Termodinâmica de adsorção de compostos fenólicos e azocorantes em nanotubos de carbono

• Autor(a) principal: Oliveira, Cassiano Rodrigues de
• Data de Publicação: 2012
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/223
• topic: Termodinâmica; Fenóis; Azocorantes; Thermodynamics; Phenols; Azo dyes; CNPQ::CIENCIAS EXATAS E DA TERRA::QUIMICA::FISICO-QUIMICA

The thermodynamics of adsorption of phenolic compounds and azo dyes onto multiwalled carbon nanotubes (MWCNT) was evaluated by isothermal titration calorimetry (ITC). The adsorption isotherms of phenolic compounds presented non-linearity and good fitness of the Langmuir and Freundlich models to experimental data. The maximum adsorption capacity of phenols followed the order: phenol < 4-chlorophenol < 2-methylphenol. The changes of standard Gibbs free energy of adsorption (&#916;adsGo), standard enthalpy of adsorption (&#916;adsH°) and standard entropy of adsorption (&#916;adsS°) were determined and the adsorption of all phenolic compounds was spontaneous, with the following order of the decrease of &#916;adsGo: phenol < 2-methylphenol < 4- chlorophenol. Microcalorimetric measurements revealed that phenolic compounds adsorption is enthalpically driven and values of &#916;adsH are on the magnitude of hydrogen bonds. Hydrogen bonds are supposed to be the driving force for the adsorption process of phenolic compounds onto MWCNT. The inductor effect, caused by the functional groups (CH3, H and Cl) presented in 2-methylphenol, phenol and 4- chlorophenol, respectively, builds different interactions between oxygenated functional groups on the MWCNT surface and phenolic hydroxyls (OH) and the resonance effects can be the reasons for distinct values of &#916;adsH. The adsorption of acid yellow 42 (AY), acid black 210 (AB) and acid green 68:1 (AG) onto MWCNT presented isotherms with non-linear shapes, with good fitness of Langmuir model and following the order of adsorption: AG < AB < AY. The adsorption of all azo dyes onto MWNCT was spontaneous, endothermic and entropically driven, according to values of &#916;adsH° and &#916;adsS° estimated from van t Hoff approximation. The amount of azo dyes adsorbed increased proportionally with ionic strength and adsorption of AY onto MWCNT decreased when the surface of the carbon nanotubes is oxidized. These observations suggest a heterogeneous surface on MWCNT. The thermodynamic properties obtained by isothermal titration calorimetry differ from those estimated by van t Hoff approximation, and show that the adsorption of azo dyes onto MWCNT is an exothermic process. The adsorption of AB, AG and AY is enthalpically driven with &#916;adsS° influence on the spontaneity of AY adsorption process. Electrostatic interactions and &#960;-&#960; dispersion forces may occur simultaneously in order to explain the general mechanism of azo dyes adsorption and the intensity of these interactions are dependent of the chemical structure of the azo dye. This work shows, for the first time, that the microcalorimetry describes the surface heterogeneity of MWCNT and, proves that the different sites of adsorption on the MWCNT surface interact distinctly with the molecules of the adsorbates, with intensities of interaction dependent of the chemical structure of the adsorbed compound.