00711 mmol L-1) and excellent surface-tension-reducing abilities. The micellization process is thermodynamically spontaneous, exothermic and entropy-driven. Moreover, the cationic gemini surfactants show excellent foaming power and efficient emulsifying abilities for white-oil/water and toluene/water mixtures. Furthermore, they exhibit significant antimicrobial activities against Escherichia coli and Staphylococcus aureus. These excellent properties are attributed to the special molecular structure of CnDBH, in which the four amides is more easily form intermolecular and intramolecular hydrogen bonds, while the hydrophobic biphenyl group is in favour of pointing towards the micellar core in order to interact with the long hydrocarbon chains during the assembly process. Consequently, they can potentially be applicable in a variety of fields as highly efficient agents.