Aggregation of Congo red with surfactants and Ag-nanoparticles in an aqueous solution.

Self aggregation, sorption, and interaction of Congo red, with cetyltrimethylammonium bromide (CTAB), sodium dodecylsulfate (SDS), Ag+ ions and silver nanoparticles have been determined spectrophotometrically. Congo red self-aggregation was identified from UV–visible spectra due to the shrinkage in an absorption band at 495nm. The shape of the absorbance spectrum changed entirely with increasing [Congo red] but wavelength maxima remain unchanged. The molar absorptivity was found to be 9804mol−1dm3cm−1 at 495nm. Absorption spectra of Congo red with Ag+ ions show an isosbestic point. The complex formation constant and difference in absorption coefficients were found to be 8.5×104mol−1dm3 and 11,764mol−1dm3cm−1, respectively. Silver nano-particles could not be used for the catalytic degradation of Congo red because it results in the formation of a strong complex with them. Sodium dodecylsulfate did not show any significant interaction with this dye. Congo red was also used as a probe to determine the critical micellar concentration of CTAB.