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

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 Δ_ads<i>H</i>⁰ 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: Δ_ads<i>H</i>⁰ = −9.13 ± 0.02 kJ mol^–1, Δ_ads<i>G</i>⁰ = −21.18 ± 0.61 kJ mol^–1, and Δ_ads<i>S</i>⁰ = 40.42 ± 0.61 J K^–1 mol^–1 for u-CNT and Δ_ads<i>H</i>⁰ = −11.49 ± 0.34 kJ mol^–1, Δ_ads<i>G</i>⁰ = −27.88 ± 0.18 kJ mol^–1, and Δ_ads<i>S</i>⁰ = 54.97 ± 0.38 J K^–1 mol^–1 for f-CNT. The process is both enthalpically and entropically driven, having a more negative Δ_ads<i>G</i>⁰ for the system based on a modified nanotube. With this work, we expect to increase the interest of researchers in the study of other solid–liquid adsorption systems using calorimetric techniques and also contribute to a more accurate characterization of the thermodynamic properties without the use of an excessive number of approximations