The surface adsorption, aggregate structure and antibacterial activity of Gemini quaternary ammonium surfactants with carboxylic counterions

A group of Gemini quaternary ammonium surfactants with the formula CnH2n+1CONH(CH2)(2)N+(CH3)(2)(CH2)(2)N+(CH3)(2)(CH2)(2)NHCOCnH H2n+1 center dot 2Y (n = 11, 13 and 15, Y = HCOO-, CH3COO- and CH3 CHOHCOO-) have been synthesized by a counterion conversion process and characterized by Fourier transform infrared spectroscopy and mass spectroscopy. Their adsorption and self-aggregation properties are investigated by surface tension, conductivity, dynamic light scattering and transmission electron microscopy (TEM) measurements. The results show that these surfactants reduce the surface tension of water to a minimum value of 26.51 mN m(-1) at a concentration of 5.72 x 10(-2) mmol l(-1). Furthermore, the increased alkyl chain length of the carboxylic counterions leads to the increased critical micelle concentration, the decreased degree of counterion binding (beta) and the decreased self-assembly tendency, but the minimum area per surfactant molecule (A(min)) adsorbed at the air-aqueous solution are similar. TEM images reveal that these surfactants self-assemble spontaneously into aggregates with veside or bilayer structures. It is also found that they have superior antibacterial activity at a concentration of 0.1 g l(-1). The high surface activity and high antibacterial activity of the Gemini quaternary ammonium salt surfactants containing different carboxylic counterions bring more possibilities for the application in the field of biomedicine.