Modified coupling agents based on thiourea, immobilized onto silica. Thermodynamics of copper adsorption Fernando J.V.E. Oliveira a , Edson C. da Silva Filho b , Maurício A. Melo Jr. a , Claudio Airoldi a, * a Institute of Chemistry, University of Campinas, UNICAMP, P.O. Box 6154, 13084-971 Campinas, SP, Brazil b Química, Universidade Federal do Piauí, 64900-000 Bom Jesus, PI, Brazil article info Article history: Received 30 September 2008 Accepted for publication 15 April 2009 Available online 23 April 2009 Keywords: Thiourea Inorganic–organic hybrids Organofunctionalization of surfaces Copper removal Calorimetry abstract New silylating agents synthesized by incorporating the thiourea molecule into precursor agents contain- ing increasing numbers of basic nitrogen atoms, 3-aminopropyltriethoxysilane, N-[3-(trimethoxysi- lyl)propyl]-ethylenediamine and N-[3-(trimethoxysilyl)propyl]diethylenetri-amine, through a solvent- free methodology were characterized before being successfully immobilized onto silica gel. The spectro- scopic 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 exo- thermic enthalpic data, which yield negative Gibbs free energies for removal of this cation from aqueous solutions by all the newly synthesized materials. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Investigations of inorganic–organic hybrid materials have sig- nificantly increased in recent years, mainly due to the attractive features related to this research field [1,2]. An important goal is the development of a sequence of reactions to design well-estab- lished molecules covalently bonded to an inorganic surface, which can then be applied for practical applications [3,4]. Silica gel is one of the inorganic polymers having the ability of attachment of desir- able molecules, giving the final materials anti-swelling properties, fast adsorption kinetics and good mechanical stabilities [3]. The sorption capacity of these chemically modified materials is closely dependent on a large variety of reactions that can be per- formed on the inorganic surface. Thus, the success of such reac- tions consists in condensing the free silanol groups on silica with the available alkoxy groups present on the most useful organosi- lane agents, in which process a strong silicon–oxygen covalent bond is formed. To attain this purpose, the covalent organic mole- cule bonded to silica must be well-chosen, in order to provide attachment of the desired basic centers. Improvement of this process depends on an expanded chain on the silylating agent itself or its introduction after bonding to enlarge the chains. In any case, the design enables attaching a series of basic centers to the main chain, for example, amino, mercapto and urea groups [4,5]. There- fore, the final solids present significant improvements in thermal and chemical stabilities, allowing this class of materials to be ap- plied for concentration of cations [3,6,7], as catalysts [8], as station- ary phases in HPLC [9], for pesticide removal [10,11] and as sensors [12]. The most common routes to synthesize desirable silanes to ob- tain the desired objective includes both homogeneous and hetero- geneous methodologies [13,14]. The first procedure is based on the reaction of an organosilane precursor with the desired molecule to yield the new agent. This product, normally in liquid form, is sub- sequently reacted with suspended silica in an inert solvent, due to the favorable condensation of the original alkoxide with the silanol groups disposed on the inorganic surface. With the heterogeneous method a precursor silylating agent is first immobilized before reacting with the same molecule to lead to the same final product. From these methodologies, the homogeneous procedure generally provides higher contents of immobilized groups in comparison to the heterogeneous route, when the same agent is considered [6,8,13]. 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2009.04.020 * Corresponding author. Tel.: +55 19 3521 3055; fax: +55 19 3521 3023. E-mail address: airoldi@iqm.unicamp.br (C. Airoldi). Surface Science 603 (2009) 2200–2206 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc