Intermolecular Interactions between Surfactants and Cationic Dyes and Effect on Antimicrobial Properties

Cationic molecules can interact with anionic and other cationic species in solutions. Electrostatic and hydrophobic interactions between two series of cationic antimicrobial dyes and several surfactants were studied using a UV-vis spectrophotometer. The dye containing more cationic charges with a short alkyl chain showed stronger binding power with surfactants than the one with less positive charge, while the ones with longer chains exhibited lower binding energy. These dyes could form complex structures with anionic surfactant species in solutions at very low concentrations of the surfactant (far below critical-micelle-forming concentrations of the surfactants), which was revealed from the changes in absorbance and wavelength of the maximum absorbance (lambda(max)) of photospectrometry. However, when the concentrations of surfactants were above their critical micelle concentrations, the dyes could redissolve and dissociate into molecular or ionic forms. Such results were confirmed by conductivity and antimicrobial functions of the solutions. These findings are useful in understanding the impact of anionic surfactants on cationic antimicrobial agents.