Modified coupling agents based on thiourea, immobilized onto silica. Thermodynamics of copper adsorption

New silylating agents synthesized by incorporating the thiourea molecule into precursor agents containing increasing numbers of basic nitrogen atoms, 3-aminopropyltriethoxysilane, N-[3-(trimethoxysilyl)propyl]-ethylenediamine and N-[3-(trimethoxysilyl)propyl]diethylenetri-amine, through a solvent-free methodology were characterized before being successfully immobilized onto silica gel. The spectroscopic results support the elemental analysis, demonstrating that the thiourea molecule reacted in such a way as to maintain free amino groups. NMR data for carbon and silicon nuclei eflucidated the structural features associated with the incorporated chains and their bonds to the silica skeleton, with formation of covalent silicon–carbon bonds. Thermogravimetric data correlated with the degree of functionalization, which was also obtained from elemental analysis. The available basic nitrogen and sulfur atoms attached to the incorporated pendant chains have the ability to coordinate copper to give 1.10, 0.47 and 1.25 mmol g−1 as maximum adsorption capacities for the samples prepared with the silanes containing one, two and three nitrogen atoms on their structures. The thermodynamic data for copper/basic center interactions at the solid/liquid interface were determined through calorimetric titration, and the set of data demonstrated favorable systems for all three newly synthesized derivatized silicas, as shown by exothermic enthalpic data, which yield negative Gibbs free energies for removal of this cation from aqueous solutions by all the newly synthesized materials.