The strong reduction in surface tension, which is characteristic of this group of active ingredients, forms the basis for antimicrobial efficiency in addition to the charge state of the active molecule. The quaternary ammonium compounds (QAV) cause an inactivation of the enzymes and a denaturation of essential cell proteins. Adsorption of the cation-active QAV on the cell surface disrupts the permeability of the microorganism cell wall, cell contents are released into the surrounding liquid and the cells are thus killed. Protein-like contamination, cellulose and other polymeric substances can affect the effectiveness of quaternary ammonium compounds. For this reason, QAV products should only be used in devices and systems that have been thoroughly cleaned previously.
Determination of the concentration of quaternary ammonium compounds (QAV) in disinfectants.
Suitable for all solutions containing quaternary ammonium compounds (QAV).
Many basic dyes (e.g. methylene blue) behave in a similar way to cation-active substances. Methylene blue forms a complex with dodecyl sulfate (anion-active surfactant), for example, which is soluble in organic solvents such as dichloromethane with blue colour. By adding quaternary ammonium compounds (QAV), this complex is decomposed and the methylene blue migrates from the dichloromethane into the aqueous phase. When titrating quaternary ammonium compounds with dodecyl sulfate, this process takes place in the opposite direction, i.e. the methylene blue migrates from the aqueous to the dichloromethane phase. The end point of the titration is reached when the aqueous and dichloromethane phases have the same colour depth.
Or to put it in other words:
Methylene blue (water-soluble dye in cationic form) forms a complex with dodecyl sulfate (anionic surfactant), which is soluble in organic solvents, e.g. dichloromethane, with blue colour. On the other hand, QAV also forms a complex with dodecyl sulfate, so that the indicator migrates into the organic phase once the equivalence point has been exceeded.