Functional Thin Films and Energy Materials

Articles

Vol. 1 (2025)

Surface Heterogeneity Effect on Azodyes Adsorption on to Multiwalled Carbon Nanotubes

Submitted
September 15, 2025
Published
2025-09-12

Abstract

The adsorption thermodynamics of the azo dyes acid yellow 42 (AY), acid black 210 (AB) and acid green 68:1 (AG) onto pristine multi-walled carbon nanotubes (MWCNTs) was evaluated by determining the following thermodynamic adsorption parameters: adsorption standard free energy (ΔadsGº), adsorption standard enthalpy (ΔadsH°) and adsorption standard entropy (ΔadsS°). The adsorption of all azo dyes onto MWCNTs was a thermodynamically spontaneous process and the decrease in ΔadsGº values follows the order AG < AB < AY. The increase of oxidized sites on MWCNT surface enabled a decrease of adsorption capacity of AY, compared with raw MWCNT. The adsorption of all azodyes is directly proportional with the ionic strength of solution. Isothermal Titration Calorimetry (ITC) data show that the adsorption process is enthalpically driven for all azodyes with contribution of entropy change in the spontaneity of AY adsorption onto MWCNT. The surface of the MWCNT has sites with different energetic potentials that allow electrostatic interactions with azo dye compounds, not only π−π dispersion interactions. The ITC technique provides a more reliable interpretation of the surface chemistry of MWCNT and of the interactions in an adsorption process in comparison with the van’t Hoff approximation.

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