Enthalpic characterization of activated carbons with different surface chemistry with organic solvents and water
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Enthalpic characterization of activated carbons with different surface chemistry with organic solvents and water Diana Hernández‑Monje1 · Liliana Giraldo1 · Juan Carlos Moreno‑Piraján2 Received: 28 August 2019 / Accepted: 1 January 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
Abstract Immersion enthalpies of activated carbons modified on their chemical surface were determined in benzene, cyclohexane, hexane, ethanol and water. Three samples of activated carbons were used: the starting sample and two samples subjected to modifications with nitric acid and heat treatment at 750 °C, respectively, in order to modify their textural properties, but mainly their surface chemistry. Subsequently, textural characterization and surface chemistry were performed by N2 adsorption isotherms, Boehm titrations and the determination of the p Hpzc. For the chemical characterization, phenolic groups predominated in the starting sample as well as a greater total acidity with respect to the sample that was subjected to heat treatment, in which the amount of oxygenated groups decreased and the total basicity increased. For the modification with nitric acid, the presence of the oxygenated groups increased. The enthalpies of immersion were exothermic (between − 21.23 and − 128.8 J g−1). Benzene was the non-polar compound with the greatest enthalpy, followed by cyclohexane and hexane, due to its atomic disposition and the interaction with the surface groups of activated carbon. For polar compounds, ethanol had higher values for the enthalpy of immersion with respect to water, due to the ability to interact with the polar and nonpolar part of the molecule, to its interaction with different oxygenated groups and finally to the formation of clusters which favor an increase in solid-solvent energy interactions. Keywords Immersion enthalpy · Surface chemistry · Activated carbon · Non-polar molecules · Polar molecules · Interaction
Introduction The enthalpy of immersion is a thermodynamic parameter that allows to determine the intensity of the interaction produced by the contact of a porous solid (activated carbon) with an adsorbate. This interaction depends on both, the properties of the solid (porosity and surface chemistry) and the liquid (size, molecular arrangement, affinity with the activated carbon, polarity). The enthalpy of immersion is determined by means of immersion calorimetry, which is a technique with high sensitivity and precision for the * Juan Carlos Moreno‑Piraján [email protected] 1
Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Sede Bogotá, Carrera 30 No. 45‑03, Bogotá, Colombia
Departamento de Química, Facultad de Ciencias, Universidad de los Andes, Carrera 1 este No 18A‑10, Bogotá, Colombia
2
measurement of small amounts of energy, in the order of 10 to 100 mJ, such as those produced in the solid–gas and solid–liquid interactions. On the other hand, it is used frequently and in different areas since it provides complementary information to the extensive studies of g
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