Structural studies of complexation of Cu(II) with aminosilane-modified silica surface in heterogeneous system in a wide range of pH

Abstract Identification of the copper(II) ions complexes deposited on the amorphous silica surface was conducted using four different research techniques. The potentiometric titration in heterogeneous system was used to establish the forms of complexes with ligands immobilized on the surface. The types of Cu(II) complexes depend on the pH value. In the pH range 4–11, the presence of the following four types of Cu(II) complexes was evidenced: CuHSGL, CuSGL, Cu(SGL)2, and Cu(SGL)2(OH). Their stability constants were found to be log βCuHSGL = 14.01, log βCuSGL = 7.51, log βCu(SGL)2 = 14.12, and log βCu(SGL)2(OH) = 4.84 for I = 0.1 M KNO3. These complexes were characterized by X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and UV–Vis methods. According to the XPS results, close to pH 6, the partly protonated ligand can additionally form surface ion compounds of the type [R-NH2+ (CH2)2 NH2][NO3–]. EPR permitted determination of the spin-Hamiltonian, showing that Cu(II) ion was in the ground state dx2-y2, and the Cu(II) ions symmetry can be described as that of deformed tetragonal octahedrons. UV–Vis measurements in a flow cell confirmed the course of reaction in the heterogeneous system between Cu(II) and diamine ligand, N-(2-amineethyl-3-aminepropyl)trimethoxysilane, immobilized on the surface of amorphous silica gel.