Urea Derivatives Anchored on Silica Gel

Methyl- and 1,3-dimethylurea were covalently bonded to silica gel, giving Sil-mu and Sil-dmu, by reacting these molecules with the (1) silanizing agent previously bonded to the surface (heterogeneous method) or (2) silanized precursor agent with the support (homogeneous method). In this last procedure the silanizing agent was characterized by elemental analyses and infrared, proton and carbon NMR spectra. The basic centers on the immobilizing molecules extract MCl2 (M = Ni, Co, Zn, Cu, Cd, Hg) from acidic water, ethanol, and acetone solutions at 298 K. The chemisorption isotherms in nonaqueous media conform to a modified Langmuir equation. The retention of cations in aqueous solutions increases with pH for Sil-dmu. The maximum retention capacity followed the sequence Cu > Ni > Hg > Zn > Cd > Co in ethanol and Cu > Zn > Co in acetone for Sil-dmu and Co > Cu > Zn in acetone for Sil-mu. With the exception of Co on both surfaces, and Ni and Cu in ethanol for Sil-dmu, the variation in enthalpy of all solvent-solute interactions on these silicas is exothermic in nature. The free energy values demonstrated the spontaneity of these interactions. Electronic spectra of solid suspended in carbon tetrachloride, collected after batchwise adsorption, indicated a tetrahedral geometry for Ni and Cu complexes.