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

•Shrinkage phenomenon due to self aggregation of Congo red•CTAB has sub-, post-, and dilution-micellar effects towards the solubilization.•Ag+ ions and AgNPs have strong complex forming affinity.•AgNPs could not be used as effective catalyst.

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 495 nm. The shape of the absorbance spectrum changed entirely with increasing [Congo red] but wavelength maxima remain unchanged. The molar absorptivity was found to be 9804 mol− 1 dm3 cm− 1 at 495 nm. 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 × 104 mol− 1 dm3 and 11,764 mol− 1 dm3 cm− 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.

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