Covalent Grafting of Organoalkoxysilanes on Silica Surfaces in Water-Rich Medium as Evidenced by 29Si NMR
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1007-S09-04
Covalent Grafting of Organoalkoxysilanes on Silica Surfaces in Water-Rich Medium as Evidenced by 29Si NMR Christian Bonhomme, Sophie De Monredon, François Ribot, and Florence Babonneau Laboratoire chimie de la matière condensée, Universite P et M Curie, 4 place Jussieu, Paris, France
ABSTRACT Despite the huge amount of experimental investigations dedicated to the grafting of organoalkoxysilanes on silica, the characterization of covalent linkages remains a difficult task from a spectroscopic point of view. In most cases, the formation of a covalent link is taken for granted and not discussed. 29Si NMR, IR or UV spectroscopies are commonly used to study the modification of silica surface. Herein we report an unambiguous method to evidence the formation of covalent links. It is based on NMR spectroscopy analysis coupled with a rigorous synthetic grafting protocol. INTRODUCTION Silica particles functionalized by organoalkoxysilanes have been extensively studied because of their applications in polymer materials [1] chromatographic separation [2], biology [3], chemical sensing [4-5] and microelectronics [6], or as additives to improve mechanical resistance [7-8]. A large variety of alkoxysilanes (and particularly trialkoxysilanes) are classically used to modify silica surface. However, the control of the final nanostructure is very difficult to achieve and is still an important subject of research. The general formula of trialkoxysilanes is RSi(OR')3, where R is a functional group and OR' is an hydrolysable group leading to a silanol group which can further condense with the silanols present at the silica surface. In such a reaction, usually under acidic conditions, the hydrolysis of the alkoxy groups occurs rather rapidly, while the condensation reaction is much slower. Consequently, the coupling agent could be covalently linked or just hydrogen bonded to the silica surface (Figure 1). Furthermore, after hydrolysis, the organosilane molecules can self-condense and form a polysiloxane network connected to the nanoparticle only through a few number of bonds rather than a well-defined monomolecular coverage in which all the organosilanes are anchored to the surface (Figure 1b). This self-condensation phenomenon can be predominant in a water-rich medium.
Figure 1. Various models of grafting of silanes on silica nanoparticles. For few years there has been an increasing pressure on the chemical industry to develop more environmentally friendly processes and companies aim to develop surface modification of silica in aqueous medium. However, the chemical and thermal stability and the properties of the cover layer are strongly influenced by the nature of the link between the organosilane and silica, and the more water in the medium, the more difficult it is to avoid self-condensation. That is why, despite the environmental issues, grafting reactions are generally performed and studied in organic solvent or in a medium containing less than 10 vol% of water. The control of the linkage between the organosilane and
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