Thermodynamic Study of Methylene Blue Adsorption on Carbon Nanotubes Using Isothermal Titration Calorimetry: A Simple and Rigorous Approach

Abstract

In this article, a thermodynamic study of the methylene blue (MB) adsorption on carbon nanotubes (CNT), a known model system, was carried out by using a simple and rigorous experimental approach based on adsorption and isothermal titration calorimetry (ITC) experiments. Considering the thermodynamics of the process, the classical approach using the van’t Hoff approximation provided endothermic values for ΔadsH0 while the ITC measurements revealed that the adsorption of MB on both unmodified and acid-modified CNTs is an exothermic process. The thermodynamic parameters for the systems were obtained using the infinite dilution regime and ITC data: ΔadsH0 = −9.13 ± 0.02 kJ mol–1, ΔadsG0 = −21.18 ± 0.61 kJ mol–1, and ΔadsS0 = 40.42 ± 0.61 J K–1 mol–1 for u-CNT and ΔadsH0 = −11.49 ± 0.34 kJ mol–1, ΔadsG0 = −27.88 ± 0.18 kJ mol–1, and ΔadsS0 = 54.97 ± 0.38 J K–1 mol–1 for f-CNT. The process is both enthalpically and entropically driven, having a more negative ΔadsG0 for the system based on a modified nanotube. W...