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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<sub>ads</sub><i>H</i><sup>0</sup> 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<sub>ads</sub><i>H</i><sup>0</sup> = −9.13 ± 0.02 kJ mol<sup>–1</sup>, Δ<sub>ads</sub><i>G</i><sup>0</sup> = −21.18 ± 0.61
kJ mol<sup>–1</sup>, and Δ<sub>ads</sub><i>S</i><sup>0</sup> = 40.42 ± 0.61 J K<sup>–1</sup> mol<sup>–1</sup> for u-CNT and Δ<sub>ads</sub><i>H</i><sup>0</sup> = −11.49 ± 0.34 kJ mol<sup>–1</sup>, Δ<sub>ads</sub><i>G</i><sup>0</sup> = −27.88
± 0.18 kJ mol<sup>–1</sup>, and Δ<sub>ads</sub><i>S</i><sup>0</sup> = 54.97 ± 0.38 J K<sup>–1</sup> mol<sup>–1</sup> for f-CNT. The process is both enthalpically
and entropically driven, having a more negative Δ<sub>ads</sub><i>G</i><sup>0</sup> 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