Enthalpic characterization of activated carbons with different surface chemistry with organic solvents and water

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 pHpzc. 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 non-polar 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.