Self-Assembled Monolayers of 12-Mercaptododecylphosphonic Acid on Titania Particles; Application to the Extraction of He
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Self-Assembled Monolayers of 12-Mercaptododecylphosphonic Acid on Titania Particles; Application to the Extraction of Heavy Metals Sébastien Anthérieu, Florence Brodard-Séverac, Gilles Guerrero* and P. Hubert Mutin UMR CNRS 5637, Université Montpellier II, Place E. Bataillon 34095 Montpellier Cedex 5, France. ABSTRACT Titania particles (P25 DEGUSSA AG) were treated by a solution of 12-mercaptododecylphosphonic acid (MDPA) in toluene to obtain an organic-inorganic hybrid material with thiol functions at the surface. This material was characterized by chemical analysis, solid state 31P MAS NMR and FTIR spectroscopies, and XPS. Reaction of the phosphonic acid end of MDPA with the TiO2 surface led to the formation of a relatively dense and well-ordered self-assembled monolayer (SAM), with a density of about 3.9 SH functions per nm2. This material was tested for the extraction of mercury in water at pH 7, 9, and 11. Chemical analysis indicated no loss of SH functionality, even at pH 11. The best extraction results were obtained at pH 9 and 11, with a metal extraction ability between 85 and 90%.
INTRODUCTION Phosphonic acids are attractive coupling molecules for the covalent anchoring of organic groups to metal oxide supports, such as titania[1-3] and alumina[4, 5], due to the stability of P-C and M-O-P bonds. Bonding to the surface involves the formation of M-O-P bonds, arising from the coordination of the phosphoryl oxygen to Lewis acid sites and the condensation of the P-OH groups with surface hydroxyl groups M-OH[6]. The reaction of long-chain alkylphosphonic acids with metal oxide supports leads to dense, well-ordered self-assembled monolayers (SAMs)[7-10] that can find applications in a wide range of fields such as catalysis, corrosion resistance, microelectronics, and chemical sensors[11, 12]. Initially, the definition of SAMs implied that the molecules assemble by non-bonding interactions with the substrate surface and among each other but actually, the term “SAMs” include strong iono-covalent interaction between the molecule and the surface too[8, 9, 11]. Then, the self-assembly refers to non-bonding interactions between long alkyl chains that induce a high grafting density and the chain organization. Hybrid materials based on mesoporous silica functionalized by thiol groups are efficient in removing mercury and other heavy metal ions from both aqueous and non-aqueous waste streams[13-15] and their extraction ability improved in alkaline media[16]. However, silicabased materials have limited stability under alkaline conditions. On the other hand, titaniaphosphonate hybrids exhibit much higher stability[17] owing to the high chemical stability of both TiO2 and the interfacial Ti-O-P bonds. The aim of this work was to prepare hybrid materials bearing SH functions stable in alkaline media, by self-assembly of 12-mercaptododecylphosphonic acid (MDPA) on titania particles. The formation of a SAM should allow the incorporation of a maximum amount of thiol functions on the surface. In the present work, the h
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