Book Description

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 and hard water cations. Despite the excellent antibacterial activity in aqueous solutions, when attached to negatively charged solid substrates (e.g. sulfonate-containing acrylic fabric), the biocidal effect of the dyes decreased significantly. It is likely that the dyes can only inhibit bacterial growth when the N active sites, which are responsible for the biocidal effect, are not blocked by inhibiting anionic compounds. Only in the case of dye leaching off from the textiles (even at concentrations below 10 ppm) excellent antibacterial properties were observed. Therefore, the dyes can not provide desired antimicrobial functions if they are combined with reactive sites of polymers. The interaction of the cationic dyes with opposite and same-charged surfactants was investigated spectrophotometrically, and the appropriate values of dye-surfactant complex formation constants, water-surfactant partition coefficients, and Gibbs free energies were estimated. 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 due to hydrophobic and electrostatic forces. In the sub-micellar regions, strong interactions occurred, hypsochromic shifts were observed, and there was a decrease in the absorption intensity. By further increasing the surfactant concentration to 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 in micellar regions.