Quaternary ammonium dyes demonstrate excellent antimicrobial ability against both Gram-negative and Gram-positive bacteria in aqueous solutions. But having active cationic sites, they could get inactivated by anionic surfactants.The interaction of the cationic dyes with opposite and same-charged surfactants was investigated by spectrophotometric and conductometric methods. While same-charged dyes and surfactants had no interaction in pre-micellar and micellar regions, the higher values of the binding constants for oppositely-charged dye-surfactant systems indicated stronger interactions. In the sub-micellar regions, hypsochromic shifts were observed and there was a decrease in the absorption intensity. In the micellar range, bathochromic shifts appeared due to the increase in dye solubility in the surfactant micelles.  At concentrations below CMC the cationic dyes lost their biocidal functions due to formation of complex with the anionic surfactant. But beyond the CMC, the dyes gradually reverted to their monomeric and dissociated states and regained their biocidal effects.

Investigation of interactions between biocidal cationic dyes and surfactants

239th National Meeting (2010)

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