Two independent routes to synthesize identical silicas by grafting ethylenimine or 2-aminoethanethiol, their cation adsorbing abilities, and thermodynamic data

The same grafted silica gel surface was obtained through two distinct procedures: (i) by immobilization of the compound 2-aminoethanethiol on precursor 3-chloropropylsilylsilica gel in a heterogeneous method, and (ii) by reacting ethylenimine with 3-mercaptopropyltrimethoxysilane to give a new silylating agent, which was anchored on the silica gel in a homogenous method. The first anchored surface gave 0.70 mmol g(-1) of molecules covalently bonded per gram of silica. This value contrasts to that obtained with the homogeneous conditions, with 1.74 mmol of pendant groups per gram of silica. Both organofunctionalized surfaces have the ability to adsorb divalent cations from aqueous nitrate solution. The adsorption was per-formed using a batchwise process and the maximum adsorption capacity followed the sequence Cu > Ni > Co and Ni > Cu > Co for surfaces prepared by the homogeneous and heterogeneous methods, respectively. Through calorimetric titrations, the thermodynamic values of cation-basic center interactions were obtained. Exothermic enthalpy results were obtained for all interactive processes. The calculated free Gibbs energy is in agreement with the spontaneity of the reactions and the positive entropy values for all reactions are favorable for this system. (C) 2002 Elsevier Science (USA).