Diazonium acid-dyes elimination via complexation with sulfonated calix[4]arene soluble in aqueous medium; on an experimental and theoretical study

This research highlights the effectiveness in employing a sulfonated para-tert-butylcalix[4]arene derivative (SCX4) as an effective molecular host for adsorption of acidic azo dyes in aqueous environment. Due to its inherent stability and rigidity, this chemical may be suitable as a host for host-guest interactions. Therefore, the sulfonated calixarene was used to study its complexation properties with the dyes, namely Acid Red 88 (AR88) and Acid Orange 7 (AO7) as adsorbing guests. Calculated formation constants (Kf) for the SCX4/dyes complexes were estimated from comparison of the absorption intensities. The formation constants were 5.1∙104 M-1 and 5.2∙104 M-1, correspondingly. Calculated stoichiometric ratios of the complexes were evaluated via Benesi-Hildebrand diagrams, which gave a ratio of 1:1. The equilibrium adsorption data of Acid Orange 7 were evaluated via three adsorption isotherm models including Freundlich, Temkin and Langmuir. The adsorption isotherms showed compatibility with three different approaches. The findings suggest that the Freundlich model better covers this adsorption process. Using a DFT-based approximation revealed the preferred structure of SCX4 and its complex with Acid Orange 7.