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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