Effect of organic and a Hofmeister series of inorganic counterions on the solvation dynamics and rotational relaxation in aqueous micelles of hexadecyltrimethylammonium surfactants

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 C _n H_2n+1CONH(CH₂)₂N⁺(CH₃)₂(CH₂)₂N⁺(CH₃)₂(CH₂)₂ NHCOC _n H_2n+1 · 2Y (n = 11, 13 and 15, Y = HCOO^-, CH₃COO^- and CH₃CHOHCOO^-) 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 × 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 (β) 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 vesicle 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.