Adsorption and aggregation properties of quaternary-ammonium-salt-based gemini surfactants with a glycinate counterion

Cationic gemini surfactants have promising bioapplications that are limited by the use of halides as counterions, which pose environmental and human health risks. This can be circumvented by using naturally occurring, highly water-soluble, and nontoxic counterions such as amino acids. In this study, we synthesized novel quaternary-ammonium-salt-based cationic gemini surfactants with glycinate as the counterion [2C_n(2-O-2) Gly, where n is the alkyl chain length, n = 10, 12, 14]. Their adsorption and aggregation properties were investigated by measuring their electrical conductivity, surface tension, and small-angle neutron scattering and compared with those of the corresponding gemini surfactants with a bromide counterion [2C_n(2-O-2) Br]. The relationship between the surface tension and concentration of 2C_n(2-O-2) Gly exhibited a unique behavior with a pronounced minimum near the critical micelle concentration. This suggests that 2C_n(2-O-2) Gly adsorbs densely at the air/water interface through hydrogen bonding between the amino nitrogen of the counterion and hydrogen atoms of water, as well as between the carboxylate oxygen of the counterion and hydrogen atoms of either the amino group or water. In an aqueous solution, 2C_n(2-O-2) Gly formed small micelles, whose structure transitioned from spherical to ellipsoidal as the concentration increased.